Rocker arms (levers) are part of the valve-actuating mechanism. A rocker arm is designed to pivot on a pivot pin or shaft that is secured to a bracket. The bracket is mounted on the cylinder head. One end of a rocker arm is in contact with the top of the valve stem, and the other end is actuated by the camshaft. In some installations where the camshaft is located near the cylinder head, the rocker arm may be actuated from the cam by the use of cam followers. Figure 4-7 illustrates a design of this type in which a cylinder head of the GM 16-278A is fitted with three rocker arms or levers. (Fig. 4-7 is a cutaway view with one rocker arm shown.) The two outer arms operate the exhaust valves, and the inner arm operates the fuel injector. Since there are four exhaust valves per cylinder, each exhaust rocker arm must operate a pair of valves through a valve bridge. The valve bridge enables the rocker arm to operate two valves simultaneously. The valve bridge in this engine is made of forged steel and has a hardened ball socket into which the ball end of the rocker arm adjusting screw fits. The valve bridge has two arms, each of which fits over an exhaust valve. The valve bridge spring keeps valve bridge tension off the valve stems until the bridge is actuated by the rocker arm. When the valve end

Figure 4-7.Valve gear (General Motors 16-278A).

of the rocker arm is forced down by the cam action, the valve bridge moves down, compressing the valve springs and opening the valves. By the time the action of the cam lobe has ceased, the valve springs will have closed the valves. The valve operating mechanism shown in figure 4-7 is representative of those in which the location of the camshaft eliminates the need for pushrods. (Note that the lobes of the cam come in direct contact with the rocker arm cam rollers.)

In installations where the camshaft is located below the cylinder head, the rocker arms are actuated by pushrods. (See fig. 4-8.) The lifters (cam followers) have rollers which are forced by the valve springs to follow the profiles of the cams. The pushrod transmits the motion from the

Figure 4-8.Valve rocker arm operation (ALCO).

roller type of lifter for intake and exhaust valve operation and are activated by their respec-tive intake and exhaust lobes of the camshaft. The intake and exhaust valves in figure 4-8 are operated by mechanisms similar to those represented in figure 4-7 with two exceptions.

The unit in figure 4-8 has no design re-quirement for an injector rocker arm because of the type of fuel system used. Also, the unit in figure 4-8 is a 4-stroke cycle unit and requires both an intake valve rocker arm and an exhaust valve rocker arm. (You should note, however, that pushrods can be used in both 2- and 4-stroke cycle en-gines.) Other parts that transfer the rotary motion of the crankshaft to operate the valve-actuating mechanism as well as accessories are referred to as ENGINE DRIVE MECHANISMS. These parts will be discussed in chapter 5 of this rate training manual.