Airplane Motors 
 
 A Course of Practical Instruction in Their 
 Care and Overhauling 
 for the use of 
 
 MILITARY AVIATORS 
 by 
 
 George E. A. Hallett, A. M. E.; A. S„ S. C. 
 
 WASHINGTON 
 Press of Gibson Bros., Inc. 
 
 1917 
 
Digitized by the Internet Archive 
 in 2017 with funding from 
 
 University of Illinois Urbana-Champaign Alternates 
 
 https://archive.org/details/airplanemotorscoOOhall 
 
 I 
 
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 629-l^ss- 
 
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 A PRACTICAL COURSE OF INSTRUCTION IN 
 THE PRINCIPLES, OVERHAULING AND 
 CARE OF AIRPLANE MOTORS. 
 
 By Geo. E. A. Hallett, A. M. E., Signal Corps, 
 Aviation School, San Diego, Calif. 
 
 INTRODUCTION. 
 
 Object of this Course. — In the case of student flyers, is to give 
 them a practical knowledge of airplane motors sufficient to 
 enable them to diagnose motor trouble when on cross-country 
 flights and to make rapid and practical repairs if possible, or 
 in any event, to be able to send an intelligent message for parts, 
 etc., and to explain to mechanicians nature of trouble and its 
 remedy. 
 
 In the case of the mechanician, the object will be to make 
 an airplane motor man out of an automobile mechanic, or 
 machinist. Parts of it should be useful to give to automobile 
 factories to help them train aviation mechanicians. The 
 course consists of: (1) A series of lectures beginning with 
 principles and covering a wide range of practical work as well 
 as bench and block work. (2) A bench course, of overhauling 
 unserviceable motors (preferably airplane motors; methods 
 explained later) . (3) Block course of installing motor on a test 
 
 block or in a fuselage; placing propeller, cranking, handling 
 switch and throttle to start motor, carburetor adjustment, 
 “trouble-shooting,” emergency repairs, inspection or “preven- 
 tion of trouble.” (4) If time permits, a study course in some 
 good gas engine book. Attached will be found a list of twenty 
 assignments in “Dyke’s Automobile and Gasoline Engine 
 Encyclopedia.” These assignments cover the automobile 
 chassis in addition to motors and will be found fairly satis- 
 factory. While this book has many typographical errors, it 
 is fairly practical. 
 
 Methods of Conducting Practical Motor Course. 
 
 The course should last from four to six weeks (18 hours per 
 week), according to the amount of material or motors avail- 
 /-able. 
 
 ( 3 ) 
 
 . >o 
 
4 
 
 Three men on a motor is an ideal number. If more are put 
 on one motor, the work progresses too fast and therefore does 
 not “soak in.” It will be possible to give a ten day course, all 
 day sessions, which will cover all the work. (Note.— Each 
 three men are called a “section.”) 
 
 Always encourage the asking of questions. 
 
 The First Few Days. 
 
 The first lecture should be given on the first day. Other 
 lectures should be given as the work progresses. The first 
 thing to do is to spread out all the tools on the benches and 
 name them, so that all will know them by the same names. 
 Give each section a simple motor. (Note. — This motor 
 can be quite obsolete, and not necessarily complete.) 
 
 Explain principles of disassembling motor as the work 
 progresses. 
 
 1. Find the number on the parts. 
 
 2. Decide in which order parts should be removed. (Note. — 
 Some parts are not accessible until others are removed.) 
 
 3. Show best methods of removing cotter pins. 
 
 4. Make marks on timing gears before disassembling. 
 
 5. Keep shims and liners in proper place (explain impor- 
 tance) . 
 
 6. Cleaning parts (especially oil passages). 
 
 7. Nomenclature of parts. 
 
 Assembling. 
 
 Explain as follows: 
 
 All parts must be carefully oiled, because the oil pump and 
 oil pipes are now empty, and it will be best to keep the idea in 
 mind that an engine may run as much as ten minutes before 
 the pump can deliver oil to all the parts. 
 
 Bearing nuts must be tight. Never loosen the nuts to 
 loosen the bearing, but add shims or scrape it out if it is too 
 tight. 
 
 Explain the following: 
 
 Making paper gaskets. 
 
 The use of shellac and graphite with these gaskets. 
 
 Grinding valves. (Note. — Grind by hand and lift off the 
 seat between each stroke.) 
 
 Test the valves with gasoline. 
 
 How to distinguish the exhaust valves from inlet valves. • 
 (Note. — The exhaust valve stems will be darkened by heat; 
 sometimes the heads are designed differently. Exhaust valve 
 
springs are always the strongest if there is any difference in 
 the two springs.) 
 
 Valves should never be ground more than necessary. If 
 there is a fair width of seat all around, it is better to leave a 
 few “pits’ ’ near the edges rather than to grind so much of 
 the valve seating away. 
 
 Explain valve timing in its simplest form, viz., put the gears 
 together by their marks. (Note. — It is not necessary to put 
 an ignition system on this engine.) 
 
 Have the class trace the water circulation and oil circulation. 
 
 Explain how to determine the direction of rotation of the 
 motor by watching the valves, as follows : 
 
 We know from the lecture on the first day, what 
 must occur inside a motor to make it run. Remember 
 that after the exhaust stroke (recognized by the exhaust 
 valve being open) we have a suction stroke (which we 
 recognize by the opening of the inlet valve). Therefore, 
 when we turn the motor in the proper direction, the inlet 
 valve will open immediately after the exhaust valve closes. 
 
 The above work will consume a different length of time 
 with different classes and motors, and therefore, I set no 
 definite time for its completion. 
 
 When this work is done, give the class motors which are 
 complete and more modern, preferably airplane motors. 
 
 It is an immense advantage if the motor can be installed 
 on a testing block or in a fuselage of an airplane and run on 
 completion of overhauling, because the class will show far 
 more interest and effort under these circumstances. 
 
 As the sections begin work on these motors, make them 
 observe all the precautions taught on the first motors, and also 
 teach them how to measure and find the valve timing (closing- 
 position of exhaust valve) either in degrees or piston position. 
 In airplane motors, the valves are usually timed by piston 
 position. 
 
 Explain that this is a motor which is new to us and we must 
 know the valve timing in order to be able to put it together 
 properly. While it is true that ordinarily we could time the 
 valves by the marks on the timing gears, remember that fre- 
 quently parts either of the gears, the crank shaft, or cam 
 shaft must be replaced, and as key ways and gear teeth are not 
 cut so that these parts are interchangeable, our marks will be 
 of no value. Therefore, we must know other methods of 
 timing the motor. We will time this motor by degrees, 
 preferably by piston position, as we assemble it, and must 
 know the valve timing. 
 
6 
 
 Explain measuring spark timing. (Find at what piston 
 position the spark occurs.) 
 
 Explain how to try the bearings for looseness before the 
 motor is disassembled, so that we will know how many shims 
 to remove before assembling them. This saves time, because 
 if it is not done it will be necessary to assemble the bearings 
 before deciding how many shims to remove. 
 
 Explain that they must note the gear clearance between 
 the crank shaft and the cam shaft, so they will know if the 
 bearings must be raised, or possibly renewed, before scraping 
 them. 
 
 Try the adjustment of the thrust bearings and oil pump 
 gears before disassembling for the same reasons. The thrust 
 bearing of an airplane motor must hold the crank shaft so 
 that it “floats” between the main bearings. The cheeks of 
 the shaft must not be able to touch the cheeks of the bearings. 
 
 In adjusting a thrust bearing, if there is, for example, a 
 3 /64th of an inch end-play in the crank shaft, the thrust bear- 
 ing should be so adjusted that it will hold the shaft so that 
 2 /64 of that clearance will be on the side which will be reduced 
 by wear in the thrust bearing. Then if the motor is a tractor, 
 the greatest amount of clearance should be left on the side of 
 the crank throw which is toward the thrust bearing, because 
 the pull of the propeller will wear the thrust bearing in that 
 direction. 
 
 Explain the testing of the crank shaft, or if possible, test 
 the crank shaft by placing it in a lathe, between centers, and 
 by using a gauge to determine to what extent the crank shaft 
 is crooked. Explain that crank shafts are not used if they 
 are more than 5 /1000 of an inch out of line or crooked, but 
 must be straightened or reground. 
 
 Then carefully measure the journals with a micrometer. 
 The journal must not be over 5 / 10000 of an inch out of round. 
 Crank shafts become crystallized in time, even in com- 
 paratively heavy engines. For instance, the big omnibus 
 companies of London set a specified number of miles that a 
 crank shaft of any certain make is allowed to be used because 
 it is found through long practice, that if used longer than this, 
 the crank shafts are liable to become crystallized and wreck 
 an entire motor when they break. At present, in this country, 
 limits are being put on the use of certain makes of crank shafts 
 in airplane motors. , For example, one crank shaft used in 
 the service must not be used more than three hundred hours 
 of flying time. 
 
7 
 
 Explain the alignment of the main bearings. Emphasize 
 the fact that the crank shaft is limber, and if we should bear 
 down on it while marking the bearings, it might be possible 
 to mark a bearing which was already 2 /1000 of an inch low, etc. 
 
 The scraping of bearings should be taught because it is 
 necessary to scrape bearings in overhauling a motor, even 
 though they are usually reamed when the motor is built. 
 If possible, give the students some old bearing caps to practice 
 scraping on. Instruct them in the use of the scraper; warn 
 them to avoid starting or stopping a stroke abruptly with the 
 scraper because this leaves little notches which the scraper will 
 jump over on the next cut. After cutting a series of parallel 
 strokes to cut away part of a bearing, the scraper should be 
 moved across the surface diagonally to smooth it up. Point 
 out that it is necessary to cut comparatively lightly on the 
 sides of the bearings. For example, if you are letting the 
 shaft down into a bearing 1 /1000 of an inch, it will be neces- 
 sary to cut a thousandth deep at the bottom of the bearing, 
 but as the shaft is not going to be moved horizontally we wull 
 not cut any metal away at the top of the sides, and only one 
 half a thousandth, half way up the sides. In other words, cut 
 deepest in the bottom of the bearing and lighter and lighter as 
 we work up the sides. The oil circulation of the motor should 
 be systematically traced and all oil passages and screens care- 
 fully cleaned. 
 
 Cutting Oil Grooves. 
 
 If the bearings are renewed in a motor, the oil grooves in the 
 new bearings should be cut in exactly the same manner that 
 they were cut in the old ones. The form and location of oil 
 grooves are usually experimented with in the motor factories 
 and should not be altered by the mechanic who does the over- 
 hauling. 
 
 Adjusting Bearings. 
 
 Before deciding how tightly our bearings should be adjusted, 
 we must consider these things. Have we a low pressure 
 lubricating system, high pressure lubricating system, worn 
 bearings (bearings worn smooth), or newly scraped bearings? 
 The modern idea of bearings in an airplane motor is to fit 
 them very loosely, from two to four thousandths of an inch, 
 and maintaining the oil film by forcing a large amount of 
 oil through the bearings under very high pressure. If we have 
 a high pressure lubrication system, the bearings can be, and 
 should be, fitted loosely. For example, they should be care- 
 
8 
 
 fully scraped and fitted to the shaft and then a two thousandth 
 shim placed in each side of the bearing to loosen it up. If 
 we have a comparatively low pressure lubrication system, 
 the bearings must be adjusted closer or they would hammer 
 out. If we have newly scraped bearings, the shaft will touch 
 the bearings only in comparatively small spots. These spots 
 wear rather fast, so the bearing loosens up quickly for a short 
 time, until the shaft has worn the bearing to a perfect “fit.” 
 Knowing this, we adjust newly scraped bearings tighter than 
 we would if we were simply adjusting bearings which were 
 already worn smooth. 
 
 Instruction in filing should be given at this stage as it is 
 frequently necessary to adjust bearings by filing the bearing 
 cap. Impress the fact that nuts on all bearings must be very 
 tight. Point out that on a “V” type motor, and also on 
 the vertical type, if the bearing cap is not pulled down very 
 tightly it will shift or “work” while the engine is running, 
 and it is only a matter of time until the main bearing studs 
 will be crystallized and broken. 
 
 When fitting new connecting rods or newly babbitted con- 
 necting rods to a crank shaft, it is necessary to test them for 
 alignment. One method is to put a bar through the wrist 
 pin end of the rod, set the rod up on the crank shaft and 
 measure from the main journal on either side up to the ends 
 of this bar, to find out whether the connecting rod stands 
 exactly perpendicular to the crank shaft. (Old connecting 
 rods can be used for this purpose.) 
 
 Disassembling and Reassembling Piston Pin Bearings. 
 
 Explain methods of avoiding the straining or distorting of 
 the pistons and also the necessity for these precautions. 
 
 Most of our modern motors use the aluminum alloy piston. 
 Due to the expansion of the pistons when hot, the wrist pins 
 must fit tightly while the pistons are cold. And as we do not 
 wish to distort the piston by forcing the wrist pin into it, 
 we sometimes heat the piston in hot oil or water and then 
 drop the wrist pin in, thus avoiding some chances of distorting 
 the piston. 
 
 The piston rings should be removed and replaced. Explain 
 methods and precautions. Explain that the lower edge of 
 the piston ring and the lower edge of the ring slot are as im- 
 portant as the face of the ring; because the rings must be free 
 in the slots and therefore the pressure and the hot gases will 
 get in behind the rings, and unless the lower edge is perfect, 
 
9 
 
 will get out under the rings. In this way rings are made 
 practically useless. Rings must be returned to their own 
 slot in the same position as they were before (same side up). 
 
 Explain fitting of piston rings and particularly the adjusting 
 of the gap in the ring to allow for expansion. Rings must 
 not be stretched or twisted. 
 
 Grinding and Testing the Valves. 
 
 Most mechanics have a tendency to grind valves to excess. 
 It is best to grind them as little as possible. In other words, 
 if the valve is badly pitted do not try to remove all the pits, 
 but grind until you have a good width of seat all the way 
 around the valve. 
 
 The valve springs should be tested while compressing them 
 to the length which they occupy while on the cylinder with the 
 valve seated. Find out what their tension is by weighing 
 them on a spring scale. 
 
 Oil up the cylinder and pistons before assembling. Be 
 sure to explain that rags must never be used for this purpose. 
 Use only the bare hand which can be easily freed from all 
 grit, and while oiling with our hands we can detect grit if 
 present. Oil the piston thoroughly, under the rings, in the 
 wrist pin and all over the outside. Oil the cylinder all over 
 the inside ; leave no dry spots. Explain precautions in slipping 
 the cylinder over the pistons. If a ring is broken, it will score 
 the cylinder. See that the cylinder gasket is not doubled over 
 on one corner, because there is danger of throwing the cylinder 
 out of line in this way. When engines have separate cylinders, 
 it is necessary to line them up carfully with a straight-edge 
 so that the inlet manifold will not have to be sprung, and be 
 under strain when bolted to the cylinders. It is usual to test 
 the manifolds after assembling by blowing into them by 
 mouth or with air pressure; then put oil on all the joints and 
 see if bubbles come through. 
 
 The water pump stuffing boxes should be carefully packed. 
 Explain the importance of polishing the shaft before packing 
 to prevent excessive wear on the packing. 
 
 Explain the necessity of putting in the largest possible 
 amount of packing, because the more packing you can get 
 into the stuffing boxes the looser it can be left without leaking 
 and less wear and less friction will result. 
 
 Explain that the stuffing boxes should never be allowed to 
 leak, because if they do, the shaft will rust, become rough, and 
 cut out the packing. 
 
10 
 
 Assemble the valve operating gear. In high speed motors, 
 it is very essential to have every part of the valve operating 
 mechanism working perfectly freely. Adjust the valve clear- 
 ance. (See Lecture No. III.) Time the cam shaft. (See 
 Lecture No. III.) Time the magneto and wire up. (See 
 Lecture No. V.) 
 
 Be sure to have every man in the class go through these 
 processes by himself and be sure he understands it. Never 
 allow them to call the process complete without checking it 
 afterwards to find out exactly what results they have obtained. 
 This is important, even with the best mechanics in airplane 
 work. Always preach the idea of preventing trouble where 
 possible. 
 
 All the way through the process of assembling a motor, 
 bear in mind the necessity of having an oil supply for all 
 moving parts run for the first few minutes, before the oil 
 pump can fill all the passages and deliver oil to all parts. 
 
 If possible, these motors which we are just finishing over- 
 hauling should be installed on a testing block or in a fuselage, 
 and run. Then, if time and material permit, widely different 
 types should be overhauled. 
 
 Block Work. 
 
 Actual power determinations are of comparatively small 
 practical value to an airplane mechanic, but the motor can 
 be run with a propeller or a club. While installing the motor, 
 impress upon the class that the switch and ground wire must- 
 be connected before the propeller is put on. In this way, the 
 motor can be made safe while the propeller is on, there- 
 by lessening the danger of some one being kicked. On 
 many engines it is necessary to place the propeller in relation 
 to the spark timing, so that the propeller will come in a posi- 
 tion which is convenient for a strong pull while the magneto is 
 in a certain position. In case it should be necessary to explain 
 this, it can be done as follows : 
 
 Place the propeller on the engine but without fastening, so 
 that it can be swung freely without revolving the engine. 
 Now stand in the proper position to crank the propeller, and 
 hold the blade in the position where it will be convenient to 
 start the pull. Note this position. We must have room to 
 pick up speed before the spark occurs, to avoid back kicks. 
 Therefore, move the propeller down about twenty degrees and 
 note this position. This is where we wish the “break” or 
 spark to occur. Next, attach the propeller lightly so the 
 
11 
 
 motor can be turned over. Open the breaker box on the mag- 
 neto, turn the propeller until a break occurs (it doesn’t matter 
 in which cylinder). Now disengage the propeller; put it in the 
 position where we said we wished the spark to occur; secure it 
 in the proper manner. This simply brings the explosion in 
 the proper position for cranking. 
 
 Precaution. 
 
 In most magnetos, when the breaker cover is removed, the 
 switch no longer short circuits the magneto; therefore, while 
 performing this operation of placing the propeller, it will be 
 wise to remove the connection between the collector brush 
 and the distributor. Or, if this is not accessible, remove the 
 wires from the distributor or from the spark plugs, because 
 there may be enough gasoline fumes in the cylinder to cause 
 an explosion and a “kick.” 
 
 Propeller Notes. 
 
 Never rock the propeller as a preparation to the final pull 
 for starting. Many people are hurt in this way. Place your 
 propeller where you intend to start the pull, raise up on tip 
 toes, and start your pull strongly. Remember you must pick 
 up a great deal of speed in the first few degrees in order to 
 “carry over” the spark. As you finish the pull or stroke, 
 manage so that you will be withdrawing your hand§. If the 
 motor kicks back, never try to resist it; simply withdraw your 
 hands instantly. Never have tools in your pockets while 
 cranking. They may fly out of your pockets and be “batted” 
 through your legs by the propeller. 
 
 The man at the propeller and the man at the switch should 
 “sound off” what they are doing, so there may he no misunder- 
 standing. Make it a rule that the man at the switch will not 
 say “closed” until the switch is closed, because if he should say 
 “closed” and then leisurely reach over to close the switch or 
 make the motor safe, the man at the propeller might work 
 faster and pull the propeller, believing the engine to be safe, 
 and get a severe kick. 
 
 It is very important to have the throttle closed or nearly 
 closed whenever the switch is open. On the testing block, 
 cranking the motor with the throttle open, to start it, will 
 only result in the motor starting violently and a possibility 
 of injuring the man cranking; but in an airplane, the danger is 
 greater because the machine is likely to start ahead and seri- 
 
12 
 
 ously injure the man cranking the propeller as he would not be 
 able to get out of the way. 
 
 After installing the motor, make the class inspect the water, 
 oil and gas supply and also the ground wire and switch. Start 
 the motor and run it slowly at first. Point out that the motor 
 must be warmed up slowly and gradually to avoid unequal 
 expansion and consequent cracking of parts. 
 
 Carburetor Adjustment on the Block. 
 
 This work is particularly valuable in training the pupil’s 
 ear. The Model “L” Schebler carburetor is suitable for this 
 work, because it is adjustable yet simple. To make the ad- 
 justment of the carburetor as simple as possible, adjust the 
 auxiliary air valve spring so that it just holds the valve 
 against the seat, before starting the motor. This leaves onty 
 the gasoline adjustments to be made. 
 
 Then emphasize the fact that the looser the auxiliary air 
 valve spring is left, the larger volume of air will pass through 
 the inlet pipes to the cylinders, giving full charges of gas to 
 the cylinders and therefore more speed. The limit to this is, 
 that if the air valve is too loose the motor will not be able to get 
 enough gasoline even with the gasoline adjustments wide open 
 and also the motor will be liable to stop when throttled down, 
 and “pick up” badly. 
 
 Have every student go through this work himself, if gasoline 
 and time are available. Teach them that they must learn 
 to make the adjustments quickly to avoid over-heating of the 
 motor in airplane work. Then disarrange the adjustments 
 after each student finishes. 
 
