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EXHAUST VALVE OPENS.

6°40'

EXHAUST VALVE CLOSES.

Fig. 41.-Outlining Position for Opening and Closing Exhaust Valves. Q. How is the camshaft driven?

A. The camshaft may be driven by means of gearing as shown at Fig. 42 or by silent chains as outlined at Fig. 43.

Q. At what speed is the camshaft driven?

A. The camshaft of a four-cycle engine is always driven at half the engine speed and always by positive gearing.

Q. What is a cam?

A. A cam is a cylinder of metal having a raised portion at one point on its periphery.

Q. What is the difference between inlet and exhaust cams? A. The difference between inlet and exhaust cams is in the cam profile as the member intended to lift the exhaust valve has a longer dwell or larger and longer raised portion because the exhaust valve is kept open longer than the inlet member.

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

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

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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. emp oyed.

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

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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

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

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