ROCKER ARMS AND PUSHRODS
ROCKER ARMS AND PUSHRODS
The principal trouble that rocker arms and pushrods may have is WEAR, which may occur in bushings, or on the pads, end fittings, or tappet adjusting screws. Worn rocker arm bushings are usually caused by lubricating oil problems. A bushing with excessive wear must be replaced. When installing a new bushing, you usually need to use a reamer for the final fit.
Wear at the points of contact on a rocker arm is generally in the form of pitted, deformed, or scored surfaces. Wear on the rocker arm pads and end fittings is greatly accelerated if lubrication is insufficient or if there is excessive tappet clearance. Pushrods are usually positioned to the cam followers and rocker arms by end fittings. The pads are the rocker arm ends that bear the valve stem or valve stem cap. When the tappet clearance is excessive, the rods shift around, greatly increasing the rate of wear of both the rocker arm and the rod contact surfaces. Worn fittings necessitate the replacement of parts. Continued use of a poor fitting and worn pushrod is likely to result in further damage to the engine, especially if the rod should come loose.
Worn tappet adjusting screws and locknuts usually make maintaining proper clearances and keeping the locknuts tight very difficult. Wear of the adjusting screws is usually caused by loose locknuts, which allow the adjusting screw to work up and down on the threads each time the valve is opened and closed. To prevent this wear, tighten the locknuts after each adjustment and check the tightness at frequent intervals. If the threads are worn, replace the entire rocker arm. Do NOT attempt to repair the threads or to use a new tappet adjusting screw except in cases of emergency.
The adjustment of the rocker arm assembly consists chiefly of adjusting the tappets for proper running clearance. The valve clearance for both intake and exhaust valves should be readjusted after overhaul. The procedure for adjusting the rocker arm tappets of a typical 4-stroke cycle engine is as follows:
1. Rotate the crankshaft and move the piston whose tappets you plan to adjust to top dead center of the compression stroke.
2. Loosen the locknut (jam nut) on the tappet screw, and insert a screwdriver in the slot of the screw.
3. Insert a feeler gauge of the proper thickness between the tappet bearing and the end of the valve stem.
4. Tighten the tappet screw (fig. 3-16) until the feeler gauge will just slide freely between the bearing and the valve stem.
5. lighten the jam nut and check the clearance. The jam nut has a tendency to increase the clearance when tightened; therefore, ALWAYS check the clearance after you tighten the jam nut.
The procedure just outlined is a preliminary, or cold engine check. Check and readjust the clearance, if necessary, after the engine has been in operation for a short time and has reached the normal operating temperature. The manufacturer’s technical manual will give the recommended valve clearances for a specific make and model of engine and will indicate whether the clearances given apply to cold or hot engines.
