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Fig. 39.-Valve Timing Diagram Showing Spaces Laid Out on Flywheel, the Dimensions Given Cor-
responding to the Number of Degrees of Crank Shaft Movement. The Dimensions Indicated in
Inches are Only Suitable for Flywheel 18 in. in Diameter, though the Timing in Degrees May Be
Used as a Guide for Many Four-Cylinder Motors.

that the valves may be timed to open and close very accurately if the engine flywheel is provided with the proper marks and a suitable trammel or other indicator is placed on the engine base to indicate the center line of the engine.

Q. What is the camshaft?

A. The small auxiliary shaft that carries the valve lifting cams and usually runs parallel with the crankshaft and which is driven by that member is called the camshaft. Some engines have but one camshaft, which carries the cams utilized in operating both inlet and exhaust valves. The "L" type cylinder engine needs but one camshaft while the power plant provided with "T" head cylinders needs two camshafts, one at each side of the motor. (Figs. 23 and 24.)

40°

INLET VALVE OPENS.

INLET VALVE CLOSES.

Fig. 40. Showing the Time of Opening and Closing of Inlet Valve Relative to Crankshaft Travel.

EXHAUST VALVE OPENS.

46°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

[graphic]

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

[graphic]

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

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