 It is well to use open exhausts (no muffler) for this work, 
 so the class will become used to the noise and also the appear- 
 ance of the exhaust under the various mixtures and conditions. 
 
 Diagnosis of Trouble. 
 
 This is a very practical subject and can be so taught that 
 it will be very valuable not only to the mechanic but also the 
 pilot. The work consists of artificially causing troubles which 
 really do occur in practice on the field, then having the class 
 find the troubles. It is necessary to have the motor, prefer- 
 ably an airplane motor, mounted on a block where it can 
 be run. The instructor should plan the troubles carefully, 
 simple ones at first, and plan so there will not be two troubles 
 giving the same symptoms at the same time. 
 
13 
 
 To get the real value out of this work, first give Lecture No. 
 VIII on system for diagnosis of trouble. Do not let the class 
 go ahead and find troubles by inspection and haphazard 
 methods, but make them reason out where the trouble is and 
 why. If necessary reason out loud for them. Never make 
 troubles by changing carburetor adjustments and never allow 
 the class to correct troubles by changing these adjustments. 
 By following this rule, you will help to combat a failing of the 
 human race, viz., to try to correct all troubles by adjusting 
 the carburetor. 
 
 The following are some of the troubles we make on the 
 Curtiss 8-cylinder engines. They may serve as a suggestion 
 for this work. 
 
 Ignition Troubles. 
 
 A Bosch magneto type DR8 is suitable for this work. 
 
 Bad plug. 
 
 Shorted spark plug. 
 
 Loosened spark plug gasket. 
 
 Spark plug wire off. 
 
 Spark plug wire shorted against motor. 
 
 Ground wire short circuited. 
 
 Magneto leads crossed. 
 
 Magneto leads disconnected 
 
 Magneto brushes missing. (Collector brush, bridge brush, 
 ground brush, and distributor brush.) 
 
 Broken insulation at spark 
 
 Dirt in breaker. 
 
 Various breaker troubles. (Points of breaker apart; out of 
 adjustment and breaker screw loose.) 
 
 Magneto firing on the wrong stroke. 
 
 Safety gap shorted. 
 
 Breaker timed wrong. (Off the key.) 
 
 Water in magneto. 
 
 Various shorts in the secondary. For example, drill a small 
 hole in the distributor arm to the middle of the core in the 
 center; insert a small wire in this hole and cover over the 
 surface. 
 
 Magneto fully retarded and key removed from driving gear 
 so magneto will shift out of time. 
 
 Magneto timed on wrong top center. 
 
 Carburetor Troubles. 
 
 (Note. — Schebler Model “L” is suitable for this work.) . 
 
 Remove the auxiliary air valve spring. Block the auxiliary 
 air valve open (with short piece of copper tubing). 
 
14 
 
 Remove float valve. 
 
 Partially obstruct float valve. 
 
 Obstruct spray nozzle, partially and completely. 
 
 Loosen joint in the inlet manifold. 
 
 Put bad gaskets in the inlet manifold joints. (Gaskets 
 which will cause a leak.) 
 
 Put water in the float valve of your carburetor. 
 
 Cause the float to stick. 
 
 Replace the lift lever spring with a weaker one, so that the 
 roller will not follow the cam. 
 
 Plug the air vent in the gas tank. 
 
 Valve Troubles. 
 
 No clearance, or valve held open. 
 
 Cam follower stuck (no lock washer on set screw). 
 
 Defective push rod. 
 
 Cam shaft gears meshed wrong. 
 
 Weak exhaust valve springs. 
 
 Cut the points off the' cam-follower-guide-set-screw, so it 
 will allow the cam follower to turn. 
 
 Piece of wire holding valve off seat. 
 
 Cam follower turned around. 
 
 Water in the cylinder. Fill your cylinder full of water while 
 the piston is at the bottom of the compression stroke, and if 
 the water gets in several of the cylinders on the same side, have 
 the class blow the water out by removing the spark plugs on 
 that side of the motor and running the motor on the other 
 four cylinders. This is an effective way of removing water 
 from the cylinders. 
 
 Inspection for Prevention of Trouble. 
 
 The instructor should loosen up and disarrange numerous 
 parts of the motor, propeller fastenings, gas feed, cooling and 
 lubrication systems, making note of all he doe§. Then call 
 the class out and explain that they are supposed to fly over two 
 hundred miles of rocky mountains with this motor today and 
 it will be necessary to inspect with the idea of preventing trouble , 
 and make note of all they find wrong. When they are through 
 compare the notes and tell the class whether or not they can 
 theoretically fly safely across the mountains; then have them 
 start and run the motor. 
 
15 
 
 Emergency Repairs. 
 
 Remove suitable parts of the motor, magneto or carburetor 
 and have the class make emergency repairs with iron wire 
 and tape. 
 
 Have each section select tools and supplies which they 
 consider most necessary to take along in an airplane on a long 
 trip. For example, tape, assorted nuts, cotters, etc., soft 
 wire, spark plugs, exhaust valve springs. Distribute the tools 
 so there will be a wrench for every part of the motor, but no 
 excess wrenches; try to find one wrench which will answer 
 many purposes. Explain that the most serious work neces- 
 sary to prepare for would be the removal of one cylinder. 
 
TEN PRACTICAL LECTURES ON AIRPLANE 
 MOTORS. 
 
 (Note. — The lecture room should be equipped with black- 
 boards, and lectures should be illustrated by diagrams on these 
 boards. Also a “cut-away” model of a four-cycle motor, 
 showing all moving parts, magnetos, carburetors, any small 
 parts which might help to illustrate the lectures.) 
 
 LECTURE I. 
 
 Subjects . — Principle of four-stroke cycle engine, heat loss, 
 and reasons for cooling. (Note. — Nomenclature of parts 
 should be taught about this time, on the model.) 
 
 It is of vital importance thoroughly to understand the ele- 
 mentary principles of the motor, because once they are mas- 
 tered, we can reason out the solution of many motor difficul- 
 ties. 
 
 1. First, let us consider the burning of city-gas, in the open, 
 unconfined. If we light the gas, as it flows out of a gas jet, it 
 burns with a small amount of heat and without generating 
 any pressure, and with no noise. Now, if we take a pipe 
 several feet long, closed at one end and put in enough gas 
 to fill it for six or eight inches, and then ignite the gas, it will 
 burn and rush violently out of the open end of the pipe. If 
 we should take the same amount of gas and place a plug 
 in the pipe, the plug will be forced out along the pipe for a 
 certain distance. Now, with the same amount of gas, if we 
 force the plug down, compress the gas into § of its original 
 space and then ignite it, we find the plug will be driven much 
 further and much faster than before. 
 
 2. In the first gas engines made, gas was drawn into the 
 cylinders and burned without being compressed. The result 
 was it required a great deal of gas to develop a given amount 
 of horse-power. In other words, the engines were very uneco- 
 nomical. Later it was found that by compressing the gas 
 before firing they were able to obtain a great deal more power 
 with the same amount of fuel. When gas is compressed, it 
 
 ( 16 ) 
 
17 
 
 will burn much more rapidly. Naturally, in a motor running 
 at the speed that our airplane motors do, gas must be burned 
 completely in a very short space of time in order to do any 
 work. Now, we can understand that for a motor to run, it 
 will be necessary, first to draw gas into the cylinder; then 
 the gas must be compressed, then it must be burned and ex- 
 panded and the burned gas cleared out of the cylinder. These 
 four things must be done in any gasoline engine. 
 
 Four-Cycle Engine. 
 
 3. Practically all engines now used in airplanes are of four- 
 cycle, or more correctly speaking, four-stroke, cycle type. 
 This word cycle practically means a “program.” The com- 
 plete “program” of events occurring in these engines could be 
 said to be composed of four “acts” or events. These four 
 events are suction, compression, expansion and exhaust. Each 
 event requires one movement, or stroke of the piston to com- 
 plete it. The first stroke will be an outward stroke of the 
 piston, drawing gas into the cylinder. This we will call 
 the suction stroke. The next stroke will be an inward stroke 
 of the piston, compressing the gas in the cylinder. That is 
 the compression stroke. Then the gas will be ignited, and as 
 it burns, it will expand and drive the piston outward. This 
 is the working or expansion stroke. The fourth stroke is an 
 inward stroke of the piston. A valve will be opened and the 
 piston will force out all the remaining hot air and smoke left 
 from the explosion, thus leaving the cylinder clean for a new 
 charge. This is the exhaust stroke. 
 
 Thus we see that we have one working stroke and then 
 three idle strokes before the next working stroke. If our 
 engine has only one cylinder, it will be necessary to have a 
 large fly-wheel which will be capable of storing up energy 
 during the one working stroke sufficient to carry the piston 
 through the three idle strokes. In an engine with more than 
 one cylinder, the other cylinders can take the place of the 
 fly-wheel. To sum up: the four cycle motor requires four 
 strokes to complete the cycle. These strokes are first, suction; 
 second, compression; third, expansion; or working stroke; 
 and fourth scavenging or exhaust stroke. 
 
 4. When we compress air, it becomes heated, if we compress 
 it rapidly enough. For instance, if we are pumping an auto- 
 mobile tire, the pump becomes warm. This is not entirely due 
 to the friction of the plunger in the pump but partly due to 
 the work of compressing the air. The same is due when we 
 
18 
 
 compress gas or a mixture of gasoline and air in an engine 
 cylinder. As we compress gas in the cylinder, it is heated 
 to a certain extent. Then we have the heat of the electric 
 spark which we use for igniting the gas. The heat of the 
 electric spark together with the heat of the compression is 
 sufficient to “ignite” the gas or start it burning. Then as the 
 gas burns its temperature and pressure will rise very rapidly. 
 If the gas were burned in the open, the temperature would go 
 quite high, but when burned in a confined space, the pressures 
 are increased as the gas burns. The increase in pressure adds 
 to the heat (as in the case of the tire pump) , as does also the burn- 
 ing of the fuel. Therefore, we obtain a temperature in a gas 
 engine cylinder between 2,000 and 2,500 degrees Fahrenheit. 
 You can think of this as a white-hot flame. 
 
 Heat Loss. 
 
 5. Let us imagine we have a cylinder and piston so arranged 
 that we may compress a charge of gas and then lock the piston 
 so that it cannot be moved, and let us also imagine that there 
 is absolutely no leakage. Now, if we ignite this compressed 
 gas, our temperature and pressure will instantly run up very 
 high. But should we leave it for five minutes before testing 
 the pressure in the cylinder, we should probably find but few 
 ounces of pressure. Now the reason for this is that the cool 
 cylinder and piston have absorbed the heat of the explosion, 
 and as the heat subsided, the pressure subsided also. In 
 other words, the hot gases have been contracted by cooling. 
 In other words, heat and pressure can be considered as prac- 
 tically the same thing in a gasoline .engine. 
 
 6. Now we will stretch our imagination a little further. We 
 will imagine that we have a cylinder which is not only capable 
 of having gas compressed in it, and then the piston locked, but 
 also can be maintained at a temperature in the neighborhood 
 of 2,500 degrees. This temperature, by the way, is far above 
 the melting point of iron. Now, if we can compress gas and 
 fire it in this cylinder at this temperature, we will find that the 
 pressure will remain very high in the cylinder as long as we 
 maintain the temperature of the cylinder. This is because 
 the cylinder and piston will not cool the hot gases after the 
 explosion. The result is there is no radiation or loss of heat 
 from the explosion which is commonly called “heat loss.” 
 
 7. With the best gasoline motors, we make use of only 
 about 20 per cent of the heat value of the fuel, or in other 
 words, we lose | of the heat or power that is in the fuel. 
 
19 
 
 8. To s^iow where this heat or power goes, suppose our fuel 
 were 100 per cent fuel value. About 5 per cent of this is 
 consumed in friction, 35 per cent is lost by radiation, mostly to 
 the water jacket, and 40 per cent goes out of the exhaust valve 
 and is lost. This leaves only about 20 per cent to be delivered 
 in the form of power. 
 
 9. These figures are only approximate but should serve to 
 give an idea of where all our power or heat goes. 
 
 10. Now, after explaining heat loss, it may seem odd that 
 we find it necessary to cool the motor. However, I have 
 already explained that the temperature of the explosion is 
 sufficient to melt an ordinary iron cylinder, and long before the 
 temperature reached that point, the piston would expand and 
 stick in the cylinder; also lubrication would be destroyed 
 by the burning of oil and the babbitted bearings would be 
 melted out. Obviously then, the motor must be maintained 
 at a temperature sufficiently low to permit lubricating oil 
 to work properly. However, there is a limit to the tempera- 
 ture at which a motor can run, which is still lower than the 
 limit set by the burning of the lubricating oil. Remember 
 that I have explained that if we compress gas it becomes 
 heated, and also we must remember that up to a certain 
 point the more we compress the gas before firing, the more 
 power we obtain for a given amount of fuel. However, if 
 we compress the gas to excessive pressure, the heat of com- 
 pression will ignite the gas, and will ignite it long before the 
 proper time, — in other words, before the piston nears the top 
 of the stroke, — and the result will be a tendency to drive the 
 piston backwards. Suppose we have an engine designed with 
 the proper amount of compression; if some portion of the cyl- 
 inder becomes red-hot or even approaches that temperature, 
 the heat of this portion, together with the heat of compression, 
 will be sufficient to ignite the gas — in other words, cause 
 what we call “pre-ignition.” 
 
 Cooling. 
 
 11. There are two methods used at present for cooling. One 
 is by air-cooling direct and the other through the medium of 
 water which is later cooled by the air. 
 
 12. To begin with, let us remember that a square inch of 
 metal will radiate or give off a certain amount of heat in a 
 given time. The more square inches of surface we have on 
 the outside of a motor cylinder, the more heat can be radiated 
 or given off in a given time. We are all familiar with the ap- 
 
20 
 
 pearance of a motorcycle cylinder. It is constructed with 
 numerous fins or ridges around the cylinder, the purpose of 
 which is to present as many square inches of surface to the air 
 as is practical. 
 
 13. Remember that the more air that passes a square inch 
 of metal, in a given time, the more heat the metal will be able 
 to give off or radiate. Therefore, we find large air-cooled 
 motors, arranged with a blower of some description which will 
 cause a large amount of air to flow by the cooled flanges on 
 the cylinders, and frequently there are jackets or housings 
 around the cylinder which will force the air to flow between 
 these cooled flanges. This applies to motors which have 
 stationary cylinders. In the case of rotary motors, which we 
 will take up later, the cylinders are passed through the air 
 constantly and therefore no fan or blower is required. 
 
 14. The other method of cooling motor cylinders is to con- 
 struct them with a double wall and fill the space between the 
 two walls with water. Then as this water is rapidly heated 
 by the explosion inside the cylinders, we must have means of 
 removing the hot water and replacing it with cool water. For 
 a stationary motor, we might use a large tank of water to draw 
 from, and pump the hot water from the cylinders back into 
 this tank; but in airplanes and automobiles it is necessary to 
 carry a small amount of water for the sake of light weight; 
 therefore we must have a means of rapidly cooling this water. 
 
 15. You all know that the way to cool a dish of hot mush 
 is to spread it out thin, in this way exposing many square 
 inches to the air. If we could carry a tank so arranged that 
 the water can be spread out over a hundred square feet and 
 not more than a sixteenth of an inch deep, this would make 
 an ideal radiator as far as cooling is concerned, but would be 
 far too bulky and we would lose water by evaporation. There- 
 fore, we use radiators of various types. These radiators are 
 so constructed that they spread the water in very thin layers, 
 usually about the thickness of heavy paper. The' 'Water is 
 enclosed in thin sheet copper, which is an excellent conductor 
 of heat. Water heats the copper and the copper is cooled 
 by the air. In a well-designed radiator, we expose a great 
 many square feet of surface to the cool air. In this way, com- 
 paratively small radiators can cool a large amount of water 
 in a short time. In an airplane, the radiator is driven con- 
 stantly through the air at a high speed, thus insuring an ample 
 supply of cool air. 
 
 16. There are two methods of taking the water from the 
 radiator to the cylinder and from the cylinder back to the 
 radiator, or in other words, circulating the water. First, we 
 
21 
 
 have the system which employs a pump of some type, usually 
 a centrifugal pump. This pump circulates the water through 
 the water jakets at the speed which is found to be best by the 
 designer of the motor. Also we have what we call the 
 Thermo-Syphon system, or in other words, heat syphon. 
 
 17. You all know that the hottest air in a room rises to the 
 ceiling and the cooler air settles in the lower part of the room. 
 This is due to the difference in weight of the hot and cold air. 
 The same thing is true with water in a tank. If you fill a 
 tank with warm water, very soon you will find comparatively 
 cool water in the bottom while the water on the surface will 
 be warm. 
 
 18. In the cooling systems of motors, this works out as 
 follows. The water in the water jacket surrounding the cyl- 
 inder is being heated and is rising. It flows upward to the top 
 of the radiator, and as it flows upward its place is taken by cool 
 water from the bottom of the radiator; also in the radiator, the 
 water is being cooled and as it cools, it becomes heavier and 
 settles to the bottom. In this way, we have the heating of 
 the water in the cylinder and the cooling of the water in the 
 radiator causing the water to circulate continually. This 
 system has the advantage of simplicity and also of keeping 
 the cylinder head comparatively hot even when the motor is 
 run slowly. However, it requires larger water jackets and 
 water passages, which mean more weight and this is probably 
 the reason it is not used in airplane work. With the pump or 
 force circulation systems we find the designers taking advan- 
 tage of this Thermo-Syphon principle in this way. Water 
 is always introduced at the bottom of the water jacket and 
 removed at the highest point of the water jacket, delivering 
 to the top of the radiator and drawing from the bottom. 
 In this way the Thermo-Syphon principle helps the pump 
 instead of hindering it. 
 
 ' LECTURE II. 
 
 COMPARISON OF AIR AND WATER COOLED TYPES. 
 
 1. The air cooled motor can run at higher temperatures 
 than the water cooled motor, and as there is less difference in 
 temperature between the cylinder walls and the heat of explo- 
 sion, there is less loss of heat by radiation or less “heat loss.” 
 The air cooled system is simpler and sometimes lighter than 
 the water cooled. It has no freezing troubles in winter, but 
 
22 
 
 as it runs hotter this means lower compression must be used, 
 because on hot days, with the atmospheric temperature 
 higher, the cylinder temperature may be several degrees 
 hotter. In other words, the designer has no definite upper 
 limit on his temperature and must make the combustion 
 chamber of a size which will give a compression suited to the 
 highest likely temperature. This means comparatively low 
 compression. With water cooled motors, as the water tem- 
 perature reaches 212 degrees (boiling point of water) the water 
 is rapidly lost. Therefore, there is quite a definite upper limit 
 to the temperature at which this type of motor can be run, 
 and this means that a comparatively high compression can 
 be used and better fuel economy obtained. 
 
 2. When a motor has excessive compression, it will run well 
 for a short time, then will lose power from pre-ignition. Air- 
 plane motors are particularly likely to be troubled with pre- 
 ignition, because of the fact that they are run continuously 
 on full throttle, which also means full compression. 
 
 Types of Motors. 
 
 3. The type of motors with which we are all familiar is 
 the vertical, where the cylinders are arranged in a row, stand- 
 ing vertically over the crank shaft. Motors with one, two, 
 three, four or six cylinders nearly always are made in this 
 type. If we attempted to make eight cylinder motors in 
 this type with the cylinders in line, it would make a very long 
 motor which would take too much room in the airplane 
 fuselage and also the crank case would have to be made quite 
 heavy in order to be stiff enough. Eight and twelve cylinder 
 engines can be built with the cylinders in line, or vertical, for 
 marine purposes, but are nof likely to be built in that way for 
 airplanes. 
 
 4. Suppose we decide to build an eight cylinder motor. 
 Now, if we can arrange the cylinders in two rows of four, side 
 by side, we cut in half the length and weight of the crank shaft, 
 cam shaft and crank case, thus making the motor lighter, 
 more compact and more rigid. 
 
 5. We have airplane motors built of still another type, 
 where the cylinders are placed around a single crank pin in 
 the same manner as the spokes of a wheel are placed around 
 the hub. We call this the radial type. This type gives a very 
 short and rigid shaft. The crank case also is a strong barrel 
 type of case not divided in the center. Also it is possible 
 with a crank shaft like this to use ball bearings satisfactorily. 
 
23 
 
 6. This type of motor is built both in the air and water 
 cooled types. In the air-cooled form it requires no fan or 
 blower, because the cylinders are so placed that the air will 
 reach all of them evenly without any blower or jacket. There 
 is a tendency for the lower cylinders to become flooded with 
 oil, but this has been satisfactorily overcome by constantly 
 pumping the oil out of the crank case and not allowing it to 
 accumulate there. This type of motor is hard to enclose in a 
 stream-line hood and therefore has not been used on high speed 
 machines. 
 
 7. We have still another type of motor built with the cylin- 
 ders arranged in the same manner as radial motors but oper- 
 ating differently. Up to the present, we have considered only 
 stationary cylinder types in which the cylinders remain 
 stationary and kick or drive the crank shaft around. Others 
 are known as rotary or, correctly speaking, rotating motors. 
 The cylinders and crank case are similar in arrangement to the 
 radial type. 
 
