A camshaft is a shaft with eccentric projec-tions called cams. The camshaft of an engine is designed to control the operation of the valves and fuel injection pump usually through various intermediate parts. On some engines, a balance shaft is used to counterbalance the rotation of the weighted camshaft and to stabilize the oscillatory impulses developed within the engine. (See fig. 4-3 for the location of the balance shaft and weights in the General Motors 6-71.)

The camshaft may be constructed in several ways. It can be forged in one piece (fig. 4-3) in which the cams themselves are integral to the shaft. (This is the most common design in small to medium engines.) A camshaft of a large engine may consist of a shaft with separate forged steel or cast iron cams keyed and shrunk on the camshaft. Another construction used on larger engines is a camshaft that is made from sections which are bolted together. Some engines have two camshafts and others have only one, depending on the design of the engine.

To reduce wear and to withstand repeated shock action, camshafts are made of low-carbon alloy steel with the cam and journal surfaces carburized (case-hardened) before the final grinding is done.

The cams are arranged on the shaft so that the proper firing order of the cylinders served can take place. If one cylinder is properly timed, the remaining cylinders are automatically in time. All cylinders will be affected if there is a change in timing. The shape of the cam determines the point of opening and closing, the speed of opening and closing, and the amount of the valve lift.

The camshaft in a 4-stroke cycle diesel engine carries the cams for actuating the intake and exhaust valves. In addition, the camshaft may carry cams for fuel injection equipment or air starting valves. In a 2-stroke cycle diesel engine there is no requirement for an intake cam due to the use of intake ports.

Figure 4-3.Cam and balancer shaft assemblies (General Motors 6-71).

Figure 4-4.Various camshaft locations.

The location of the camshaft differs in various engines. The camshaft may be located low (near the crankshaft) and may use long pushrods (fig. 4-1), or the camshaft may be located at the cylinder head level without pushrods (fig. 4-2). Variations of camshaft location are shown in figure 4-4.