Q. How are cams attached to the camshaft?

A. The cams may be separate forgings assembled on a piece of straight shaft and held from turning by means of keys and taper pins driven through both cam and shaft. Or they may be formed integrally with the camshaft by either a forging or machinery process.

Q. How do cams raise the valves?

A. The point of a cam raises the valve stem through the medium of a plunger or cam follower which rides on the cam and follows the profile. As will be seen at Fig. 44, when the point of the cam is not in contact with the roller on the valve operating plunger, that member

Fig. 42.-Front View of Engine Showing Gearing for Operating Cam Shaft.

Fig. 43.-Front View for Cadillac Motor Showing Silent Chain for Camshaft and Water Pump Drive.

projects down and is nearly in contact with the circular portion of the cam profile. As soon as the projection moves around so that it hits the roller, that member is raised and rides over the projecting nose of the cam, thereby raising the valve plunger and the valve that it operates.

Q. What is the best form of valve plunger?

A. The best form of valve plunger to use depends entirely upon

cam profile. The preferred construction is to use a hollow tubular member, to secure lightness, provided with a hardened roller at its lower end that follows the cam contour with a rolling motion. Where the cam construction permits a mushroom plunger is sometimes employed.

Q. How are valve plunger guides fastened?

A. Valve plunger guides are attached to the engine base by any one of three commonly used methods. That most widely applied is

Fig. 44.-Defining Method by Which Valve Operating Cam Raises the Plunger Between Camshaft and Valve Stem. A-Plunger Just Engaging Point at Cam. B-Plunger Fully Raised. C-Plunger Leaving Point of Cam.

to bolt them down with small cap screws. Optional methods are the use of a stirrup or double yoke member which is held by one bolt and which holds two valve plunger guides or by screwing the guide bushings into suitable tapped holes in the engine base.

Q. What keeps the valves against their seats?

A. The valves are kept seated by means of coil springs which are under compression and which always oppose cam action.

Q. Which valve has the strongest spring?

A. The strength of the valve springs depends on methods of

Fig. 45.-Piston, Piston Rings, Wrist Pin, and Connecting Rod of Typical Gasoline Motor.

valve operation. If the inlet valve is operated by suction only, the spring is comparatively light and the exhaust valve has the strongest spring. When mechanical valves are employed and both are the same, the exhaust and inlet valve springs are of equal strength to facilitate replacement and to make them interchangeable during manufacturing.

Q. How strong should valve springs be?

A. Valve springs should be just strong enough to hold the valve firmly against its seat and to return it to a closed position at the maximum speed of the motor just as soon as the cam roller leaves the raised portion of the valve operating cam. If valve springs are too strong, considerable power will be consumed in raising them, whereas if too weak the valve operation and consequently the engine action will be erratic.

Q. Name advantages of enclosed valve gear.

A. On modern motors the tendency is to enclose the valve operating plungers and springs, in chambers formed in the cylinder castings by means of cover plates. This is desirable practice because it keeps dirt and grit away from the valve operating plungers and valve stems and insures superior quietness of operation. (Fig. 23.)

Q. What is the piston and of what material is it made?

A. The piston is a reciprocating cylindrical member that moves in the cylinder and which transforms the power of the explosion to mechanical energy. Pistons are usually made of close grained gray iron of approximately the same mixture as the cylinder iron though where great lightness is desired, as on aeroplane motors, steel may be employed. (Fig. 45.)

Q. What are the piston bosses?

A. The piston bosses are projections cast inside of the piston to hold the wrist pin or member that supports the upper end of the connecting rod. (Fig. 46.)

Q. What are the piston rings?

A piston is usually provided with a series of grooves in which rings of cast iron are mounted to form a packing. The piston must