 8. In the rotary type of engine, the crank shaft is bolted to 
 the airplane and the propeller is attached to the crank case, 
 just the reverse of the practice with stationary cylinder motors. 
 The crank shaft then stands still and the cylinders drive 
 themselves around it. With this type of motor, we have 
 to deal with centrifugal force. In other words, there is a 
 strong tendency for the cylinders to be thrown away from the 
 center of the engine. This makes a steel construction neces- 
 sary, and in order to reduce this strain as much as possible, 
 the cylinders are built of steel with very thin walls of very light 
 construction. 
 
 9. The rotary engines combining the principles which tend 
 to lighten the weight, as in radial motors, with an extremely 
 light all-steel construction, are the lightest motors now in use. 
 But, unfortunately, they consume so much fuel and oil that 
 if we weigh them with fuel for a long flight, they are heavier 
 that the stationary cylinder types; but for short flights, they 
 remain the lightest. The reasons for their excessive fuel 
 consumption are, first, that as they are air cooled they have 
 comparatively low compression; and also that they require 
 between ten and twelve per cent of the power they develop to 
 rotate the engine, or drive the cylinders through the air. 
 
 Types of Cylinders. 
 
 10. There are two main types of cylinders used. They 
 differ in location and operation of their valves, and also in the 
 form of their combustion chambers. The “L” head type has 
 
24 
 
 the valves side by side in a pocket at one side of the combus- 
 tion chamber. The valves are operated by simple cam fol- 
 lowers or plugs riding upon the cam shaft which is in the crank 
 case. The cams raise the followers which raise the valves, 
 making a simple valve gear. This design makes room for 
 long valve stem guides and springs, and there is a straight 
 thrust on the valve stem and no side thrust which tends to 
 wear the guides. 
 
 11. The valve in the head type of cylinder has the valves 
 placed either vertically or at a slight angle apart, with the 
 heads of the valve seating in the head of the cylinder. This 
 gives a combustion chamber that has no pockets, but the valve 
 must be operated by push rods and rocker arms or else the 
 cam shaft must be placed along the top of the cylinder with 
 small rocker arms operating the valve direct from the cam. 
 In this case the cam shaft is held in an oil-tight housing. 
 
 12. Now I will ask you to recall the explanation of heat loss 
 in the first lecture. You will remember that a large amount 
 of heat was lost by radiation, or by the interior surfaces of the 
 combustion chamber absorbing heat. The more square inches 
 of surface exposed to the heat in the combustion chamber, the 
 more heat will be absorbed and the greater will be the heat 
 loss. Evidently then, the combustion chamber which exposes 
 the least surface to the heat will cause the least heat loss. This 
 also works another way; the cylinder exposing the least sur- 
 face to the heat of combustion in the combustion chamber 
 will require the least cooling. This may mean reduced weight 
 in the cooling system. In the case of high speed motors, it is 
 extremely important to have very direct gas passages. In 
 other words, the gas should be able to go straight into the 
 cylinder and straight out without having to flow around any 
 corners or through pockets in the side of the cylinders. The 
 “valve-in-the-head” type of cylinder gives a more direct gas 
 passage, which means such a cylinder can receive a more com- 
 plete charge of gas at high speed and the exhaust gases can be 
 more completely expelled or scavenged. 
 
 Spark Timing or Spark Advance. 
 
 13. Up to the present, we have considered ignition of gas as 
 occurring when the piston is at the top of its stroke. It is 
 important to ignite the gas or start it burning at such a time 
 that it will be completely burned and ready to do work by the 
 time the piston starts down. If our motor is run at an ex- 
 tremely slow speed we can ignite the gas when the piston is at 
 
25 
 
 the top of its stroke, and as the combustion or burning of the 
 fuel is quite rapid, the fuel would be completely burned by the 
 time the piston starts down. But, if the motor is run at high 
 speed, the crank shaft will turn many degrees during the time 
 that the gas is burning and if we ignite the fuel when the piston 
 is already at the top of the stroke, the gas will not be completely 
 burned and the maximum pressure will not be obtained until 
 the piston is already very far down on the working stroke. 
 Therefore we will not get all the power from this explosion. 
 
 14. In high speed motors, the spark must occur far enough 
 before the piston reaches the top of its stroke so that the fuel 
 will be completely burned and the maximum pressure and heat 
 obtained by the time that the piston starts down. Therefore, 
 the faster our motor runs the earlier the spark must occur; also, 
 it makes considerable difference to the spark timing if we use 
 different shaped combustion chambers. 
 
 15. For example, if we use the “L” type of cylinder with 
 one spark plug at one side, the flame will have a long way to 
 travel to pass through the entire charge of gas. This means 
 that it requires a long time for the gas to be completely burned 
 and such a type of cylinder will require an earlier spark, or 
 more spark advance, as we say. 
 
 16. It is desirable, then, to use the most compact combus- 
 tion chamber we can and we frequently use the two spark plugs 
 in each combustion chamber as far apart as possible. In this 
 case, lighting the gas at two points, it requires less time for 
 complete combustion. 
 
 17. The “ valve-in-the-head ” type of cylinder has the most 
 compact combustion chamber of any type at present. I wish 
 to state here that when we crank our motor by hand we are 
 turning it over at a very low speed and therefore we set the 
 spark to occur very late, frequently after the piston has 
 already started down, because, if we did not, it would be possi- 
 ble for the explosion to drive the piston backwards, because of 
 its slow movement. This is known as a “ back-kick” and is 
 always dangerous to the man cranking the motor. 
 
 Multi-Cylinder Engines. 
 
 18. You will remember that with the single cylinder engine, 
 we found that we had one working stroke followed by three 
 idle strokes, and a fly-wheel was necessary in order to carry 
 the engine through the three idle strokes. With this engine, 
 our power comes in jerks. There is not a steady flow of power 
 from the engine, and, if there is a heavy piston and connecting 
 
26 
 
 rod, moving up and down, with nothing to balance it, the motor 
 causes considerable vibration. While this type of motor can 
 be balanced with a counterweight on the crank shaft, it never 
 works very smoothly. 
 
 19. Now, if we build a four-cylinder engine, we will arrange 
 the cylinders to fire once for every stroke of the piston. In 
 this case, there is a much steadier flow of power, or in other 
 words, a much more even torque, and it is usually arranged 
 so that there are two pistons moving upwards and two moving 
 downwards at the same time, thus partially balancing the 
 engine. 
 
 20. With the six-cylinder engine, we have still more even 
 torque and better balance, and with the eight or twelve cylin- 
 der engine, we have a very steady flow of power, and compara- 
 tively good balance and smooth running. 
 
 21. Now the advantages of a multi-cylinder motor over a 
 motor with fewer cylinders are as follows: First, each piston 
 and connecting rod is smaller and therefore lighter. These 
 parts are called reciprocating parts because of their back-and- 
 forth motion, and it requires a great deal of power to start and 
 stop these parts in a high-speed motor. Therefore, it is essen- 
 tial that they be as light as possible. When we have few cyl- 
 inders of large diameter we have much more difficulty due to 
 unequal expansion. With smaller cylinders there is less of 
 this difficulty. With a multi-cylinder engine we have little 
 loss of speed and power when olie cylinder fails for any reason. 
 
 22. In all multi-cylinder engines all the cylinders fire during 
 one complete cycle (2 revolutions) and nearly all of them are 
 arranged so that the cylinders fire at even intervals. 
 
 23. For example, a four-cylinder engine fires four times in 
 two revolutions, or twice in one revolution, which means the 
 cylinders fire one-half a revolution or 180 degrees apart. A 
 six cylinder engine fires every 120 degrees; an eight cylinder, 
 every 90 degrees and a 12 cylinder every 60 degrees. 
 
 24. The length of the working stroke is from the top center 
 to the time the exhaust valve opens. Let us say for example, 
 135 degrees. If the four cylinder engine fires every 180 
 degrees, there is an interval of 180 degrees minus 135 degrees, 
 or 45 degrees when there is no pressure to turn the crank shaft 
 ahead and the fly wheel must do it. 
 
 25. In a six cylinder engine, the cylinders fire every 120 
 degrees, which is less than the length of the working stroke, so 
 the working strokes overlap 15 degrees. This means smooth 
 running. In an eight or twelve cylinder engine the working 
 strokes overlap still more. 
 
27 
 
 26. In all high-speed airplane motors, it is quite a problem 
 to cool the piston head. In fact it is usually this part of the 
 engine which limits the amount of compression we can use. 
 The piston head is cooled mainly by the heat flowing through 
 its metal to the comparatively cool cylinder wall. If the 
 piston is large, the heat has further to travel from the center 
 to the sides, and it is quite difficult to keep this piston head 
 below the temperature at which it would ignite a charge of 
 gas under compression. The only way we can use very large 
 sized pistons is to reduce the amount of compression. By so 
 doing, the charge is less easily ignited by hot parts and less 
 heat will be generated with the low compression, but it means 
 less fuel economy. 
 
 27. The advent of the aluminum alloy piston has been a 
 help in the solving of this problem because aluminum con- 
 ducts heat about three times as fast as iron or steel, which 
 were formerly used for pistons. 
 
 28. In practice, the multi-cylinder, small bore engines are 
 not necessarily more efficient or economical than the larger 
 bore engines because they expose more surface to absorb heat 
 in proportion to the cubic feet of gas handled. Also there is 
 more friction in the multi-cylinder motor. 
 
 Lubrication. 
 
 29. Lubrication consists in introducing some substance, for 
 example, oil, between two rubbing surfaces to reduce the fric- 
 tion and wear that otherwise would occur. No matter how 
 smooth a metal surface may appear to sight and to touch, it 
 is in reality covered with tiny ridges and hollows that are 
 easily seen under a microscope. So when two clean, smooth 
 metal surfaces are placed together and caused to slide over 
 each other, these little ridges engage each other, or interlock, 
 and naturally some of the projections are torn loose from each 
 piece. This tearing away of metal is known as wear, and the 
 resistance to the tearing is known as friction. When oil or 
 grease is put between the two surfaces, it fills the little hollows 
 and forms a thin film or layer that prevents the metal sur- 
 faces from actually touching each other except at the highest 
 points of the largest of these microscopic ridges. The result 
 is a smaller number of these ridges torn loose; therefore, less 
 wear, less friction and less heat generated. 
 
 30. This oil film must be maintained in bearings. If it is 
 not, excessive wear always results. In large, heavy duty 
 machinery, it was found that the oil film could be maintained 
 
28 
 
 by making the bearings large in area so that the pressure per 
 square inch would not exceed a certain point, beyond which 
 the oil film would be destroyed. 
 
 31. In airplane motors, it is impossible to use bearings of 
 this size on account of weight. Therefore, it has been found 
 possible to maintain the oil film by the use of a large volume 
 of oil pumped through the bearings under very high pressure ; 
 but space must be left in the bearings for this oil film, and also 
 it is necessary to leave space and clearance in the bearings so 
 that oil can be pumped through. 
 
 32. You can easily imagine that a heavy film of oil could not 
 be forced through a bearing which had only a thousandth of 
 an inch clearance. Motors using these high pressure systems 
 frequently have the bearings from two thousandths to four 
 thousandths of an inch loose, thus allowing the crank shaft to 
 run on an oil film instead of on babbit metal. This means less 
 friction and less wear on the bearings, but it requires a high 
 oil pressure. 
 
 33. I can now point out that in deciding how tight to adjust 
 the bearings of a motor, we must first consider the nature of 
 the oil system. If we have a high oil pressure system, we can 
 fit the bearings quite loosely. In fact, we must fit them loosely 
 or excessive wear and excessive oil pressure will result. But 
 should we have a motor using a splash system, gravity feed 
 system or very low pressure, we could not leave the bearings 
 loose because, if we did, the bearings would hammer out 
 quickly. With such lubrication systems, we must adjust the 
 bearings closer. 
 
 34. Another thing to be considered before adjusting the 
 bearings is whether we are simply adjusting bearings which 
 have been run and are worn to a perfect bearing surface or 
 whether the bearings in question have been newly scraped 
 or fitted, and have not worn to a perfect bearing surface. 
 In the case of the worn bearings, they will not loosen up rap- 
 idly, and therefore cannot be adjusted as tightly as the bear- 
 ings which have been scraped, have not a perfect bearing 
 surface, and will therefore loosen up rapidly at first. 
 
 35. Nearly all of our modern airplane motors use this high 
 oil pressure system, generally having a gear pump in the crank 
 case delivering oil to some tube running the full length of the 
 crank case which acts as an oil distributor. Sometimes the 
 cam shaft is used for this purpose. In any case, we find oil 
 ducts leading from this tube to the main bearings of the crank 
 shaft, and here we must see to it that the oil holes in these 
 bearings register with the oil holes in the crank shaft, as oil is 
 
29 
 
 usually expected to flow into the crank shaft through the 
 arms of the crank, out of the crank pins and connecting rod 
 bearings. The oil is thrown from the connecting rod bear- 
 ings up on the piston and cylinder walls and also the wrist pins 
 or piston pins. The older motors always had dip pans or 
 splash pans, arranged under the crank shaft in such a way that 
 the connecting rods could dip into the oil and splash it to all 
 parts of the motor, but as the airplane motor operates in all 
 positions, such systems proved unsatisfactory, because, when 
 in some positions, the oil from these splash pans would flood 
 the cylinders and combustion chambers, causing various 
 troubles, such as foul spark plugs and carbon deposits. 
 
 36. Our modern engines have no splash pans and have a 
 pump which will keep all excess oil drained from the crank 
 chamber and usually have a small oil sump under the crank 
 case but not connected to it in such a way as to allow the oil to 
 flow up into it. In other words, the oil is pumped into the 
 lubrication system and the crank case is constantly pumped 
 dry so that there will be no accumulation of oil in the crank 
 case, which might flood the cylinders in the case of the motor 
 operating in extreme positions. 
 
 37. Oils used in motors are usually the mineral oils. Castor 
 oil has been used in some types of motors where the oil is not 
 
 \ used over and over again. Castor oil, if used over and over, 
 will congeal or thicken. It is useful in rotary motors because 
 it is not mixed with gasoline in crank case and because 
 of its high viscosity or ability to cling to the cylinder walls 
 under high temperature and under the influence of centrifugal 
 force encountered in that type of motor. It is finally thrown 
 out of the exhaust valves and lost, which accounts for the high 
 oil consumption of this type. The desirable qualities for 
 motor oil are first of all that it should burn only at very high 
 temperatures. In other words, it is said to have a high fire 
 test. This is necessary to stand the high temperatures in 
 the motor cylinders. Next, it should maintain its viscosity or 
 body at high temperatures and after using, it should show very 
 small percentage of decomposition so that it will be suitable 
 for use over and over again in the motor. Also it should have 
 a low carbon content so that it will not leave excessive depos- 
 its of carbon in the combustion chamber. As a rule, a heavier 
 oil deposits more carbon than the lighter oils. Lighter oils 
 must be used in very cold climates because heavier oils become 
 too thick, and in tropical climates the heavier oils must be used 
 as the lighter oils fail to retain sufficient body under high tem- 
 peratures. 
 
30 
 
 38. The oil in a motor must be changed from time to time, 
 that is, all the old oil from the lubricating system must be 
 removed and new oil must replace it. Some of the reasons for 
 this are, first, that the oil in being thrown from the crank pin 
 or connecting rod bearings comes in contact with the under 
 side of the piston head and becomes partly burned. In this 
 way, it is turned black and has a large amount of carbon in it. 
 Also it becomes full of metal particles from the wear of the 
 bearings; and still another reason more recently discovered is 
 that when we are using low grade fuels in our motors, a cer- 
 tain proportion of this fuel fails to burn and mixes with the 
 lubricating oil, thus thinning the oil and impairing its lubri- 
 cating qualities. In other words, it is poor economy to try 
 to use oil too long. 
 
 LECTURE III. 
 
 VALVE TIMING. 
 
 1 . The valves of a motor must open and close at exactly the 
 right time in order to let the new gas in and the burned gas 
 out of the cylinder at the right time ; also it is important to let 
 a good f fully charge of gas in and to scavenge or clean out the 
 burned gas as completely as possible. 
 
 2. Beginning with the working stroke: at the beginning of 
 the stroke, the pressure is often between three hundred and 
 four hundred pounds to the square inch (approximately four 
 times the pressure of compression). As the piston is forced 
 down in the cylinder, the gas expands and pressure becomes 
 less and less. 
 
 3. Finally a point is reached where the pressure is down to 
 fifty or sixty pounds per square inch and the crank is at an 
 angle where the pressure has but little effect on it, and there 
 is not much to gain by keeping it in the cylinder any longer. 
 There is also a large amount of cylinder wall exposed, to absorb 
 heat; and if the gas is kept in the cylinder over-heating will 
 result. Therefore it is usual to open the exhaust valve some 
 distance before the piston reaches the bottom of its stroke 
 or the crank reaches “bottom center.” 
 
 4. There is another reason for opening the exhaust valve 
 before bottom center. We wish all the pressure in the cylin- 
 der to rush out before the piston starts up, because, if it did 
 not, the piston would have to be forced up against it and a 
 loss of power would result. 
 
31 
 
 5. The exhaust valve is kept open all the way up the ex- 
 haust stroke and slightly after the top center so the piston can 
 push out as much of the burned gas as possible. If it is held 
 open too long, the piston will draw back some of the burned 
 gas as it is started down, and if closed too soon, the cylinder 
 will not be completely scavenged. So we see that the clos- 
 ing of the exhaust valve must be quite accurately timed. The 
 inlet valve usually opens at the same time the exhaust valve 
 closes or a few degrees later, because we wish to use the full 
 length of the suction stroke. 
 
 6. The piston, goes down very rapidly in a high speed motor 
 and there will not be time for a sufficient amount of mixture 
 to pass through the inlet valve to give the cylinder a full 
 charge. In other words, there will still be suction, or a vacu- 
 um, in the cylinder at the end of this stroke. Therefore, the 
 inlet valve is held open for a considerable period after the pis- 
 ton reaches the end of its stroke and starts up on the compres- 
 sion stroke. Although the piston is travelling upwards, gas 
 will continue to flow into the cylinder on account of the suc- 
 tion remaining. 
 
 7. There is another reason why the gas will continue to flow 
 into the cylinder. There is a large column of gas in the inlet 
 manifold which has considerable weight, and during the down- 
 ward stroke of the piston, this column of gas attains consider- 
 able momentum and this momentum will continue to force 
 gas into the cylinder, even after there is no more suction in the 
 cylinder. So we see why a motor will get a more complete 
 charge of gas if the inlet is held open some distance after bot- 
 tom center. (Note. — The faster the motor runs, the longer 
 the valve is held open.) 
 
 8. The exhaust valve opens from forty to fifty-five degrees 
 before bottom center on different motors. (The crank can 
 be set to this angle by using a protractor, and the piston posi- 
 tion measured while the crank is in this position, so we will 
 know the correct piston position when we come to time the 
 valves. It is usually more convenient to time airplane engines 
 by piston position than by degrees or crank angle because they 
 use no fly wheel.) 
 
 9. The exhaust valve closes on top center or within fifteen 
 degrees after top center, varying on different motors. The 
 inlet valve opens at the same time the exhaust valve closes 
 or possibly five or ten degrees later. (In a few engines, the 
 inlet valve opens a few degrees before the exhaust valve closes.) 
 The inlet valve closes from thirty to fifty degrees past bottom 
 center. 
 
32 
 
 10. The above timing refers to all stationary cylinder types 
 of four-cycle engines now used in airplanes, but is not correct 
 for the Gnome rotary engine. 
 
 Method of Timing Valves. 
 
 11. The process of timing the cam shaft in a motor consists 
 in, first, placing the crank and piston of one cylinder in the 
 correct position for the exhaust valve to close. (Note. — We 
 always time the crank shaft by the closing of the exhaust valve 
 because this must be more accurately located than any other 
 operation that the cam shaft performs.) 
 
 12. Second, we turn the cam shaft separately in the direc- 
 tion in which it runs until it just allows the exhaust valve 
 to close. Third, we mesh the gears or connect the two shafts 
 together. If we remember this simple explanation, we are 
 not likely to go wrong on the valve timing. 
 
 13. The exact method of meshing gears or of placing the 
 crank shaft and the cam shaft will vary somewhat in different 
 motors, but the principle remains the same. 
 
 14. It is very important to adjust the valve clearances before 
 timing the cam shaft, because a small variation in clearance 
 makes a large variation in valve timing. If we should time 
 the cam shaft without previously adjusting the clearance, then 
 adjust the clearance afterwards, this would throw our valve 
 timing out or make it incorrect. 
 
