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Q. What do sudden grinding noises in transmission case or rear axle indicate?

A. Grinding noises are generally caused by gearing out of adjustment or by particles of foreign matter such as metallic particles from broken bearings or stripped gear teeth working in between the gears from time to time.

Note.-For complete analysis of troubles see "The Modern Gasoline Automobile."By Pagé.

LESSON THIRTY-TWO

REPAIRING POWER PLANT GROUP

Q. What are the main causes of lost power?

A. The principal causes of lost power are irregular engine operation, loss of compression if engine fires regularly, overheating and carbon deposits.

Q. Describe method of testing compression.

A. The easiest method of testing the compression is to turn the crank shaft over by means of the starting crank with all compression release cocks closed and spark plugs in place. The amount of resistance to cranking can be taken as an indication of the compression in the cylinders. A cylinder with good compression will offer decided resistance against rotation of the crank shaft, whereas one lacking in compression will offer no perceptibly greater resistance to the crank shaft than does bearing friction. If on turning the starting handle of a multiple cylinder engine one finds some cylinders that have good compression while others permit the gas to escape it is imperative that those having less compression should be found as these are the ones that need attention.

The amount of compression in a cylinder can be easily determined by various forms of pressure gauges that can be screwed into the cylinder head in place of a spark plug or compression relief cock. A gauge of this character is similar in construction to those used for showing the pressure of steam and the amount of compression is usually indicated on the dial. The various cylinders may be tested in turn and the compression should average at least 65 or 70 pounds per square inch in each cylinder. If no pressure gauge is at hand one may test each cylinder separately by removing the spark plugs or opening the relief cocks on all cylinders except that in which the compression is to be tested. Starting with the first cylinder each

of the other units is tested in turn and it is imperative that the compression be equal or nearly the same in all cylinders. The use of a pressure gauge is recommended because this gives a positive determination of the condition of the various members depended on to hold the gas under compression in the cylinder.

Q. What is the first thing to examine if compression is weak?

A. The first point to look at is the various gaskets and packings on top of the combustion chamber, such as those around the spark plug or under the valve chamber caps. It is easy to determine if

Valve Spring

Fulcrum

Spring Lifter

Adjustable Support

Fig. 277.Methods of Relieving Valve Spring Tension to Remove

Valve Stem Key.

these leak by squirting oil around the joints before cranking the engine. Any compression leak will be indicated by air bubbles. The next thing to examine are the inlet and exhaust valves. Valves that seat poorly are the main cause of poor compression and this defective condition is easily remedied, as a rule, by grinding the valves to a correct seating.

Q. Describe process of valve removal.

A. In order to remove the valves from the conventional form of cylinder it is first necessary to unscrew the valve chamber cap at the top of the cylinder casting and also to remove the valve spring enclosing plates if these members are carried in a housing. The valves are kept seated by coil springs which expand against a key of some kind put through the valve stem and held in place by some form of collar that takes the valve spring thrust. In order to remove the valve it is first necessary to compress the spring sufficiently to draw the valve key out of the valve stem. This permits the valve to be lifted out through the top of the valve chamber.

A number of simple and easily operated devices are offered to facilitate valve removal. Some of these are shown at Figs. 277

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Fig. 217A.-How Forked Lever is Employed to Raise Valve Spring.

and 277A. That at A Fig. 277A consists of a simple hook carrying a chain and a notched lever which is adapted to fit the links of the chain. The method of using is so clearly shown in illustration that explanation of its action is almost unnecessary. The function of the hook member is to hold the valve head down against its seat while the spring is compressed by the spring lifting lever placed under the collar at the lower end of the valve stem. By bearing down on the long arm of the lever the collar and the spring which bears against it is easily raised and the key may be withdrawn from the stem. The form shown at B is equally simple in its application and consists of a frame bar having a screw at the upper end and a fork at the lower end. By turning the hand screw the valve spring may be compressed with but little exertion and the valve key removed. The form shown at Fig. 277 is a simple fulcrumed lever carried by an adjustable support that permits raising the valve spring though some method of holding the valve head down against the seat must be used in combination with this tool. The usual method is to interpose a block of wood between the valve head and the valve chamber cap while the spring is being compressed to permit one to remove the keys.

Q. Enumerate principal valve defects.

A. The exhaust valves usually suffer more than the intake members because they are constantly exposed to the corroding effect of the hot exhaust gases.

Exhaust valve heads will warp and the seats will be scored or pitted sooner than inlet valves. Sometimes the valve head may flake and these particles of metal will get between the valve and seat and keep the head from seating correctly. Inlet valves are not so apt to warp as the exhaust valves because they are exposed to the comparatively cool charges of gas coming from carburetor. Some valves are made with cast iron heads riveted to a steel stem. These very often give trouble because the heads will crack or break or may become loose on the stem. Valve stems are sometimes bent or they may become gummed in the guides so they do not move freely. The principal valve trouble is defective seating due to wear between the valve head and the valve seat and is generally easily remedied by grinding the valves to a new seating.

Q. Outline process of valve grinding.

A. Valve grinding is a simple process, though it must be carried on carefully in order to secure good results. The valve heads of most cars will be provided either with two small holes, as shown at A, Fig. 278, or with a screw driver slot, as shown at B, Fig. 278. Tools

may be made to fit the valve head at one end and the ordinary form of carpenters' bitstock at the other. The process of valve grinding itself consists of coating the valve and seat with some

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