The direct mechanical lift air timing mechanism includes cams, pushrods, and rocker arms. These parts are subject to the same failures as engine cams, pushrods, and rocker arms. Therefore, you can find the causes of trouble in the actuating gear and the necessary maintenance procedures under information covering similar engine parts.
Most troubles are a result of improper adjustment.
Generally, this involves the lift of the starting air cam or the timing of the air starting valve. The starting air cam must lift the air starting valve enough to give a proper clearance between the cam and the cam valve follower when the engine is running. If there is not enough clearance between these two parts, hot gases will flow between the valve and the valve seat, overheating them. Since the starting air cam regulates the opening of the air starting valve, check those with adjustable cam lobes frequently to ensure that the adjusting screws are tight. Obtain the proper values for lift, tappet clearance, and time of valve opening for a direct mechanical lift timing mechanism from the manufacturer s technical manual for the particular engine.