 15. Now I will explain in detail the process of timing the 
 cam shaft on a Curtiss 0X2 Motor, eight-cylinder, “V” type. 
 First, we will adjust the valve clearance. At the present time, 
 these clearances are supposed to be ten thousandths of an 
 inch on each valve. Then we will remove the cam-shaft-gear- 
 retaining-screw and pull the gear partly off the cam shaft, far 
 enough so that it no longer engages with the crank shaft gear 
 but still engages the key on the cam shaft. This makes it 
 possible to turn either shaft independently. Now we will 
 remove the spark plug from No. 1 cylinder and insert a rod 
 or scale, and turn the crank shaft in the direction of rotation 
 until we place the piston exactly on top center. We now have 
 a point to measure from. The instruction book for this 
 motor will tell us that the exhaust valve should close one 
 thirty-second of an inch past the top center. Therefore, we 
 will make a mark on our rod or scale, just even with the top of 
 the spark plug hole, and will measure one thirty-second of 
 an inch up from this point and make another mark. As the 
 book told us the valve should close one thirty-second of an 
 
33 
 
 inch after top center we will turn the crank ahead until the 
 piston has gone down one thirty-second of an inch. We can 
 now see that the crank shaft and piston in No. 1 cylinder are 
 in the correct position for the exhaust valve to close. 
 
 16. Second, we will turn the cam shaft in the direction m 
 which the cam shaft revolves until the exhaust valve is open, 
 and keep on turning until the exhaust valve is just seated . By 
 this I mean: not until we have full clearance at the exhaust 
 valve, but just until the tappet-screw in the rocker arm is just 
 leaving the end of the valve stem. Another method of deter- 
 mining this, is previously to insert a cigarette paper between 
 the tappet-screw and the valve stem and when the cam shaft 
 is moved far enough to allow this paper to be slipped out with- 
 out tearing, that will indicate that the valve is just seated. 
 (Note. — A cigarette paper runs one one-thousandth to two 
 one-thousandths of an inch in thickness.) 
 
 17. Now we can see that our cam shaft is in the position 
 where it is allowing the exhaust valve to close, or that the 
 exhaust valve is just closed. Therefore, we are ready for the 
 third step, or meshing the gears. All that is necessary to do 
 this, is to push the gear on to the shaft, but it is possible that 
 after we have placed both our shafts, the gear teeth may not 
 be in position to mesh. In this case, we will have to decide 
 whether we prefer to shift the cam shaft a fraction of a tooth 
 forward or backward. Probably, shifting it backward will be 
 safer because it is important that we do not allow the exhaust 
 valve to close before top center. Then, having meshed the 
 gears, the valve timing will be correct for the entire engine, be- 
 cause the cams are so spaced on the cam shaft that having 
 timed one cam correctly the rest will follow in proper order. 
 In airplane work, when we are timing the cam shaft or the 
 magneto, we do not consider the process complete until we 
 have “checked” the timing. The simplest way to check the 
 exhaust valve timing is to crank the motor in direction of rota- 
 tion until the exhaust valve is just closing. Find this point 
 accurately as I have just explained, then place the rod through 
 the spark plug hole and make a mark. Now, this mark repre- 
 sents the place where the valve actually does close. Then, if 
 this is supposed to be after top center, let us back the engine 
 until we reach top center. Then, make another mark. By 
 measuring the distance between these marks, we know 
 exactly where the exhaust valve is closing. 
 
 18. The same method of checking applies to spark timing with 
 the exception that we watch the breaker points instead of 
 the exhaust valve clearances. 
 
34 
 
 19. Occasionally a bad cam shaft is received from the 
 factory. If we had a mysterious trouble in the engine that 
 no one could find the cause of, it would be well to check the 
 valve timing in every cylinder of the motor, and check not 
 only the closing of the exhaust valves, but evety operation 
 which the cam shaft performs. 
 
 LECTURE IV. 
 
 FUEL AND CARBURETION. 
 
 1. Nearly all the fuels now in use for internal combustion 
 motors are distilled from petroleum. Alcohol is not used to 
 any extent as yet. I will explain briefly the methods of dis- 
 tilling lighter fuels from petroleum. The petroleum is heated 
 usually by means of steam pipes in order to avoid danger of 
 fire, and when heated to a comparatively low temperature, at 
 first the lighter elements come off' in the form of vapor . The first 
 vapor to come off, when condensed, will be the liquid known 
 as ether. If the petroleum is heated to a higher temperature, 
 high test gasoline will come off in the form of vapor. Then, 
 by heating the petroleum to a greater temperature, low test 
 gasoline will come off. Now as to the next products, that will 
 depend upon the kind of crude oil used. 
 
 2. There are two general classes of crude oil or petroleum. 
 Most of the eastern oil is what is known as paraffine base oil. 
 Most of the western is known as asphalt base oil. In the case 
 of the paraffine base, after the low test gas, next comes kero- 
 sene, usually in large quantities. In the case of the asphalt 
 base, the next product below gasoline is distillate or naphtha. 
 After kerosene or distillate we get light lubricating oils, and 
 by heating the crude oil to a still higher temperature we obtain 
 cylinder oils or heavier lubricating oils and so on through the 
 oils until we come to the tar products and vaseline. The 
 chemists divide these various products into many different 
 varieties. 
 
 3. I will explain what is meant by high test and low test 
 gasoline. High test gasoline is very light and volatile and 
 evaporates very rapidly if left open, while low T test is heavier, 
 less volatile and will not evaporate so easih r The gasolines 
 are tested by the Baume gravity scale. Formerly the gasoline 
 of high test would show 72 degrees on test, but at present, ordi- 
 nary automobile gasoline tests about 60 de rees. 
 
35 
 
 4. As the demands for gasoline have increased in the last 
 few years, the oil refineries have been forced to mix many of 
 the heavier fuels with gasoline. For instance, distillate, and 
 then in order to make the gas test properly, they have added 
 some of the higher test fuels. In other words, the present day 
 gasoline may be a mixture of everything above gasoline and 
 a good deal that is below, and mixed in such proportions that 
 the result will serve as gasoline. 
 
 5. There is also another method of obtaining gasoline. 
 This is by compressing natural gas to a very high pressure and 
 practically wringing liquid gasoline from it. For instance, 
 after oil wells have been pumped dry, they often yield gas 
 which is useful for this purpose and in certain districts there 
 are gas wells which are useful in the same way. This gasoline 
 is known as “casing-head” gasoline. 
 
 6. Carburetion is the process of mixing fuel with the air. 
 Gasoline will burn rapidly and cleanly only when mixed with a 
 large proportion of air. Now the problem is to mix the gaso- 
 line and air rapidly in a comparatively small and light mixing 
 device. That is the first part of the problem, we will say. 
 Originally, with the first gasoline engines, the gasoline used 
 was of high test and would combine or mix with the air very 
 easily. In fact it was only necessary to draw air over a large 
 surface wetted with gasoline and the air would become satu- 
 rated with the gasoline vapor, and was readily burned in the 
 motor. But the present day gasoline will not mix in this 
 manner. 
 
 7. There are two methods for mixing gasoline with air. 
 The first is to break the gas into very fine spray, and the other 
 is to vaporize it by heat. Usually a combination of these two 
 methods is used. 
 
 8. I will now try to make plain the method of breaking the 
 gasoline into a fine spray. Let us start with the stationary 
 gasoline engine, running at a constant speed. We will have a 
 straight pipe, for instance, 1J inch in diameter, running to the 
 gas engine cylinder so that the piston will draw air through 
 this pipe during the suction stroke. Then we will have a 
 nozzle or jet in this pipe, placed in Such a way that all air 
 must pass it. We will feed gasoline to this nozzle from a float 
 chamber located beside and just outside the pipe. This float 
 chamber will be so arranged that when the gasoline reaches a 
 certain height, for instance, even with the top of the nozzle, the 
 float will close the valve and stop the gasoline from rising any 
 higher. This is a simple device for the purpose of maintaining 
 the gasoline at a constant level. With an arrangement like 
 
36 
 
 this, the gas will feed through the nozzle equally well with the 
 tank full or nearly empty. Now as the air goes through this 
 pipe to the cylinder, it will pass the spray nozzle with consid- 
 erable speed and will pick up or suck up a certain amount of 
 gasoline from the nozzle. In this way the gas will come out 
 of the nozzle in a fine spray and will be fairly well mixed with 
 the air by the time it reaches the cylinder, and during the com- 
 pression stroke the heat of the cylinder and the heat of com- 
 pression will help unite the particles of gasoline with the air. 
 
 9. By using a simple carburetor, such as I have described, 
 if we should try to slow the motor down by closing the throttle 
 valve between the spray nozzle and the cylinder, the air w r ould 
 flow past the nozzle so slowly that it would no longer pick up 
 gasoline, or in any event, it would not flow rapidly enough to 
 break up the gas, even if it should draw some out of the noz- 
 zle. So we see that our carburetor is not suitable for vari- 
 able speed. 
 
 10. Our next step will be to put a choke tube in this carbu- 
 retor, or in other words, a restricted air passage at the point 
 where the spray nozzle is located. By this means we can make 
 the air passage so small that even when the engine is running 
 quite slowly there will be sufficient air velocity at the spray 
 nozzle to pick up and break up the fuel. 
 
 11. Having decided to restrict this air passage at this point, 
 the next problem is of what form shall we make this choke 
 tube. We must choose a form of choke tube which, wffiile it 
 is small enough to get the desired air velocity at low speed, 
 will still permit a large amount of air to flow through. We 
 find that the Venturi form of air passage is best adapted to 
 this purpose. It is practically an air nozzle so formed that 
 the side in which the air enters is at rather an abrupt angle, 
 but the side through which the air leaves is at a gradual angle, 
 or, in other words, allows the air to expand gradually on the 
 other side. This has the following advantages: first, that 
 this form of opening allows more air to pass through for a 
 given size than other forms, and also that as the air is expand- 
 ing as it passes through, there is a tendency for the jet of fuel 
 from the nozzle to burst apart into a fine spray. 
 
 12. Now, having put this choke tube, or throat, as it is 
 sometimes called, in our carburetor, we will see how it works. 
 Evidently our motor will run nicely at slow speed, but wffien 
 we open the throttle to allow the motor to run very fast, we 
 will have trouble becasue, as the suction of the engine increases, 
 we find that the flow of gas from the spray nozzle will increase 
 faster than the flow of air through the choke tube. This is in 
 
37 
 
 accordance with a certain law of physics. This means that 
 our mixture of gasoline and air will become richer as the 
 engine speeds up; richer means that there will be too large a 
 proportion of gasoline for the amount of air. Therefore, we 
 must find some means of diluting this mixture at high speeds. 
 We will make a large air opening between the spray nozzle and 
 the cylinder and we will put a valve in this opening which will 
 be held closed by a light spring. This spring will hold the 
 valve closed while we are running slowly, but as the motor 
 runs faster and the mixture tends to become richer, the suc- 
 tion will open the valve and allow enough air to come in to 
 dilute the mixture and make it right, provided the spring is 
 properly adjusted. This we will call an auxiliary air passage 
 and auxiliary air valve. The air passage in which the nozzle 
 is located should be called the main air passage. 
 
 13. This diluting of the mixture at high speeds is one of 
 the problems of carburetor work. One form of carburetor 
 accomplishes this by having two jets, one working on the 
 principle which I have described so that it delivers a richer 
 and richer mixture as the speed is increased, and the other 
 arranged on a different principle so that it delivers a rarer and 
 rarer mixture; in this way the two jets compensate each other. 
 We will take up this particular carburetor later. 
 
 14. We find many forms of carburetors, but they are nearly 
 all made on these principles that I have explained. 
 
 15. The action of the spring on the auxiliary air valve is 
 considered faulty because when we suddenly open the throttle, 
 there is a tendency for the valve to jump suddenly and move 
 too far off its seat. The result is that it temporarily admits too 
 much air, “ starving ’ ’ the motor. To correct this, some manu- 
 facturers use what is known as a dash pot to steady the action 
 of the auxiliary air valve. This is simply a piston working in 
 gasoline, or sometimes in air, in much the same manner as the 
 piston in a shock-absorber. As the air or gasoline is forced to 
 pass through a comparatively small hole in the piston, the 
 piston cannot move rapidly. Some designers of carburetors 
 go still further and in addition to using the dash pot to pre- 
 vent the air valve from opening too far or too suddenly, they 
 use what is called a metering pin, which is an arrangement 
 whereby the opening of the auxiliary air valve will tempora- 
 rily feed an extra supply of gasoline to avoid the tendency of 
 starving the motor when the throttle is suddenly opened. 
 
 16. Some carburetors have fixed jets of certain wire-gauge 
 sizes which are screwed into the carburetor and cannot be 
 adjusted. Others have nozzles with a pin point valve screwed 
 
38 
 
 in or out of them to change the size of the opening and the 
 amount of gasoline flowing through them. These are called 
 needle valves. Some carburetors have the needle valve con- 
 nected to the throttle by mechanical means, in such way that 
 the needle valve will be opened slightly as the throttle is 
 opened, or, in other words, as the speed of the motor is 
 increased. 
 
 Carburetor Adjustment. 
 
 17. Let it be clearly understood that when a mixture con- 
 tains too large a proportion of gasoline, it is called rich and the 
 motor is said to be flooded, and when it contains too small a 
 proportion of gasoline, the mixture is rare, and the motor is 
 said to be starved. 
 
 18. When a motor is cold it usually requires a rich mixture 
 to make it start easily. This is because the gasoline does not 
 combine readily with the air. In order to accomplish this 
 temporary richness of mixture for starting, most carbu- 
 retors have a device, usually called a priming or flushing pin, 
 which will, when operated, depress the float or hold the float 
 down in the float chamber of the carburetor, allowing the gaso- 
 line to rise so high that it will overflow through the spray 
 nozzle of the carburetor, thus wetting the inside of the carbu- 
 retor with gasoline and making it easy for the air passing 
 through to pick up a heavy charge of fuel. 
 
 19. Another method is to have some form of valve between 
 the spray nozzle and the place where the air enters the carbu- 
 retor. By closing this valve we get greatly increased suction 
 in the spray nozzle, thus picking up an excess of gasoline. 
 Such a device is usually called a choker. It is usually oper- 
 ated from the pilot’s seat while the motor is being cranked. 
 
 20. Most carburetors have two gasoline adjustments and 
 an air adjustment; one gasoline adjustment for low speed and 
 one for high speed. In addition to this, we have a little set 
 screw which allows the throttle valve to close more or less 
 completely, according to its adjustment. This is to permit 
 the motor to idle at the desired speed. 
 
 21. Before adjusting any carburetor, the motor must be 
 thoroughly warmed up and ignition system must be perfect. 
 The valve operation and compression must be perfect, and 
 fuel must flow in a generous stream to the carburetor. Also 
 the carburetor float chamber and spray nozzle must be clear. 
 If there are any loose joints in the pipe between the carburetor 
 and the cylinders, too much air will leak in and dilute the 
 mixture. Therefore, these joints must be tight. 
 
39 
 
 22. I will try to explain in a general way how carburetors 
 are adjusted. First, all adjustments should be in a medium 
 position. In other words, half-way between the most and 
 the least, of either air or gas according to their functions. 
 The low speed gasoline adjustment should be opened two or 
 three turns. The engine should be started and thoroughly 
 warmed up. Then we will adjust the low speed gasoline 
 adjustment and after we have adjusted it so that we are 
 getting the best results, we can adjust the high speed gasoline 
 adjustment while the motor is running at high speed. Then 
 having our mixture right, we can adjust the screw which limits 
 the closing of the throttle until the motor runs at the desired 
 speed for slow speed. If we try to make the motor run too 
 slow, it will stop, and if we allow it to run too fast it will make 
 landing of the airplane difficult and there will be danger of 
 the machine running into and injuring the man who cranks 
 the propeller to start the motor. 
 
 Now we have made our gasoline adjustment suit the adjust- 
 ment of the auxiliary air valve. If the auxiliary air valve is 
 unnecessarily tight our motor will throttle down nicely and 
 respond nicely when opened quickly but will fail to show the 
 proper speed. Then, in that case, by loosening the auxiliary 
 air valve spring, or allowing the auxiliary air valve to open 
 further, we will get a larger volume of air through the carbu- 
 retor, giving the motor more complete charges of gas, and 
 gain higher speed. Accordingly, the gasoline adjustments will 
 have to be reversed to suit this change in the air adjustment. 
 In most of these carburetors we can make our gasoline adjust- 
 ments suit almost any air adjustment; and in airplane work, 
 the amount of speed or revolutions per minute we can make is 
 the most important part of our adjustment; therefore, we 
 generally aim to have the air valve open as far as possible. 
 The limit to this is that if the air valve opens too far our spring 
 is too weak, and the motor will take air through this valve 
 instead of through the main air passage when we throttle 
 the motor down, and also it will fail to respond when we 
 open the throttle quickly. So let us see that we have the 
 auxiliary air valve adjusted so that it will just stay seated or 
 closed when the motor is running very slowly or idle, but not 
 tighter than is necessary. 
 
 23. Most carburetors are provided with a means of drawing 
 hot air from the outside of the exhaust pipe, for the heat helps 
 to evaporate the particles of gasoline, or in other words, helps 
 the mixing of the gas with the air. 
 
40 
 
 24. If cold air only is used, or if the motor itself is cold, the 
 gasoline goes into the cylinders in “chunks,” and therefore 
 it is never completely mixed with the air and is never completely 
 burned. The result is that a larger amount of gasoline must 
 be fed to the motor. When heat is used, or when the motor is 
 hot, the heat causes the gasoline to combine more thoroughly 
 with the air and less fuel can be used. The result is that if 
 we should adjust the carburetor properly when the motor is 
 cold, the mixture would become too rich when the motor was 
 thoroughly warmed up, which would make poor economy and 
 cause trouble. If we adjust the carburetor properly while the 
 motor is hot, then the motor may be difficult to start when it is 
 cold and may run badly until it is well warmed up, but this can 
 be taken care of by using the choker, which I have described 
 before. The main thing is to have a correct, economical and 
 clean mixture after the motor is heated up. 
 
 25. If the mixture is too rich, it burns very slowly. The 
 result is that the charge will be completely burned and the 
 maximum temperature reached only after the piston has de- 
 scended in the cylinder for a considerable distance. Thus, the 
 greatest heat occurs at a time when there is a large amount of 
 surface exposed to absorb it. The result is over-heating of 
 the motor; also the heat will be very great at the time the 
 exhaust valve opens, which would burn the exhaust valve and 
 overheat the exhaust pipe. We lose power also, because the 
 maximum pressure occurs late, after the piston has already 
 completed part of the working stroke. 
 
 If the mixture is too rare, first of all, we lose power and have 
 excessive vibration. Next, if it is still rarer, the motor will 
 “spit” through the carburetor. This is known as a back-fire. 
 It is supposed to be caused in this manner: This rare mixture 
 also burns very slowly, so slowly in fact that there will be a 
 flame remaining in the cylinder even after the exhaust stroke, 
 and when the inlet valve opens to admit a new charge, the 
 flame remaining in the cylinder will ignite the gas and the flame 
 will travel down the inlet pipe to the carburetor. This is 
 dangerous because it is likely to set fire to the airplane, espe- 
 cially if the gasoline pipes are leaking slightly, near the car- 
 buretor, which they often are. 
 
 The Zenith Carburetor. 
 
 26. This carburetor is used all over the world on airplane 
 engines. It is of the so-called non-ad justable type. The only 
 method of adjusting the carburetor is to remove the jets or 
 
41 
 
 the choke tube and replace them with other sizes. These 
 carburetors are fitted with the proper size jets and chokes at the 
 factory where the motors are tested, and never require further 
 adjustment. The Zenith is of the type of carburetor having 
 no auxiliary air valve but using the compensating jet prin- 
 ciple. In other w6rds, it has a main jet which delivers a 
 richer and richer mixture as the suction is increased and a com- 
 pensating jet which is arranged to deliver a rarer and rarer 
 mixture as the suction is increased. To make the operation 
 of these two jets plain, we will suppose that I am sucking 
 lemonade out of a bottle, which corresponds to the “float 
 chamber,” with a straw corresponding to the main jet. The 
 harder I suck on the straw, the more lemonade I can get 
 (richer and richer mixture). Now, for the compensating jet. 
 Suppose you pour lemonade from a bottle, drop by drop 
 into an open glass, this glass corresponding to the “well” in 
 the carburetor. I would suck from the glass or “well” through 
 a straw (corresponding to the priming tube or cap jet). No 
 matter how hard I suck, I get only the amount that you drop 
 into the glass, and some air. As 1 increase the suction in the 
 straw, and cannot get any more gas, I get more and more air, 
 or, the mixture I get will be thinner, rarer and rarer. 
 
 27. Now, I will try to explain the operation of the carbu- 
 retor. (The names of the parts on the carburetor should be 
 taught on the model at this point.) As we come to start* the 
 motor, we find that the gasoline has gradually flowed through 
 the compensating jet and has filled the “well” to a point even 
 with the top of the main jet. If the motor is cold, we must 
 close the throttle to take the charge and as the pistons can 
 draw air only through the priming tube, they will suck up 
 liquid in the well through the priming tube until the well is 
 empty, thus giving us a rich mixture for starting the cold 
 motor. 
 
