Lesson 1/Learning Event 1
Camshaft. The camshaft rotates on machined bearing surfaces fitted in the engine block. Offset portions,
called cams (one for each valve), are also machined on the shaft. The high point of a camshaft cam is the lobe
or toe of the cam while the lowest part is the base circle or heel. The camshaft is mounted in the engine block
so the cams are located directly under the valve stems or tappets.
Tappets. The tappets are round, bar-like parts that have a threaded hole in one end to receive an adjusting
bolt. Tappets are also commonly called cam followers or valve lifters. They are installed in bored holes in the
engine block between the camshaft cams and the valve stems.
The tappet adjusting screw is adjusted so there is a slight amount of clearance between the valve
stem and the screw head. This adjustment must be made with the tappet resting on the base circle
of the cam.
If the camshaft is turned, the cam lobe will push the tappet up in its bore. In turn, the tappet
contacts the valve stem, compressing the valve spring and opening the valve. As the camshaft
continues to rotate, the cam lobe moves on past the tappet, and the spring forces the valve closed.
The amount the valve opens and how long it stays open for a given engine speed depends on the
shape of the cam lobe.
. We now
have a way
of opening and
closing the valves, but how
is the camshaft
what speed is it turned? What controls the time that the valve opens? Since the crankshaft is always turning
when the engine is running, it seems only logical to use the spinning crankshaft to turn the camshaft. This can
be done by two timing gears. One gear is keyed to the crankshaft and meshes with the second gear which is
keyed to the camshaft.
Camshaft Speed. For reasons that will be explained later when discussing engine operation, the
camshaft must turn half as fast as the crankshaft. To do this, a small timing gear is used on the
crankshaft and a larger gear with twice as many teeth is used on the camshaft. With this
arrangement, turning the crankshaft two turns will rotate the camshaft one turn.