 28. Now if we open the switch and start the motor, it will 
 run slowly with the throttle closed or nearly closed, and it 
 will still be sucking only through the priming tube. But as 
 the compensating jet supplies gasoline to the well and priming 
 tube in very small quantities, the pistons will draw far more 
 air than gasoline through the priming tubes. Therefore we 
 can think of this priming tube as a miniature carburetor for 
 running the motor idle. Now, if we open the throttle slightly, 
 the suction will be decreased in the priming tube and increased 
 in the choke tube of the carburetor in the main air passage. 
 Therefore the suction will naturally be transferred to the cap- 
 jet, or outer jet, placed in the choke tube. We are now draw- 
 
42 
 
 ing this same amount of gasoline and some air from the “well’' 
 and compensating jet, but through a new route, viz., the cap- 
 jet. Then as we open the throttle a little more, the air 
 velocity through the choke tube will increase and the main jet 
 begin to deliver a small amount of gasoline. This main jet, 
 you will remember, is the type which will deliver more and 
 more gas as the suction is increased. We can call this main 
 jet our high-speed jet. 
 
 29. As we use the priming tube for slow running, it is not 
 necessary to have the choke tube very small, therefore the 
 suction \till not increase around the main jet to the extent 
 that it does in most carburetors. Also, the main jet is not 
 quite large enough to supply the gas needed at high speed and 
 relies partly on the jet which is fed from the compensating 
 jet, and as this latter jet delivers a rarer and rarer mixture, it 
 will offset any tendency on the part of the main jet to deliver 
 a richer and richer mixture; thus, we will say, the jets compen- 
 sate each other. 
 
 30. This carburetor is provided with the usual screw adjust- 
 ment to limit the closing of the throttle valve in order to make 
 the motor idle at the proper speed, and also there is a small 
 air adjustment which acts on the priming tube and affects 
 the running of the motor only at the idling speed. 
 
 31. To take a charge when the motor is cold, the throttle 
 should be completely closed, and when the motor is not cold, 
 it should be very slightly opened, because, if we took a charge 
 with the throttle closed, while the motor was hot, there would 
 be danger of getting entirely too much gasoline in the cylin- 
 ders, or flooding the motor. That is, there would be such 
 a large proportion of gasoline to air that it would be impos- 
 sible to ignite the mixture. 
 
 Action of Gasoline in the Inlet Pipes and Manifold. 
 
 32. After adjusting our carburetor to deliver the proper 
 mixture of gasoline and air under all conditions, our problem 
 is not yet completely solved. We have still to solve the prob- 
 lem of keeping the gas mixed. As the mixture leaves the 
 carburetor, it consists of air and a finely divided spray of 
 gasoline. If we could see this, it would resemble steam from a 
 tea-kettle. Now, this mixture must be kept rapidly moving 
 or it will condense, that is, the liquid will gather together in 
 puddles in all the low places on the way to the cylinders. 
 
 33. One method of avoiding this condensation is by allowing 
 hot air to flow to the carburetor or by heating the intake 
 
43 
 
 manifold from the outside so that the walls will be sufficiently 
 hot to vaporize the liquid fuel which comes in contact with 
 these walls. It is important to have the manifold of the same 
 inside diameter- as the out-let of the carburetor because if it 
 is larger the mixture will flow more slowly than it would in the 
 carburetor and condensation will be the result. Allowing hot 
 air to flow through the carburetor helps the gasoline spray to 
 evaporate or combine with the air, but we must remember 
 that if the mixture is heated too much, it goes into the cylinder 
 in a hot and expanded condition, and, therefore, we don’t get 
 as much of it in the cylinder to compress. In other words, we 
 do not get a good, full charge, and there is less oxygen to the 
 cubic foot of the heated mixture. 
 
 34. Another problem connected with the handling of the 
 mixture of gasoline and air is that with certain motors there is 
 a tendency for the gas to reciprocate in the manifold, or, in 
 other words, to jump back and forth. This is especially true 
 in a six-cylinder vertical motor with the usual firing order. 
 The gas will be drawn towards one end of the engine by one 
 cylinder and next it will be drawn in the opposite direction 
 by a cylinder in the other end of the motor ; thus a very hard 
 condition has to be overcome and causes uneven gas distribu- 
 tion and condensation. The same condition exists in eight- 
 cylinder “ V ” type motors if a single carburetor is used, because 
 there is a variation in the suction from the two sides of the 
 motor and a tendency for the gas to reciprocate between the 
 two sides. 
 
 35. Most of the modern airplane engines of six-cylinder 
 vertical and eight and twelve cylinder “ V ” type motors use a 
 divided inlet manifold and a duplex or double-barrel carbu- 
 retor. This effectually prevents any possibility of the gas 
 reciprocating from one half of the motor to the other because 
 they get their gas from separate sources. The “ duplex” 
 carburetors have a single float chamber to supply the two sets 
 of jets. 
 
 THINGS TO BE REMEMBERED ABOUT 
 CARBURETORS. 
 
 36. It is impossible to get a correct adjustment on a car- 
 buretor unless everything else about the motor is right. If a 
 carburetor seems to need very exact adjustment, or, in other 
 words, the motor seems to be sensitive, it is a pretty sure 
 sign that something is not quite right. For instance, the 
 ignition may be poor, or there may be air leaks in the inlet 
 
44 
 
 manifold, or dirt in the carburetor, and under these circum- 
 stances, no one can get a good adjustment. If a carburetor 
 d ips gas, that is, seems to leak, it may be that the float is set 
 too high and shuts off the gasoline only after it is high enough 
 to overflow through the spray nozzle. Or it may be that there 
 is dirt under the float valve which keeps it from seating 
 properly. It may be a defective or leaky float valve. Fre- 
 quently, when the carburetor drips gasoline there is nothing 
 the matter, and the gasoline dripping out is simply due to the 
 fact that after we stop the motor the gas which was in the 
 manifold or inlet pipe will condense and drip out through the 
 carburetor. Therefore, when our carburetor leaks, we must 
 notice whether or not it occurs as soon as we turn on the gaso- 
 line before starting the motor, in which case it would be 
 trouble with the float; or whether it only leaks after running 
 the engine, in which case it would be due to condensation and 
 unavoidable. 
 
 37. Back-firing in a carburetor may be caused by too rare 
 mixture or by an extremely rich one. Usually, however, it is 
 a rare mixture. Either mixture can burn so slowly that there 
 will be enough flame left in the cylinder, even after the scaveng- 
 ing stroke, to ignite the new incoming charge of gas when the 
 inlet valve opens. Sometimes the flame will reach the car- 
 buretor or make a noise. A leaky inlet valve can also cause 
 a back-fire if it leaks badly enough or sticks open. An engine 
 can “pop” from two causes, either from back-fire in the 
 carburetor, which would indicate lack of gas or too much air 
 or possibly inlet valve trouble. The other cause for “pop- 
 ping” can be a very rich mixture or a very late spark. But 
 there is a great difference in these two because in the case of 
 the former the pop will occur in the carburetor and in the case 
 of the latter, it will be in the exhaust ports or exhaust pipes. 
 So it is necessary to determine where the pop is occurring be- 
 fore attempting lo find the cause. 
 
 COLD WEATHER STARTING. 
 
 38. In cojd weather many people prime a motor with gaso- 
 line direct in the cylinder to help start it. There are objec- 
 tions to this method. For instance, if the motor fails to start 
 on the first priming, we are likely to find that we are losing 
 compression because the gasoline has washed the oil off the 
 piston rings, and when the motor does finally start there will 
 be danger of scoring or scratching the cylinder walls, on account 
 of the oil not being there to lubricate. 
 
45 
 
 39. A better method of priming the motor is to turn it over 
 as fast as you can with the switch safe or closed, and the 
 throttle half open, and while this is being done, completely 
 stop the auxiliary air opening with one hand and nearly stop 
 the main air passage with the other. This will cause high air 
 velocity and suction at the spray nozzle. This method gets 
 the gas into the cylinder in the form of a vapor or spray. The 
 result is, the motor is more apt to start and there is less danger 
 of scoring the cylinders. Frequently it is possible to know 
 when you have a charge in the motor because you can see the 
 vapor coming out of the exhaust pipes by looking very closely. 
 This applies to the case where the motor is cold. If the motor 
 is hot there would be a certain amount of vapor present from 
 other causes. Another good method of priming is to squirt 
 gas through a cock in the top of the intake manifold or at the 
 highest point of the manifold. In this way, the gasoline will 
 spread over the interior walls of the pipe and expose a large 
 wetted surface for the air to pass over. In extreme cases, it 
 is a help to saturate- a piece of rag with “ ether” and hold over 
 the air intakes of the carburetor while taking a charge. The 
 motor can be started in this way when it refuses to start in any 
 other way. 
 
 40. Nine times out of ten when the motor seems to have 
 carburetor trouble the trouble is somewhere else. It seems as 
 though the human race is prone to blame the carburetor for 
 all troubles, possibly because it is the easiest thing to “ monkey” 
 with. Many times I have seen people adjusting a carburetor 
 to correct a trouble caused by the magneto being wired up 
 wrong. If the engine is missing or acting badly in any way 
 and we believe the carburetor is to blame, we can find out or 
 prove it in the following manner. If we have a carburetor of 
 the auxiliary air valve type, simply run the engine at the speed 
 where it is giving trouble and raise the air valve slightly off 
 its seat with one finger, thus temporarily making the mixture 
 rarer; and if this does not correct the trouble try holding the 
 hand partially over the air intake to make the mixture tem- 
 porarily richer. If neither of these operations stops the miss- 
 ing or trouble, the fault absolutely is not in the carburetor, and 
 if raising the air valve or thinning the mixture would seem to 
 help the engine, that might only mean that there was dirt 
 under the float valve allowing gasoline to rise too high in the 
 float chamber, and would not necessarily mean that the ad- 
 justments of the carburetor should be changed. It might also 
 mean that we should have had more cold air and less heated 
 air. If conditions seem to be improved by putting our hand 
 over the air intake or making the mixture richer that might 
 
45 
 
 mean that we had air leaks in the inlet manifold, no air vent 
 in the gas tank, poor flow of gasoline to the carburetor through 
 feed pipes, water in the gasoline, dirt in the spray nozzle, or 
 weak exhaust valve springs. The chances are that there 
 would be no real necessity for altering the carburetor adjust- 
 ments to feed more gas. 
 
 41. Remember that we must not leave gasoline in the float 
 chamber of a carburetor if the engine is not going to be run 
 for a week or so, because some grades of gasoline when evapo- 
 rating in a carburetor under these circumstances will leave a 
 deposit which resembles wax or soft soap, and naturally when 
 gasoline is turned on and the engine started, this deposit will clog 
 some of the fine gasoline passages. In any case it is wise to 
 inspect and clean the carburetor after the motor has been 
 standing for some time without running. 
 
 42. Make it a rule never to connect the feed pipe to the 
 carburetor without first allowing gasoline to run through the 
 pipe to clean the pipe or flush it out and to prove the volume 
 of the flow. If there is any doubt as to the amount of the 
 flow through the pipes being sufficient you can prove it in this 
 way: Suppose your motor burns ten gallons of gasoline per 
 hour. You should not be satisfied with the flow of gas unless 
 it will flow twice that fast, or two gallons in six minutes. 
 When running gas through the pipe for the purpose of cleaning 
 hold the end of the pipe as low as possible to insure all heavy 
 dirt and water coming out of the end of the pipe, and when 
 testing the amount of flow try to have the end of the pipe at 
 the same height as where it connects with the carburetor in 
 order to make it a fair test. To clean out the carburetor, we 
 should begin at the gasoline tank and see that there is a good 
 air vent in the tank, because gasoline cannot flow out of the 
 tank unless air is able to come in and take its place. (Note. — 
 This refers to gravity feed.) 
 
 43. Then we should disassemble and clean out the float 
 chamber, inspecting the float mechanism to see that nothing 
 is working loose. We should clean out the strainers, wherever 
 they may be located; clean the jets. A special socket wrench 
 should be used to remove them for cleaning. Always remove 
 one at a time, clean it and replace it firmly before removing 
 another. See that a gasket is on it, but don’t change the 
 thickness of the gasket as this would affect the flow from the 
 jet. Sand, water, or rust flakes are likely to be found in the 
 jets. In the Zenith carburetor, there are plugs under the jets 
 which make them accessible. These plugs also have a cup in 
 them which catches dirt, and they should be cleaned out before 
 being put back in place. 
 
LECTURE V. 
 
 MAGNETOS. 
 
 1. In speaking of electric currents, we speak of the flow and 
 the pressure in much the same way in which we would speak 
 of the amount of water flowing through a pipe or of the pres- 
 sure of water in the pipe. In electricity the amount of flow 
 is measured in amperage , or number of amperes; and the pressure 
 is called voltage or, we speak of the number of volts in a circuit. 
 Then a high pressure current would be called a high voltage 
 current. Low pressure would be called low voltage. We 
 also use the expressions high tension and low tension. For 
 ignition in a motor, we require high pressure or high tension 
 current, which will be able to force its way through the space 
 between the spark plug points. A high tension magneto does 
 three things: it generates a current, “steps it up” and dis- 
 tributes it, or “hands it out” to the proper cylinder at the 
 proper time. The so-called low tension magneto generates 
 a low tension current only, and has a simple armature com- 
 posed of a soft iron core with a single winding of wire. 
 
 2. The high tension magneto has a double-wound arma- 
 ture. It is constructed with a soft iron core, then a primary 
 winding composed of a few turns of coarse wire next to the coce. 
 Outside thus comes a secondary or outer winding composed 
 of very fine wire and having perhaps a thousand times as many 
 turns as the primary winding. (Note. — The names of the 
 parts on a model should be taught at this point.) As the ar- 
 mature revolves between the ends of the magnets or in the 
 magnetic field, a low tension current is generated in the pri- 
 mary winding . W e call this the prim ary circuit (prim ary circuit 
 not useful for ignition), and by suddenly stopping this current, 
 a strong wave of high tension current will be “induced” in 
 the outer or secondary winding. This high tension current 
 is collected by the collector ring and carried by the collector 
 brush to the distributor where these waves of current, which 
 we call “sparks,” are handed out to the right cylinder at the 
 right time. 
 
 3. Now in order suddenly to stop or break this primary 
 current it must flow through the interruptor or breaker. This 
 device makes contact for a short time and allows the current 
 to flow through, then quickly separates the contact points, 
 thus breaking the circuit. Now, when a current is flowing 
 
 U7) 
 
48 
 
 through two points, it possesses a certain amount of momen- 
 tum or speed and when the points are separated it tries to 
 jump the gap and keep flowing a fraction of a second longer in 
 this way. But if it does continue to flow we will not have made 
 a quick and complete break in the primary current and the 
 spark will be weak, and this flowing after the points are sep- 
 arated (sparking) will burn away the platinum breaker points > 
 which means that frequent adjusting and filing will be necessary 
 to keep them true. This tendency of the current to keep flowing 
 after the break occurs is taken care of or corrected in the high 
 tension magneto by a device called the condenser, which is 
 capable of temporarily absorbing the current, which would 
 otherwise try to jump between the points when they are sepa- 
 rated. When the condenser of a magneto fails, we find the 
 breaker points burning or pitting, and we find the spark becom- 
 ing weak. An electric current must always make a complete 
 circuit or round trip, returning to the place it starts from. 
 I will now explain the routes of the circuits in the magneto. 
 You will see that we have two complete and separate circuits 
 in a high tension magneto. 
 
 The Primary Circuit. 
 
 4. The primary current is generated in the inner or primary 
 winding armature and flows from one end of this winding to 
 the breaker and condenser, from there to ground or frame of 
 the engine and through the frame of the magneto to the core 
 of the armature. The other end of the primary winding 
 is connected or grounded on this core, so in that way the 
 current returns to the place where it started. When the 
 breaker points separate, the current no longer flow r s through 
 them but is absorbed by the condenser. 
 
 The Secondary Circuit. 
 
 5. One end of the secondary winding is grounded to the 
 core of the armature. The current is induced in this winding 
 at the time of the break in the primary circuit and flows to 
 the collector ring, and through the collector brush, through the 
 connection between the collector brush and the distributor and 
 from the distributor to the central or insulated electrodes of 
 the spark plug; from there it jumps to- the outer or grounded 
 portion of the spark plug through the frame of the motor and 
 magneto back to the core of the armature, completing the cir- 
 cuit. This secondary circuit is of very high pressure, some- 
 
49 
 
 thing like 30,000 volts, and must therefore be very heavily 
 insulated. If one of the wires running to the spark plugs 
 should be broken and the ends separated, there would be no 
 outlet for this particular wave of secondary current and it 
 would jump to the ground through the easiest channel. 
 Usually it would jump through the insulation of the armature, 
 puncturing or ruining the insulation. To avoid this, a safety 
 valve is provided which is called the “safety spark gap.” 
 This is a gap provided on the magneto which has the points 
 separated a great deal farther than the points of the spark 
 plug, but close enough so that the resistance will be less than 
 the resistance of the insulation in the armature. In other 
 words, it will be easier for the spark to jump the safety gap 
 than to jump through the insulation in the armature, but it 
 will be much harder to jump the safety gap than the gap in 
 the spark plugs. Then if we find a spark occurring in this 
 safety gap, that should tell us that the secondary current 
 finds no way to get to the ground. These safety gaps are 
 enclosed in fine brass gauze or screen to prevent them from 
 igniting any gasoline vapor which might be present under the 
 hood of the engine. 
 
 6. When we wish to stop the magneto we cannot use a 
 clutch to disconnect it so it will no longer turn, because when 
 we let the clutch in or connect the magneto up again, it would 
 be out of step, or out of time with the motor. The method 
 for stopping the magneto is to “cut out” the breaker. In 
 other words, we take the primary circuit and connect it to the 
 ground before it reaches the breaker. In this way, the primary 
 current is not broken or ruptured by the breaker, and therefore 
 we get practically no secondary current. 
 
 7. In the Bosch high tension magneto the armature gener- 
 ates two waves of primary current per revolution. It is 
 necessary to have the break in the primary occur at the time 
 when the current is the strongest, and this will mean at the 
 highest point of the two waves I have referred to. In other 
 words, the correct time to have the break occur is when the 
 armature is just leaving the pole pieces on the ends of the 
 magnets. The exact measurement between the armature and 
 pole pieces varies with different magnetos for engines of dif- 
 ferent number of cylinders, but ordinarily this position of the 
 armature occurs when there is an air space of one-eighth 
 of an inch between the armature and the pole pieces. As 
 airplane motors are run most of the time at full speed there is 
 seldom any necessity for changing the time of the spark, but 
 on some machines the spark is retarded to make the cranking 
 
50 
 
 of the motor safer. In all ignition systems, the spark is 
 advanced or retarded by changing the time of the break in the 
 primary circuit. In the high tension magneto this is accom- 
 plished by shifting the cams that operate the breaker. In the 
 Bosch magneto, if the breaker housing that carries these cams 
 is in the fully advanced position, the break will occur with the 
 armature in the ideal position that I have spoken of, but if 
 we retard the spark the break occurs at a time when the 
 armature has moved away from the pole pieces a considerable 
 distance, perhaps half or three-quarters of an inch. The 
 result is that this type of magneto gives the strongest spark 
 in the full advance position and a weaker spark in the full re- 
 tarded position. Sometimes the spark in this position is so 
 weak that the motor cannot be started on it. It is therefore 
 well to start the engine with the spark as far advanced as we 
 can have it without making the motor kick back. Some 
 magnetos overcome this difficulty of having a weak spark in 
 the retarded position by rocking the magnets with the breaker 
 housing, or in other words, moving the pole pieces at the same 
 time we move the breaker cams, so that the break will occur 
 with the armature in the ideal position anywhere from full 
 advance to full retard. The Eismann, Mea and Dixie mag- 
 netos are of this type. Some magnetos are arranged so that 
 there are four positions of the armature or rotor, per revolu- 
 tion, in which conditions are right to have the break occur. 
 These magnetos then deliver four sparks per revolution (of 
 the magneto), and such magnetos run at one-half the speed of 
 the other types. 
 
 Care of the Magneto. 
 
 8. First of all, do not over-oil the magneto. Clean oil 
 would not do very much harm in a magneto, but oil as we know 
 it around a motor is never clean. It always contains some 
 carbon or metal particles which make it a fair conductor of 
 electricity, and for this reason it can short-circuit a magneto. 
 Also if we had oil between the breaker points it would string 
 out when the points separate and supply a path for the cur- 
 rent to flow through, thus preventing a quick and complete 
 break in the primary circuit. The breaker mechanism of all 
 magnetos is arranged to run without oil and should not be 
 oiled. The ball-bearings on the armature and distributor 
 bearings must be oiled, but only every thousand miles or once 
 a week, and then with a few drops of high-grade, light oil. In 
 cleaning the distributor we sometimes find the contacts or 
 the segments blackened and it becomes necessary to brighten 
 
51 
 
 them up. This should be done with brass polish, pumice 
 stone powder, whiting or crocus cloth, but never with sand 
 paper or emery cloth no matter how fine they are because the 
 grit from these cloths would become embedded in the soft 
 insulation material of the distributor and cause scratching or 
 tearing. In many magnetos, there are numerous carbon 
 brushes which are apt to become glazed, and when found in 
 such condition they should have the glaze scraped off, leaving 
 them dull. All these carbon brushes have light springs behind 
 them. Never stretch these springs to cause a firmer contact 
 because it is unnecessary, and if the contact is made firmer 
 the carbon brush will wear faster, spreading a streak of carbon 
 powder around the part it touches, and wull be likely to short- 
 circuit the magneto in this way. After cleaning the distribu- 
 tor, oil can be rubbed around it with the finger; then the oil 
 should be wiped lightly with a clean rag, leaving only an oily 
 appearance which is about the right condition. If too much 
 oil is left in the distributor, it will form a paste with the powder 
 from the carbon brushes and conduct the current from one 
 contact to the next. If the distributor is left too dry, as it 
 would be after washing with gasoline, cutting and tearing 
 would result. If the magneto is driven by a gear direct on the 
 armature shaft, great care must be taken to be sure that the 
 gears mesh properly. If the gears mesh too loosely they can 
 jerk back and forth and strain the magneto, but if the gears 
 mesh too tightly the bearings on the armature shaft of the 
 magneto will be ruined in a very short time, putting the mag- 
 neto out of commission. Gear teeth should never “bottom” — 
 that is, the points of the teeth should never touch the bottoms 
 of the spaces in the other gear, but a clearance of from 1/64 
 to 1/32 of an inch must be left between the gears. 
 
 9. When inspecting or cleaning a magneto never remove the 
 end plates which carry the armature bearings because they are 
 accurately fitted to hold the armature just .002” from the pole 
 pieces on all sides. Never remove the magnets from the 
 magneto or the armature, because if this is done, some of the 
 magnetism or “pull” will be lost from the magnets. These 
 two operations should only be performed by a well-equipped 
 magneto expert. It is not necessary to do these things in the 
 field. 
 
 Testing the Magneto. 
 
 10. First, suppose we have the magneto on the bench. By 
 taking hold of the magneto shaft where the coupling or gear 
 would go, turn the magneto in its proper direction (usually 
 
52 
 
 indicated by an arrow on the magneto). Notice how much 
 pull or resistance it offers as you turn it over. This should 
 feel a good deal like the compression in a motor; and by feeling 
 a magneto in this way which is known to be up to full strength, 
 and then feeling your magneto, you can form an idea as to the 
 condition or strength of the magnets. If the magneto is on the 
 engine and we wish to test it to see if it throws a spark, the 
 best place to test it will be from the collector brush (which 
 collects the secondary current from the armature) and make a 
 spark jump from this brush-holder to the frame of the engine 
 by holding a screw-driver against the engine and leaning it so 
 it passes close to the brush-holder. The magneto should 
 throw a spark at least an eighth of an inch while the engine is 
 being cranked by hand, but we must remember one important 
 thing. Whenever conditions are right for the magneto to 
 deliver a spark at the collector brush it will also deliver sparks 
 to the spark plugs in the cylinders. This would make the 
 cranking of the motor dangerous; therefore, we must either 
 remove all the wires from the spark plugs, which would be 
 quite a job, or the more convenient methbd is td remove the 
 connection between the collector brush and the distributor, in 
 order to safeguard the man who is cranking. Also remember 
 that the switch must be open when we are testing the magneto. 
 
 11. If we should fail to find the spark the first thing to do is 
 to remove the ground wire or short-circuit wire from the mag- 
 neto, because if there is any defect or short-circuit in the 
 ground wire or in the switch, it would have the effect of con- 
 tinually shorting the magneto even when the switch is open. 
 So if we get a spark only after disconnecting the ground wire, 
 we can be sure that the ground wire in some way made contact 
 with the frame of the engine when the switch is open. 
 
 12. Let us clearly understand that when the switch is 
 “closed” the primary current from the magneto flows through 
 the switch into the ground or frame of the engine instead of 
 flowing through the breaker of the magneto and therefore we 
 get no spark in the cylinders and we say that the motor is safe. 
 When the switch is “open” that means that there is no path 
 for the primary current to flow directly to the ground and it 
 must flow through the breaker. In this way a spark will be 
 delivered. But should our ground wire chafe against some 
 metal part in the airplane so that the wire itself makes actual 
 contact with this metal, the condition would be the same as 
 having the switch closed all the time. If we test the magneto 
 and find no spark, the next thing we should examine after 
 testing our ground wire should be the breaker points. See 
 
I 
 
 53 
 
 that they open the correct distance for that particular kind 
 of a magneto. (Note. — The Bosch magneto requires .015" 
 gap between the breaker points; the Berling requires .016" to 
 .20" and the Dixie requires .020".) The breaker points 
 must open the correct gap, no more and no less. Any change 
 in the adjustment of the breaker will cause the break to occur 
 with the armature a different distance from the pole pieces, and 
 may have considerable effect on the working of the magneto. 
 The breaker points should be clean and smooth. If we find 
 them badly burned we can true them up with a jeweler’s 
 file and re-adjust them so that they open the correct distance 
 or separate the correct distance. However, if they are badly 
 burned we should endeavor to get another magneto, because 
 the fact that they are burning shows that the condenser is 
 not working properly. 
 
 13. I have found by experience that we can tell more about 
 the actual strength of a magneto by the appearance of the 
 spark plug points than by any other means we have on the 
 field. The correct distance between spark plug points for 
 Bosch magneto and Dixie magneto is 1/64" or .015"; for a 
 Berling magneto, .030". As long as these spark plugs are 
 properly adjusted, if the magneto is up to full strength the heat 
 of the spark will burn these points to a whitish appearance 
 between the tips, and when the magneto begins to get weak 
 from any cause, it will fail to burn the plugs in this manner. 
 Also if the spark plug points are too wide apart this whitish 
 appearance will not be noted. I have found that by carefully 
 watching the spark plug points, I have often been able to 
 remove a magneto which was becoming weak before the 
 engine had missed a shot, in this way preventing trouble. Also, 
 suppose that in an eight-cylinder motor seven of our spark 
 plugs show this white appearance between the tips and the 
 eighth does not. If all the plug points are adjusted equally, 
 this will indicate that the insulation in this spark plug is 
 defective and usually if you will break open such a spark 
 plug you will find a crack in the porcelain which will usually 
 be blackened by smoke. This will prove that you were right 
 in removing the plug. 
 
 14. Remember that in ignition work, it is the heat of the 
 spark, rather than the size of the spark, which counts. If you 
 are out somewhere with a motor and find that the magneto is 
 becoming weak and have no replacement for it, you can gener- 
 ally cause it to burn the spark plug points white by closing the 
 points together slightly; for instance, closing them in to .010"; 
 in this way you will be able to keep the magneto in commission 
 
54 
 
 for several hours longer, sometimes six or ten hours longer. 
 Another method of keeping the magneto in commission when 
 it begins to weaken is to retard the breaker housing slightly. 
 This applies to the Bosch and Berling magnetos and will often 
 enable you to get home with a motor or machine. You must 
 not retard the spark too far or overheating of the motor will 
 result. 
 
 15. Remember that magnetos give trouble mostly at high speed , 
 not at low speed , and when you can get a spark from a magneto 
 by testing it in the manner I have described the motor will 
 usually start well and run all right up to about one-third of its 
 speed, but may not run at all at high speed. In other words, 
 a magneto will start a motor unless it is completely “knocked 
 out.” When ordering a new magneto, it is necessary to state 
 make, and type of magneto, and number of cylinders on the 
 motor. Also it is necessary to state whether you require 
 a “clockwise” or “anti-clockwise” magneto. A magneto is 
 said to revolve “clockwise” or “anti-clockwise” as seen when 
 looking at the gear or coupling end of the armature or rotor 
 shaft; this does not refer to the direction of rotation of the 
 distributor. 
 
 Timing the Magneto. 
 
 16. Timing the magneto consists of three operations. First, 
 we must place the motor or crank shaft in the proper position 
 for the spark to occur. Second, we must place the magneto in 
 the position where it is delivering a spark, and third, we must 
 mesh the gears or connect the coupling. 
 
 Placing the Motor. 
 
 17. First of all, it is usual to time a magneto for the No. 1 
 cylinder of a motor. This is not necessary, but it is the con- 
 ventional practice. Therefore, we will place the piston of 
 No. 1 cylinder on the right stroke. Naturally the spark must 
 occur on the compression stroke, the only stroke when there 
 is gas in the cylinder and compressed ready to fire. Now the ■- 
 question is, how shall we know when No. 1 piston is on the 
 compression stroke? If we will turn the engine over in the 
 proper direction or in the direction in which it runs until we 
 see the inlet valve open, we know that we are at the beginning 
 of the suction stroke and when we see the inlet valve close we 
 will know that the piston is at the end of the suction stroke I 
 and at the beginning of the compression stroke. Now, if we I 
 will put a rod or screw driver down through the spark plug 
 hole on the head of the piston we can follow the piston as it 
 
55 
 
 comes up on the compression stroke and find the top center or 
 highest point of the piston travel. It is best to find this point 
 accurately , to measure from. 
 
 18. Airplane motors are usually timed in full advanced 
 position because they are seldom retarded. In other words, 
 the advanced position is the most important. Now, the ques- 
 tion is how much advance shall we give this motor, or, how far 
 before top center shall the spark occur? The amount of ad- 
 vance depends on the speed at which the motor runs, also on 
 the type of combustion chamber, whether the motor is of high 
 or low compression, and whether there is one or two spark 
 plugs per cylinder. The faster the motor runs the greater 
 advance we must give the spark or the earlier the spark must 
 occur. If we have a compact combustion chamber, of the 
 valve in the head type, or if we have two sparks per cylinder 
 simultaneously, less time will be required for the flame to travel 
 through the entire charge and therefore less advance will be 
 required. An experienced motor man can judge fairly closely 
 where the spark should occur on any motor if he knows these 
 conditions. But most men will do well to refer to the instruc- 
 tion book for their particular motor or to get the information 
 from a superior. Whenever we disassemble a motor or remove 
 a magneto we must make it a point to determine by measure- 
 ment where the spark occurs so that we will have this infor- 
 mation for that particular motor. 
 
 19. As airplane motors seldom have fly wheels the spark 
 timing is generally given in inches of piston travel. Instead 
 of saying the spark should occur so many degrees before top 
 center, it is usual to say the spark must occur at a certain 
 fraction of an inch before top center, meaning for instance, a 
 quarter of an inch before the piston reaches the top of its 
 stroke. Then to complete the process of placing the motor, 
 suppose our instructions for the motor we are working on are 
 to set the spark 5/16" before top center. We have already 
 placed our piston at the top center of the compression stroke. 
 We will now take a scale or rule and measure 5/16" upon our 
 screw driver or rod and make a mark. Then we will back the 
 motor until the piston has gone down this 5 /16". The piston 
 in No. 1 cylinder is now in the correct position and on the 
 right stroke for the spark to occur. 
 
 Placing the Magneto. 
 
 20. We must first decide which distributor contact we will 
 call No. 1. Frequently, a figure "1" will appear in a little 
 window injdie distributor cover when this contact is made. 
 
56 
 
 The breaker breaks once for each contact of the distributor. 
 Turn the magneto until the distributor brush is on No. 1 con- 
 tact. We will now turn the magneto carefully a few degrees 
 until the breaker points just begin to separate. Just as soon as 
 we can see that the points are separating, that is when the 
 spark occurs. Another method is to previously insert a cigar- 
 ette paper between the breaker points. This kind of paper is 
 only .001" thick and when it will slip out it will show us when 
 the points are separated just that amount. That is perhaps 
 the most accurate way to "get the break." We must also be 
 sure that the breaker housing is in the advanced position 
 because we have no desire ever to make the spark occur earlier 
 than 5 /16" before top center in this particular motor. Now 
 placing the distributor and breaker points we can mesh the 
 gear or connect the coupling. Then our magneto will be timed. 
 Sometimes, when the magneto has been accurately placed, we 
 find that the gears will not quite mesh but require turning in 
 one direction or the other perhaps half a tooth. Many mag- 
 netos are provided with adjustable couplings to take care of such 
 a situation, but if our magneto is not so provided, we will have 
 to decide whether we prefer to have the spark occur half a 
 tooth earlier or half a tooth later. If the motor is using a 
 rather large amount of advance perhaps half a tooth later 
 would be better; and if our motor happens to carry rather 
 little spark advance, a half tooth earlier would do no harm. 
 Where two magnetos are used, they must be timed exactly 
 alike or "synchronized." 
 
 Wiring up the Magneto. 
 
 21. The cylinders receive the sparks in the same order that 
 they get their gas through the inlet valves. When the 
 designer of a motor designs the cam-shaft, he arranges it to 
 open the inlet valves and deliver gas to the cylinders in some 
 certain order. For example, a 4-cylinder engine never fires in 
 numerical order. It will fire either 1-2-4-3 or 1-3-4-2. If it 
 should fire 1-2-3-4 it would have a great amount of vibration 
 and the strains on the crank shaft would be excessive. It is 
 important then for the motor to have a certain firing order to 
 make it run smoothly and also to make it draw gas through 
 the inlet manifold without jerking the gas back and forth or 
 causing it to reciprocate. If the engine fires in the same order 
 in which the inlet valves open, obviously the way to find out 
 the firing order of any motor is to crank it over and note 
 in which order the inlet valves open. For example, we have 
 
57 
 
 a six-cylinder motor. We will start with No. 1 cylinder and 
 turn the engine over until No. 1 inlet valve is just opening. 
 Then we will put down the figure 1 on our paper. Now we 
 can turn the motor over a few degrees and see which inlet valve 
 opens next. It may be No. 4. Then the next may be No. 
 3, 6, 2 and 5. Remember that we must not take for granted 
 that all motors of one make have the same firing order because 
 some may be right hand motors and some left (normal or 
 anti-normal rotation), and also the manufacturer may have 
 found that he could improve the running of his motor by chang- 
 ing the firing order. The point I want to make clear is this: 
 Remember the method of finding the firing order rather than try- 
 ing to remember the firing order. It is but a few minutes 
 work to turn the motor over and note the actual firing order, 
 and by doing so you will save yourself trouble some time. 
 The magneto delivers the sparks in numerical order, 1, 2, 3, 4, 
 etc., because as the distributor brush moves around it touches 
 one contact after the other, but try to think of the distributor 
 terminals not as No. 1, 2, 3, but as first, second, third, etc. 
 
 22. In timing the magneto, we time it for No. 1 cylinder. 
 So we can connect No. 1 terminal on the distributor to No. 1 
 spark plug, and the second terminal on the distributor will 
 connect to the second cylinder in the firing order, and the third 
 distributor terminal to the third in the firing order, and so on. 
 
 Magneto 1st 2d 3d 4th 
 
 Cylinder Nos 1 4 3 2 
 
 23. Above all, in timing a magneto, do not forget to place 
 the engine on the right stroke. Before timing the cam shaft, 
 the strokes on the pistons were merely up strokes, and down 
 strokes, but since timing the cam shaft, each stroke has a name, 
 viz., suction stroke, compression stroke, working stroke, and 
 exhaust stroke. The spark must occur near the top of the 
 compression stroke, when there is gas in the cylinder and it is 
 compressed ready to fire. This only occurs every alternate 
 time the piston is up. If the spark occurred on the other top 
 center no explosion would occur as there would be no com- 
 pression and no gas in the cylinder, and if you should go so far 
 as to start the motor, you would not get so much as a “shot” 
 out of it. 
 
LECTURE VI. 
 
 SPARK PLUGS. 
 
 1. A spark plug is composed of an outer steel shell which 
 screws in the cylinder and a core of insulation material usually 
 made of some form of porcelain with a wire running down the 
 center. This wire is known as the central electrode. The high 
 pressure current or “spark” from our magneto is delivered to 
 this central electrode and jumps from its inner end to some point 
 arranged on the steel shell of the plug, which, being in contact 
 with the cylinder, is grounded. When mica insulation is used 
 it has a tendency to become oil-soaked in time, and with the 
 porcelain insulations there is danger of their cracking from 
 great heat or rapid changes in temperature. The electrodes 
 are usually made of some metal which does not scale easily 
 under high temperatures. The scale would be in the nature 
 of an insulator if it could form. Platinum, iridium and nickel 
 alloys are generally used for electrodes. The proper adjust- 
 ment of the gap between the electrodes at the inner end of the 
 spark plugs should be accurately made. Most of the high 
 tension magnetos require a gap of 1/64" or .015" and nearly 
 all modern battery systems require a gap of about .030" 
 or approximately 1/32". With high tension magnetos if 
 the spark plug points are set too close together the heat of the 
 spark is greater than when the points are far apart. If the 
 spark plug points are too close the heat of the spark will be 
 sufficient to fuse or melt the tip and little globules or bubbles 
 of metal are formed on the tips which are liable to connect or 
 short-circuit them, and if the spark plug points are too far 
 apart it will be difficult for the magneto to throw a spark 
 across them when the engine is running at low speed and the 
 magneto also running at low speed. Because a spark will 
 jump 3/16" outside the cylinder does not mean that it will 
 jump that far inside. Remember that the spark inside the 
 cylinder occurs while the gas is compressed to 90 or 100 
 pounds to the square inch, and under these conditions can 
 jump only about one-third as far as it can jump outside the 
 cylinder where the air is at atmospheric pressure. 
 
 2. I have pointed out that it is the heat of the spark rather 
 than the size of the spark which counts and also that the 
 magneto makes a hotter spark when the points are close than 
 when they are far apart, and there are other arguments in favor 
 
 ( 58 ) 
 
59 
 
 of having the plug points close together. If the plug points 
 are far apart, the resistance to the flow of the current will be 
 high and it will not require very much dirt or oil on the surface 
 of the insulation to provide an easier path for the current 
 than jumping the gap. In other words, with the plug points 
 farther apart, the plug will be more easily short-circuited. I 
 have often seen spark plugs with cracked insulation begin to 
 miss at high speed and by taking those plugs and putting the 
 points closer together, thus reducing the resistance at the 
 points, the current would once more jump the gap and the 
 cylinder continue to fire. Under these circumstances, the 
 plug would fire until sufficient carbon accumulated in the crack 
 in the porcelain to provide an easier path for the current than 
 the reduced gap in the plug. It is always best to adjust spark 
 plug points accurately to a gage, because in a multi-cylinder 
 motor the heat of the spark should be unifo rm in all cylinders. 
 
 3. Oceasionally we find a motor, or one cylinder of a motor, 
 which floods with oil and continually fouls or short-circuits 
 spark plugs. Under these conditions, a single pointed spark 
 plug should be used, or if you are using a three-point plug in all 
 the cylinders, break off two of the points in this particular 
 cylinder. The reason for this is that in a three-point plug 
 while the spark is jumping between a certain pair of points 
 and burning them clean, oil and carbon are accumulating 
 between the points where no spark is occurring. In a single 
 point plug, there is a strong tendency for the spark to keep the 
 points clean, due to its great heat. Also the points should be 
 close together and in this way the current will be able to jump 
 through the oil instead of flowing through it. In spark plugs 
 it is a great problem to carry the heat away from the central 
 electrode fast enough so that this part will not become red hot 
 or incandescent. In some spark plugs the points do become 
 red hot and cause pre-ignition. This will usually show up as 
 follows: You can start your motor on a test block or in a 
 machine and it will run smoothly for two or three or possibly 
 ten or fifteen minutes, and then begin to jerk or jar a little bit. 
 In many cases, it will begin to shake steadily and regularly 
 appearing as excessive vibration. Usually the speed of the 
 motor will be slightly reduced. The thing to do in a case like 
 this is to experiment with some kind of spark plug having 
 shorter and thicker electrodes. 
 
 Care of Spark Plugs. 
 
 4. When we remove spark plugs for cleaning we should 
 gently scrape the carbon away, being careful not to wedge 
 
60 
 
 anything between the shell of the plug and the insulation, 
 because we might crack the insulation in this manner. Adjust 
 the spark plugs hy your gage and note the appearance of the 
 points. They should be burned whitish in color between the 
 points. If they are not, it probably indicates that the points 
 are set too far part or that the magneto is weak. If one plug 
 in the set fails to show this whitish appearance, it should be 
 replaced, because the insulation is probably defective. If you 
 find small, metallic beads or bubbles on the points, they 
 should be adjusted .002" or .003" farther apart. 
 
 5. It is well to use new spark plug gaskets frequently. By 
 so doing it will be unnecessary to screw the plug in very tightly 
 and the danger of straining or cracking the insulation by pull- 
 ing hard on the shell of the plug with your wrench will be 
 greatly reduced. Never drop spark plugs, because there is 
 danger of cracking the porcelain; and always shake them to 
 see if you hear any rattle in the porcelain. Unfortunately 
 there is often no way in which we can detect the crack in the 
 plug unless it is by noting that the spark fails to burn the points 
 white or that the cylinder mises at high speed. It is an excel- 
 lent plan to put a few drops of oil on the spark plug thread be- 
 fore screwing it into the cylinder. If you ever have one stick 
 in a cylinder, you will never fail to do this. 
 
 LECTURE VII. 
 
 INSPECTION OR “PREVENTION” OF TROUBLE. 
 
 1. In airplane motor work, if the motor fails, the aviator 
 usually fails to accomplish whatever he started out to do. 
 This means that it is important to prevent trouble by means of 
 careful planning, thorough inspection and forethought, rather 
 than to wait until the trouble occurs and then figure out a way 
 to fix it. Inspect in a systematic way. Begin by inspecting 
 the gas feed. To do this, first examine the gas tank and see 
 that there is a good air vent, and see that the air vent is not 
 plugged up, because gasoline will not flow from the tank very 
 long unless air can get into take the place of the gasoline. 
 Next, test the flow from the pipe where it connects with the 
 carburetor. Clean the settlers and strainers in the gasoline 
 line. Clean the carburetor thoroughly, including the jets. 
 Then examine the intake manifolds, for air leaks or loose joints. 
 It is important to examine the throttle controls. The best 
 
61 
 
 way is to have someone sit in the pilot’s seat and operate the 
 throttle, then stand where you can see the carburetor and be 
 sure that when he operates the throttle that it opens all the 
 way and be very sure that when he closes the throttle it closes 
 all the way. This is exceedingly important because it is quite 
 possible to install a new motor or carburetor in a machine and 
 the throttle controls may not fit. The result may be that 
 the throttle will not come anywhere near closing. Then when 
 the mechanic cranks the engine, the machine may start ahead 
 so that the propeller will strike him. Next let us inspect the 
 ignition. See that the ground wire or short circuit-wire con- 
 nections are tight and examine its condition throughout its 
 entire length. See that it does not vibrate or chafe against 
 any metal parts, because after the insulation wears through, 
 the wire will short-circuit against metal parts of the machine, 
 and the effect will be the same as having the switch closed all 
 the time. Sometimes a nail or tack will have been driven 
 through the ground wire accidentally. This will do the same 
 thing. See that the connections of the wire are firm and not 
 likely to work loose, because if they should work loose, the 
 pilot will have no way to stop the spark occurring in the cylin- 
 ders, making it necessary to shut off the gasoline to stop the 
 motor and making it dangerous for the man who cranks the 
 propeller. 
 
 2. Examine the switch; see that there are no loose parts. 
 Sometimes trouble will occur from a strand coming loose on 
 a wire where it is attached inside the switch, and these strands 
 may touch the opposite part of the switch and short-circuit it. 
 Sometimes the switch will become corroded so that it will not 
 make a contact and the motor will fail to stop when we close 
 the switch, or it might be dangerous to the man cranking the 
 propeller. Always remember that on Bosch and Berling 
 magnetos and most other makes when the breaker cap or cover 
 is removed so that you can see the breaker points operate, it 
 will be dangerous to crank the motor because you contact for 
 short circuiting the magneto is usually in this cap or cover. 
 Therefore, if you wish to crank the motor to see how the 
 breaker points operate, always remove the connection between 
 the distributor and the collector brush, or remove the spark 
 plugs. Next, we can clean the magneto. First clean the 
 breaker points. The breaker should not be adjusted unless 
 they are considerably wrong, and if possible, they should be 
 trued up with a jeweler’s file when readjusted. The reason 
 is because after the points are adjusted they seldom come to- 
 gether true and must be squared up with a file. The best 
 
62 
 
 policy is to leave the breaker points alone in most cases, but 
 if you find them burned badly or find that they are badly 
 in need of adjustment, it must of course be done. Clean the 
 distributor; the best way to do this is to wipe it with an 
 oily rag and then wipe off all the superfluous oil with a dry rag, 
 leaving it slightly oily looking. Examine the entire magneto 
 from the outside and see that everything is tight. Next re- 
 move the spark plugs. Clean and adjust the points and see 
 if they are receiving the proper strength of spark. See if they 
 are burning white. Be sure to examine the plug for defect 
 or crack in the insulation. Examine the secondary wire, 
 running from the distributor to the spark plugs and see that 
 they are not chafing against any moving parts, such as the 
 rocker arms or push-rods. 
 
 2. Remember that these wires carry high pressure current 
 and any injury to the insulation is likely to cause a leak or 
 short through the frame of the motor. It is well to avoid run- 
 ning these wires close to metal parts because the rubber, in 
 time, becomes cracked by the weather, and if we have a foggy 
 morning, making things damp, the current may leak through 
 these cracks and jump to the cylinders or ground. Next we 
 can inspect the valve operating gear. In high speed motors, 
 all parts of the valve operating gear must work freely. There 
 must be no rubbing or binding. Remember that the valve 
 springs must overcome the inertia of the heavy valves and 
 return them to their seats in an extremely short space of time. 
 When the motor is running fast, they must also overcome the 
 inertia from the push rods and rocker arms or other parts of 
 the valve gear and return them to their normal positions. It 
 requires a surprising amount of power to do this, and if there 
 is any excess friction or binding in the valve operating gear, 
 the spring will fail to do it, and inaccurate valve action will 
 result. 
 
 3. Cam followers are usually shaped on the end in such a 
 way that they must be held in a certain position to ride the 
 cams properly. Frequently they are held by some form of set 
 screw, and therefore it should be inspected from time to time 
 to see if it is wearing.- If a cam follower should turn around 
 it will usually ruin both the cam followers, and, what is worse, 
 the cam shaft. 
 
 4. The tension of the exhaust valve springs must be correct, 
 or they are liable to become weakened due to their great ten- 
 sion in some cases, and also due to the fact that they are sub- 
 ject to considerable heat in some motors. In one motor that 
 I know of, if an exhaust pipe gasket blows out, the exhaust 
 flame will come in contact with the exhaust spring, thus draw- 
 
r>3 
 
 ing the temper from the steel and allowing the spring to col- 
 lapse. These springs are always tested when the motor is 
 being overhauled, but sometimes they weaken while the motor 
 is in the machine, and can usually be detected by running the 
 motor at its slowest idling speed and listening. The valve 
 with weak springs will usually suck open, or open automatically 
 during the suction stroke and will usually make a buzzing 
 sound as it does this. Sometimes it can be detected by putting 
 the finger on each exhaust valve spring in turn and the one 
 which is sucking off its seat will be vibrating in a peculiar 
 manner. When these exhaust valve springs become weak, air 
 will be drawn through the valves during the suction stroke 
 and will flow into the cylinder and also into the intake mani- 
 fold and spoil the mixture for all cylinders on that part of the 
 manifold. Its action is very similar to that of a loose joint 
 admitting air in the manifold. On the Curtiss eight-cylinder 
 one hundred horse power motors a pull-down spring is used 
 for opening the inlet valve. This spring must be at least ten 
 pounds heavier than the intake valve spring or else the valve 
 will not open properly and a “ clicking” sound will be heard 
 while the engine is running slowly. 
 
 5. Then, next, we should adjust the valve clearance. The 
 designer of engines decides that he wishes his valve to operate 
 with a certain clearance. He designs the cams to give the 
 correct valve timing with this clearance. Then the engineers 
 who test the motor at the factory find what clearance they 
 must give the valves when they are cold, in order to have the 
 correct clearance when they are hot. This clearance is pub- 
 lished for all motors and we should not adjust the clearance 
 on any motor unless we know what the correct clearance is. 
 Having found out what the correct clearance is for our motor, 
 the next important step is to place the motor correctly for 
 adjusting clearance. This must be done for each cylinder in 
 the motor. The best way to do this is to crank the motor in 
 direction of rotation until the inlet valve of the No. 1 cylinder 
 is just closed, then turn the propeller 90 degrees further. This 
 will place the cam followers on a neutral part of the cam and 
 we can now adjust the clearance. This applies to all four- 
 cycle engines, no matter what the type. Always use a thick- 
 ness gauge or “feeler” for adjusting these clearances. When 
 you realize that a thousandth of an inch variation in valve 
 clearance may make four or five degrees difference in the 
 timing, you will see the importance of adjusting valve clearance 
 accurately and uniformly. 
 
64 
 
 6. Next, we should try the compression of the motor to see 
 if all the valves are seated properly. The best way to do this 
 is to remove a spark plug from each cylinder. Then put one 
 in one cylinder at a time so that there will be no doubt as to 
 which cylinder is on compression. One good way is to swing 
 the propeller up against the compression hard enough so that 
 the compression will bounce it back again several times, to see 
 how many times it can be done before the air has leaked by 
 the piston or valves enough to let it turn on over freely. 
 
 7. Leaky exhaust valves can frequently be heard by listen- 
 ing at the exhaust pipes. Leaky inlet valves can sometimes 
 be heard by listening in the carburetor, but when trying this, 
 have the carburetor or throttle partly open, because many 
 carburetors make a wheezing sound when the throttle is closed. 
 
 8. Then examine each part, nut and screw, for tightness. 
 For instance, examine every cylinder nut; then each lock-nut 
 on valve adjustments and so on in a systematic way, otherwise 
 the loose ones will be overlooked nine times out of ten. Exam- 
 ine the engine bed bolts to see if they are tight and locked. 
 Examine the water connections. See that there is no chance 
 of the hose sucking shut in the line between the bottom of the 
 radiator and water pump. Examine all copper tubing and 
 brass tubing carrying oil, air, or gasoline; see that they are free 
 from vibration or swinging. See that they are not pinched or 
 likely to be; see that they cannot be chafed by any part. For 
 instance, where the pipes run through an aluminum bulk-head 
 in the machine, sometimes a hole is cut small and then the pipe 
 will rub against the edge of the aluminum. In this case, there 
 is danger of the aluminum wearing through the copper pipe 
 in time. 
 
 9. Inspect the Tachometer drive. Inspect the propeller. 
 See that it is on tightly and locked securely. The entire 
 power output of the engine is transmitted through the pro- 
 peller hub, and unless it is very tightly set on the shaft it will 
 “work” or vibrate and it is liable to shear the keys or loosen 
 the hub. This should be very carefully watched. A rapidly 
 revolving propeller has as much ability to do damage as a 
 large charge of dynamite and should certainly be watched. 
 
 10. Never “rock” the propeller as a preparation to the 
 pull for starting. Many men are hurt in this way. Place 
 your propeller where you intend to start the pull, raise up on 
 “tip-toes” and start your pull strongly. Remember you must 
 pick up a great deal of speed in the first few degrees in order to 
 “carry-over” the spark. As you finish the pull or stroke, 
 manage so that you will be withdrawing your hands. If the 
 
65 
 
 motor “kicks-back” never try to resist it, simply withdraw 
 your hands instantly. Never have tools in your pockets 
 while cranking, they may fly out of your pockets and be 
 “batted” through your legs by the propeller. 
 
 11. The man at the propeller and the man at the switch 
 should “sound off” what they are doing, so there will be no 
 misunderstanding. Make it a rule that the man at the 
 switch will not say “closed” until the switch is closed, because 
 if he should say closed, and then leisurely reach over to close 
 the switch, or make the motor safe, the man at the propeller 
 might work faster and pull the propeller, believing the motor 
 to be safe, and get a severe kick. 
 
 Propeller Notes. 
 
 A system for diagnosing trouble or “trouble shooting.” 
 
 1 . When a motor is not working right, we must first classify 
 our trouble or decide what the motor is doing. For example, 
 does it miss, cut out, slow down, back-fire, fail to start, fail to 
 stop, or what? Then, at what speed does the trouble occur? 
 Many troubles show up only at certain speeds and therefore 
 we should notice at what speed the trouble occurs. 
 
 2. Next, if possible, notice what part of the motor is giving 
 trouble. For example, on a “V” type motor, is the right side 
 missing or the left side? Notice which cylinder is giving the 
 trouble. Knowing which cylinder is giving trouble, we can 
 find the trouble comparatively quickly. Some kinds of 
 trouble can cause the entire motor to give trouble but could 
 not cause just one cylinder to give trouble. These could be 
 called “general” troubles. Another kind can cause one or 
 two cylinders to miss or give trouble but could not cause a 
 general trouble. These we call “local” troubles. 
 
 3. For example, suppose we say our motor is missing. 
 Now, in addition to noticing at what speed the motor is miss- 
 ing, let us note the nature of the miss. For instance, is one cyl- 
 inder missing occasionally, one cylinder missing steadily, one 
 side of the motor cutting out, or the whole engine cutting out, 
 as it would do if we intermittently pulled the switch? Or is 
 it a scattering miss, which means first one cylinder on one side 
 and then another cylinder on the other side missing, but not 
 regularly? Suppose we decide that we have one cylinder miss- 
 ing occasionally. Some of the likely causes of this trouble 
 would be, trouble from oil, slightly cracked porcelain in spark 
 plug, parts binding in the valve action or need of adjustment 
 in the valve clearance; it might be water on the spark plug. 
 
66 
 
 Suppose we decide that we have one cylinder missing steadily. 
 This is likely to be caused by one of the secondary wires or 
 leads, running from the distributor to the spark plug, being 
 disconnected at either end; a broken spark plug or a burned 
 exhaust valve spring, etc. Suppose we decide that one side of 
 the motor is giving trouble. If this trouble occurs at high 
 speed it is most likely to be caused by magneto troubles (this 
 can sometimes be remedied by slightly retarding the magneto) 
 or it may be an inlet valve stuck open. If the trouble occurs 
 at low speed, it is pretty sure to be caused by an air leak in the 
 inlet manifold on this side of the engine, a weak exhaust valve 
 spring, which has the same effect, or possibly an exhaust valve 
 held or stuck open. 
 
 4. Magnetos will cause one side of a “V” type motor, or 
 every alternate cylinder in a six cylinder motor, to cut out at 
 high speed, and this is sometimes caused by the breaker points 
 opening a different distance on the two cams; sometimes by 
 the weakening of the magnets due to vibration or other causes, 
 and sometimes by wear in the armature bearings. 
 
 5. Suppose we decide that our motor is “cutting out.” 
 Now this is a “general” trouble and is likely to be caused by 
 some troubles in the common source of gas. For instance, in 
 the gas feed, by an obstruction in the spray nozzle, or water 
 in the gasoline. Or the trouble may be in the source of igni- 
 tion, not in the distributor where the current is handed out to 
 the individual cylinders, but probably in the primary circuit 
 in the magneto. It may be in the secondary circuit, but if it 
 is, the trouble will be between the armature and the distributor. 
 
 6. Occasionally an inlet valve stuck open will cause such a 
 violent back-fire as to make the whole motor “cut out,” 
 because when a back-fire occurs it burns the gas in the mani- 
 fold and the other cylinders draw a mixture which is partly 
 burned gas. 
 
 7. Now, we may decide that we have a “scattering miss” 
 in our motor. This is harder to find, and frequently we will 
 catch one cylinder in the act of missing and believe we have 
 found a local trouble. This scattering miss is most likely to 
 be caused by a weak magneto, if it occurs at high speed; but 
 it can mean many things. It is usually a combination of 
 several slight disorders. If we find that we have a scattering 
 miss at high speed, perhaps the best thing to do is to examine 
 the spark plug points to see if they show the proper heat or 
 strength of spark, and if they do not, we should put on a new 
 magneto, if possible. If a magneto is not obtainable, we can 
 either close the spark plug points together a few thousandths 
 
67 
 
 of an inch, or in extreme cases, we may slightly retard the 
 magneto, which will help in some instances. But if we find 
 the appearance of the spark plug such that we may decide the 
 spark is all right, then the best thing is thoroughly to inspect 
 the motor as I have described. 
 
 8. To find out which cylinder is missing (if our motor has 
 open exhausts ports or exhaust pipes for each cylinder), 
 the simplest way is to hold a stick in front of each exhaust 
 in turn. In this way, we can readily see when a cylinder 
 misses explosions. Frequently we can watch the appearance 
 of the exhaust as it comes out of the pipe, and at the same in- 
 stant that we hear the motor miss, we may see a slight differ- 
 ence in the exhaust, lighter or darker in one cylinder. This 
 will probably be the cylinder that is missing. 
 
 9. When seated in an airplane with a “V” type motor, it is 
 a good plan to listen intently to the sounds coming from the 
 exhaust on one side of the engine at a time. With practice, 
 you can concentrate on the sounds coming from one side and 
 in this way you can tell a great deal more about the operation 
 of the motor than by listening to the entire jumble of sounds. 
 
 10. Sometimes when looking at the exhaust of a motor, you. 
 will see more flame in one cylinder than in another, or different 
 colored flames. This may be an indication of air leaks in the 
 manifold or weak exhaust valve springs, but it is quite likely 
 to mean that the motor is cold or that the manifolds do not 
 distribute the gas evenly. This is something that a mechanic 
 on the field cannot correct. It is well, however, to watch the 
 exhaust of your motor each day and if a change takes place — 
 if, for instance, one side becomes clearer and the other side 
 darker, it may indicate than an air leak has developed or that 
 there is a weak exhaust valve spring. White smoke is always 
 due to an excessive amount of lubricating oil, but always 
 remember that oil on a plug can be the cause of a cylinder 
 missing by fouling the spark plug, or it can be an effect. In 
 other words, if a spark plug broke and failed to deliver a spark, 
 oil would accumulate in the cylinder and not be burned and 
 in this way would be an effect and not a cause. 
 
 Troubles and Some of their Probable Causes. 
 
 11. Signs of overheating. 
 
 (1) Slowing down. 
 
 (2) Radiator steaming. Possibly we notice a smell of burn- 
 ing oil or hot rubber. Frequently as the motor begins to over- 
 heat, vibration will increase, making the motor seem to run 
 
68 
 
 harshly. Sometimes a smell of too much gas or smell of rich 
 mixture will be noticeable. In any case, stop the motor as 
 soon as you can when this is noted, because continuing to run 
 a motor when it is overheated will usually ruin it. When feel- 
 ing the motor to see if it is overheated, remember that it must 
 be felt immediately after stopping the engine, because after 
 the motor is allowed to stand a few minutes the outside be- 
 comes considerably hotter. The reason for this is that the 
 water does not circulate when the motor is stopped and be- 
 comes highly heated, in turn heating the outer wall of the water 
 jacket and making the motor seem very hot. The radiator, if 
 felt immediately after stopping the motor, will probably feel 
 about “blood warmth” at the bottom and quite hot at the top, 
 but after standing for some time, it may be quite hot all over. 
 
 Some of the Causes of Overheating. 
 
 12. Some of the causes of overheating are: 
 
 I. Obstructed water connections. For example, dirt or 
 rust flakes in the pipes. Or frequently in using hose connec- 
 tions when the pipe is thrust into the hose, the end of the 
 pipe will ‘catch and “curl” the inner layer of fabric back into 
 the hose, in this way at least partially shutting off the water. 
 
 II. Broken piston rings. This will cause overheating by 
 allowing the hot gases to pass down between the piston and 
 cylinder walls. 
 
 III. Leaky radiator. 
 
 IV. Oil in the radiator. This will cause overheating by 
 preventing the water from coming in actual contact with the 
 metal of the radiator which is cooled by the air. If we take 
 two pieces of iron and heat them to a black heat and put oil 
 on one of them, then drop both of them in a pan of water, we 
 may be able to pick the clean one up immediately, but the one 
 which is oily will remain hot a great deal longer. Therefore, 
 never use an oily measure to fill your radiator. 
 
 V. A rich mixture. 
 
 VI. Early or late spark. The rich mixture and the late 
 spark cause overheating by causing the maximum temperature 
 to occur late, at a time when there is a great amount of cylin- 
 der wall exposed to absorb heat, and therefore this surface 
 will be harder to cool. 
 
 VII. Failure of oil pump or oil supply. This causes over- 
 heating on account of excessive friction in dry bearings. 
 
 VIII. Tight motor or new parts. If the bearings are tight 
 in a motor or there are some tight-fitting parts, the extra 
 friction may cause overheating. 
 
69 
 
 IX. Hot weather. The cooling systems on airplanes are 
 as small and light as possible, and while they serve to cool 
 the motor in reasonable weather, they may give trouble from 
 overheating on exceptionally hot days, simply because the size 
 of the radiator and the amount of water is not sufficient for 
 these extreme conditions. 
 
 X. Excessive deposits of carbon or any other cause of 
 pre-ignition will cause overheating. 
 
 13. Causes of loss of power: 
 
 I. Pre-ignition. Overheating causes loss of power by caus- 
 ing pre-ignition in many cases. Remember that pre-ignition 
 means ignition occurring from some hot parts of the cylinder 
 or hot particles of carbon, and occurs far earlier than the spark 
 would occur, causing a tendency for the motor to work against 
 itself. 
 
 II. Tight motor. 
 
 III. Bad valve or spark timing. 
 
 IV. Broken piston rings. 
 
 V. Stoppage in fuel line or carburetor. 
 
 VI. Waste of rags in the carburetor. Motors in airplanes 
 draw a large amount of air through the carburetor, and if a 
 bunch of rags were carelessly left under the hood, it would 
 easily be sucked up by the carburetor and would be liable to 
 make serious trouble. 
 
 VII. Leaky valves or need of overhauling. 
 
 VIII. Weak valve springs. 
 
 IX. High altitude. When a motor is running in high alti- 
 tude, the atmospheric pressure being very much less than at 
 sea level, the piston will not draw a complete charge. In 
 other words, there is less difference in pressure inside the cylin- 
 der and outside, therefore less air will rush in through the 
 carburetor, and the motor never shows as much power in 
 high altitudes as at sea level. This can be partly corrected 
 by opening an auxiliary valve in an effort to increase the 
 amount of air flowing into the cylinders. 
 
 Vibration. 
 
 14. Some of the causes of vibration are: 
 
 I. Pre-ignition. 
 
 II. Air leaks in the inlet manifolds. This causes uneven 
 mixture in the different cylinders and therefore a different 
 action in the different cylinders and uneven explosions. 
 
 III. Weak valve springs. These cause vibration by admit- 
 ting air in the same way as the intake manifold leaks. 
 
70 
 
 IV. Broken piston rings. These cause uneven compression. 
 
 V. Rare mixture. 
 
 VI. The magneto breaker housing may be put on crooked 
 or worn, so that the breaker separates different distances on 
 the two cams, causing uneven spark timing. 
 
 VII. The propeller may be out of balance, warped or “flut- 
 tering.” If the propeller is built too lightly it will flutter. 
 This cannot be corrected. 
 
 VIII. Engine-bed bolts loose. 
 
 IX. Engine parts of uneven weight. For example, some 
 pistons heavier than others. These parts must be as nearly 
 as possible the same weight, usually within one quarter of an 
 ounce. 
 
 Loss of Compression. 
 
 15. Some of the causes are: 
 
 I. Valve held open, or no clearance. 
 
 II. Leaky valves or valves needing grinding. Sometimes 
 the valves have chunks of carbon holding them off their seats. 
 
 III. Scored cylinders. 
 
 IV. Cracked pistons or cylinders. 
 
 V. Worn or broken piston rings. 
 
 VI. Cylinder dry or out of oil. For example, if our motor 
 has been standing in the storeroom without beingjyun, all the 
 oil may drain off the pistons and cylinders, leaving them com- 
 paratively dry, and poor compression will result. Some- 
 times in priming a motor with gasoline on a cold morning, we 
 will get an excess of gasoline in the cylinders and wash the 
 lubrication out of it.. In either case, the remedy is to put in 
 some extra oil through the spark plug hole. 
 
 VII. Loose spark plug or bad spark plug gasket. 
 
 Failure to Throttle Down, or Stopping When Throttled. 
 
 16. Some of the causes are: 
 
 I. Throttle may be stuck or shifted in the carburetor. 
 
 II. Throttle controls may be adjusted wrong, not allowing 
 the carburetor throttle to close. 
 
 III. Very bad air leaks in manifold or weak exhaust valve 
 springs. In these cases, the excessive amount of air leaking 
 in will either stop the motor when it is throttled down or 
 cause it to idle too fast. The latter trouble would be in case 
 the mixture had been too rich. Then the excess of air would 
 thin this mixture down and make more of it. The result would 
 be the motor would idle too fast. The best way to find air 
 
71 
 
 leaks in the inlet manifold is to run the motor throttled and 
 put oil on each point with oil can. It will be sucked in where 
 the leak is. Don’t use gasoline, because there is danger of 
 fire from the exhaust. It is possible to get enough suction for 
 this test by closing the throttle and turning the motor over 
 by hand. 
 
 IY. Spark plug points may be much too wide apart. In 
 this case, it is difficult for the magneto to throw a spark across 
 the points at low speed. 
 
 V. Throttle closed too far. Nearly all carburetors have an 
 adjustment to limit the closing of the throttle, usually on 
 the throttle arm. 
 
 VI. Too much or too little gas, or in other words, imperfect 
 low speed gas adjustment. 
 
 VII. Auxiliary air valve may be loose or stuck open. 
 
 Back-Firing. 
 
 17. A back-fire is caused either by the inlet valve being held 
 open, spark occurring while the valve is open, or by a rare 
 mixture. 
 
 18. Let us take these causes, one at a time, and reason them 
 out. 
 
 If the inlet valve is held open, that might be caused by 
 lack of clearance, cam followers stuck, or valve stuck open. 
 If the spark is occurring while the inlet valve is open, it might 
 be caused by the magneto being timed wrong, the valve timed 
 wrong, or the magneto leads (secondary wires) being crossed 
 or connected to wrong cylinder. If the back-fire is caused by 
 a rare mixture, remember that the rare mixture may be from either 
 too much air or not enough gas. If it is too much air, the aux- 
 iliary air valve may be stuck open, or the spring may be weak. 
 You might have air leaks in the inlet manifold or weak exhaust 
 valve springs admitting air. If the motor is not getting 
 enough gas, this might be due to a clogged air vent in the gas 
 tank, water in the gasoline pipe, or in the jets. It might mean 
 that the motor was cold or that hot air was needed. 
 
 Loud Exhaust. 
 
 19. If the motor is making a loud exhaust, the question is, 
 what can cause it? The noise that we call the exhaust is 
 pressure escaping through the exhaust valve when it opens. 
 An unusually loud exhaust means there is an unusual amount 
 of pressure at the time the valve opens. This might be caused 
 by the valve opening while there is high pressure; that would 
 
72 
 
 mean opening early. The valve might be stuck open; a cam 
 follower turned around; or if all the cylinders are making a loud 
 exhaust, it might be the cam shaft is out of time. 
 
 20. Now the other way to think of it is that there may be 
 high pressure at the time the valve opens normally. This 
 might be caused by a rich, slow-burning mixture, or from a 
 float valve being held open by dirt. Possibly the rich mixture 
 could be caused by the tube through which the carburetor 
 sucks the warm air, being sucked shut. A late spark can cause 
 a loud exhaust by causing the maximum pressure to occur late, 
 in the same way that the rich mixture would cause it. 
 
 21. If all cylinders make a loud exhaust, it may mean that 
 the magneto is timed wrong. Sometimes a loud exhaust in 
 one or two cylinders may be caused by the magneto leads or 
 spark plug wires being crossed or mixed up. 
 
 Failure to Start. 
 
 22. If the motor fails to start the first thing to do is to 
 inspect the gas feed carefully. Prove that the gas flows 
 through the passage where it joins the carburetor, and prove 
 that it flows through the jets by running a wire through them. 
 If the trouble is not found then, test the magneto by “ground- 
 ing” a screw driver against a cylinder. Place it close to the 
 top of the collector brush or any place where we can get at the 
 secondary current before it reaches the distributor, to see if 
 we get a spark. If we fail to get a spark, remove the ground 
 wire and try it again. If we still get no spark, thoroughly in- 
 spect the magneto and if no spark can be obtained, we must 
 have a new magneto, or at least a magneto expert. 
 
 23. If the magneto tests out all right, the next most likely 
 cause of failure to start would be that the motor is “flooded;” 
 or in other words, in trying to start, we may have drawn so 
 much gasoline into the cylinders that it cannot burn. In this 
 case, we must shut off the gasoline and close the switch, open 
 the throttle and endeavor to get a charge of clean air into the 
 cylinders; or in other words, “air the motor out.” 
 
 24. If the motor has been flooded, it will probably start 
 under these conditions. If we have found all these things all 
 right, then the trouble must be that either the spark or valve 
 timing is incorrect. In that case, check the spark timing; 
 turn the engine over until the inlet valve on one cylinder has 
 just closed; remove the spark plug and insert a screw-driver or 
 wire and follow the piston up to the top of the stroke; back it 
 down from one-quarter to one-half an inch. Now by following 
 
73 
 
 the spark plug wire of that cylinder to the distributor, we 
 should find the distributor brush touching the contact to which 
 this wire is connected. 
 
 25 . If this is correct, the spark timing is not at fault, be- 
 cause if the magneto was timed close enough so that we would 
 find the distributor in contact by this method, it would be 
 near enough for the motor to start, or at least for it to “kick” 
 one way or the other, certainly on the right stroke. If we 
 have time, we can measure the magneto timing more accurately 
 to see if the amount of advance is correct, but it is seldom that 
 we find any great mistake in the amount of the magneto ad- 
 vance, while we often find the magneto firing on the wrong 
 stroke. 
 
 26. If the valve timing is suspected, it is only necessary to 
 turn the engine over and find out by watching the exhaust 
 valve, and by putting a rod through the spark plug hole, 
 where the exhaust valve closes. The exhaust valve must close 
 on, or slightly after, the top center. 
 
 Failure to Stop. 
 
 27. Sometimes when we close the switch, the motor keeps 
 on running or “kicking.” If it merely continues to kick, the 
 chances are that the motor is hot, or overheated. Possibly 
 some particles of carbon, spark plug points or exhaust valve 
 remain red hot (or incandescent), and are igniting the charges. 
 If the motor continues to run smoothly after we close the 
 switch, it is likely that the ground wire has become discon- 
 nected or the switch fails to make contact. Possibly the 
 breaker cap or cover is off and not making contact. Some- 
 times the breaker housing is not put on far enough. In this 
 way the breaker cover is held away from the breaker, so that 
 it cannot short circuit it. 
 
 LECTURE IX. 
 
 INSTALLATION, CARE, REMOVAL AND STORAGE OF 
 MOTOR. 
 
 1 . In placing a rope around the motor preparatory to raising 
 it for installation in the fuselage, be careful to keep the rope 
 away from any part it might injure. The water jackets on 
 airplane engines are usually delicate and the rope must not 
 press against them. Priming cocks are easily broken off. 
 Rocker arms are likely to be bent, and small copper or brass 
 
74 
 
 tubing could be smashed flat by a rope. The timbers or beds 
 on which the motor will be placed in the^airplane must be 
 level and parallel, and the bolts holding them in the machine 
 must be tight. 
 
 2. After the motor is placed in them machine, the switch and 
 ground wire should be connected up first, so that the motor 
 may be made “safe” while the propeller is being installed. 
 In making up the water connections, be careful that the ends 
 of pipes do not “turn in” the inner layer of fabric in the hose, 
 thus obstructing the flow of water. If tape is used on these 
 water joints, it must be shellacked afterwards, because if the 
 tape is not coated with shellac it will unwrap and deteriorate 
 from the effects of the hot weather. Make it a rule never to 
 connect the gasoline to the carburetor without first allowing 
 gasoline to flow through the pipe for the purpose of flushing or 
 cleaning the pipe and also to prove the flow of gasoline. 
 
 3. Check the adjustment of throttle and spark retard wires 
 or rods to see that they are properly adjusted to the motor. 
 The throttle must open fully and close fully, and when the 
 magneto is pulled to the retard position, be sure to see that the 
 spring will pull back into the full advance position, because 
 if it fails to do this, the motor will probably overheat. 
 
 Testing the Motor on a Testing Block. 
 
 4. Airplane motors are tested after overhauling before being 
 installed in an airplane. 
 
 5. Around flying fields we seldom have opportunity to test 
 a motor for horse power output, but generally put a standard 
 propeller on the motor or a “club” (dummy) propeller which 
 will turn at the same speed as the propeller. If the motor 
 turns this standard propeller or “ club ” to the required number 
 of revolutions its power output is considered satisfactory. 
 
 6. While running on the block with this propeller or “club” 
 the motor must: 
 
 A. — Run up to specified revolutions. 
 
 B. — Throttle down and pick up smoothly when the throttle 
 is opened. 
 
 C. — Not accumulate oil in the cylinders or foul spark plugs. 
 
 D. — Reveal a proper oil-pressure for that particular motor, 
 so that it will not throw oil. The motor must cool properly 
 and not have excessive vibration. Must not slow down after 
 running half an hour or so, or seem to need very accurate 
 adjustment, and must not be “sensitive.” Compression must 
 be even and motor must turn freely. 
 
75 
 
 ^ Care of the Motor. 
 
 7. To give you some idea of how an airplane motor is cared 
 for, I will tell what is done to one make of motor used to con- 
 siderable extent in the Government Schools. 
 
 8. The motor receives a thorough and careful inspection and 
 cleaning daily. After each five hours running, the valve 
 clearance is carefully adjusted to a gauge, spark plugs are 
 removed, inspected and adjusted. All the oil is removed 
 from the oil pump and renewed after fifteen hours running. 
 This is because the oil comes in contact with the hot piston 
 heads and deteriorates from the heat; also becomes filled with 
 chunks of carbon and a certain amount of liquid fuel con- 
 denses in the cylinder and mixes with the cylinder oil im- 
 pairing its lubricating qualities. Remember that if a motor 
 is run too long without overhauling, when it is finally removed 
 from a machine it will require an expensive rebuilding instead 
 of overhauling. 
 
 Removal of Motor. 
 
 9. Make it a rule to remove the propeller first and ground 
 wire connections last. This is so that the motor may be made 
 “safe” as long as the propeller is on it. Be sure that every- 
 thing is disconnected before raising the motor with the “chain” 
 fall or tackle. 
 
 Storage. 
 
 10. While the motor is in storage, be sure there is no gas 
 left in the carburetor. If gas is left in the float chamber of the 
 carburetor it will evaporate and leave a deposit of some kind 
 that is likely to clog the gas passages when the engine is used 
 again. Oil should be put in the cylinders through the spark 
 plug holes, and the motor turned over by hand a few times, 
 at least each week, to prevent rust in the cylinders; and the 
 valve stems should be oiled with an oil can. 
 
 Overhauling. 
 
 11. Overhauling is a very elastic word. It can mean merely 
 disassembly and reassembly of a motor, or it can mean very 
 carefully taking a motor apart and noticing each place where it 
 is wearing or rubbing and investigating the cause, and careful 
 planning to correct and improve conditions while assembling 
 the motor. 
 
76 
 
 12. One important thing is to trace the lubrication system 
 from the oil pump or through the motor and back to the sump. 
 After learning the route through which it travels, all passages 
 should be carefully cleaned out by forcing gasoline through 
 them to avoid any possibility of stoppage in the oil circulation. 
 
 13. After a motor is overhauled, it is best to make two or 
 three short flights, for instance 10 or 15 minute flights, before 
 a long flight is made, and the motor should be examined or 
 felt after each of these short flights. It is far better, if pos- 
 sible, to install the motor on a block and test it thoroughly 
 before installing it in an airplane. 
 
 LECTURE X. 
 
 DIFFERENCES BETWEEN AIRPLANE AND AUTOMO- 
 BILE ENGINES. 
 
 1. First of all, airplane engines are always built lightly, 
 and this means that the crank case is not very rigid. The 
 timbers or beds on which the engine sits must be carefully 
 lined up and be perfectly true so that the crank case will not 
 be sprung out of shape when it is bolted to them. 
 
 2. Airplane engines run at nearly full capacity all the time. 
 This means that we have full compression, maximum tempera- 
 tures and maximum pressures, also maximum vibration all the 
 while. Lubrication of moving parts becomes a greater prob- 
 lem under these conditions. For example, in a racing auto- 
 mobile, the driver throttles down his motor to a certain extent 
 on the turns. This will allow the crank shaft to ease up on its 
 bearings slightly, allowing the oil film to be renewed under the 
 shaft. In the airplane motor there is no such easing up on 
 turns. Airplane engines run at high temperatures because of 
 the necessarily light weight of the cooling systems. In other 
 words, the cooling systems are very close to the limit. Air- 
 plane motors are subject to strains caused by the propeller. 
 Even the best built airplane propellers vibrate or flutter to a 
 certain extent and all this vibration goes through the motor. 
 Usually the crank shaft gets it first and when flying, the gusty 
 winds impose great strains on the propeller and crank shaft 
 due to the uneven air through which the propeller is working. 
 As the airplane is “pushed” or “lead” through the air by its 
 crank shaft in most cases, it is necessary to have a substantial 
 thrust bearing on the crank shaft to take care of these strains. 
 
77 
 
 3. It is somewhat difficult to tune up an airplane motor on 
 the ground for the following reasons; The motor cannot he 
 run long on the ground without overheating because the air- 
 plane is stationary and the propeller throws little or no air 
 through the radiator. (The central part of the propeller is 
 not designed to do any work or throw any air to amount to 
 anything and the result is that it throws practically no air 
 through the radiator when the machine is standing still.) 
 The result is, if we are tuning up the motor we can run it with 
 throttle open for short intervals. 
 
 4. In some machines there is a difference in the atmospheric 
 conditions around the carburetor when the machine is stand- 
 ing or flying. For instance, when the machine is flying, there 
 may be either a decrease or an increase in the atmospheric 
 pressure, due to the location of the carburetor, and this makes 
 carburetor adjustment on the ground difficult in some instances. 
 
 5. Airplane engines must run at high altitudes and under 
 these conditions encounter very low temperatures and very 
 low atmospheric pressure, which means that the cylinders 
 will not get a complete charge of gas. A motor loses a large 
 percentage of its power, even at a 10,000 foot altitude. The 
 reason is that there is less air pressure outside the motor; 
 therefore less air or mixture rushing through the carburetor, 
 and into the cylinder during the suction stroke. One of the 
 remedies for this is to open an extra air passage in the inlet 
 manifold to allow a larger volume of air to enter the cylinders. 
 Other methods are being experimented with. 
 
 6. Airplane engines must run at all angles. This means 
 there must be plenty of “fall” from the gasoline tank to the 
 carburetor. The oil sumps of the crank case must have the 
 oil continually pumped out of them because a large accumula- 
 tion of oil in the crank case would flood the cylinders under 
 certain flying conditions. Some of the exhibition flyers have 
 had carburetors especially arranged to feed gasoline while the 
 airplane is upside down. Such devices are not in common 
 use, however, 
 
 7. You probably know that if we take “hay-wire” or soft 
 iron wire and bend it several times it becomes brittle and 
 breaks. This is known as crystallization. In airplanes, if 
 the engine vibrates, it sends little strains through the entire 
 airplane which bend the metal parts ever so slightly. As 
 these vibrations occur frequently and for long periods, we find 
 the metal in time becomes crystallized in the same way as the 
 soft iron wire I spoke of. The result may be that while the 
 machine is “taxying” along on the ground to the hangar, 
 
78 
 
 fittings may snap simply because thay are crystallized. I 
 mention this so that you will know the importance of keeping 
 a motor running smoothly. On an airplane motor the com- 
 pression in all cylinders must be uniform. Moving parts must 
 all weigh alike. Spring tension must be uniform. Spark 
 plug points must be adjusted at uniform gaps. If more than 
 one magneto is used, they must be carefully synchronized or 
 timed together and certainly all the plugs must be of the same 
 make and type. 
 
 8. Airplane engines must be free from vibration, because 
 engine vibration rapidly crystallizes all the metal parts in the 
 airplane, which means that they are liable to break without 
 being subjected to any great strains. 
 
 9. Most American airplane engines are built with the car- 
 buretor very low. While this gives a long inlet manifold 
 which has some disadvantages, it has the advantage of placing 
 the carburetor where it is easily fed from a tank located in the 
 fuselage of the airplane. Many European machines have a 
 gas tank placed in or under the top plane, but this necessi- 
 tates pipes running up to this tank and consequently increased 
 danger from leaks and possible fire. 
 
 10. A good many air pressure gas feed systems are used with 
 good success; and also gasoline pumps, pumping an excessive 
 amount of gasoline to a tank which will feed the carburetor by 
 gravity. This tank has an over-flow pipe to the main storage 
 tank, to take care of the excess delivered by the pump. Re- 
 member that gasoline tanks in airplanes must have “ baffle 
 plates” to keep the gasoline from washing from one end to the 
 other. One reason for this is that if gasoline could rush from 
 one end to the other, it might seriously affect the balance of the 
 airplane. Air vents in gasoline tanks must be so arranged that 
 should gasoline splash out of them it could not be ignited by the 
 hot gases from the exhaust pipes. Frequently these vents 
 have a tube running down through the bottom of the fuselage 
 for safety. 
 
 11. Up to the present time most airplane motors have been 
 equipped with exhaust “ stacks” (short pipes). The purpose 
 of these stacks is to carry the gases away from the fuselage, 
 and also to have a slight syphon effect to scavenge the cylin- 
 ders, but at present many airplanes are equipped with pipes 
 to carry the exhaust gases and sounds above the upper plane. 
 
 12. It is difficult to use mufflers on airplane engines, partly 
 due to the fact that a muffler reduces the power of a motor, 
 but more particularly because an engine is more difficult to 
 
79 
 
 cool if it is equipped with a muffler, and the exhaust is more 
 liable to burn. « 
 
 13. The gaskets between the exhaust pipes and cylinders 
 are important. If they blow out, there is danger of the flame 
 coming through and burning some part of the airplane, or in 
 some motors, this flame could ruin the exhaust valve springs 
 by overheating them and causing them to collapse. 
 
 14. Gasoline strainers and settlers must be arranged so that 
 they will work properly with the airplane in various angles. 
 Many settlers now in use fail in this respect. 
 
 BACK-FIRING. 
 
 15. Back-firing in the carburetor is particularly dangerous 
 in airplanes. Some manufacturers are experimenting with 
 devices to prevent back-firing. If an airplane catches fire, it 
 is a very dangerous situation for the pilot because he cannot 
 step out on the ground and use the fire extinguisher. If he is 
 up in the air he is likely to be burned badly before he can land 
 the machine, therefore, every precaution should be taken to 
 avoid chance of fire.