Whether the causes of broken valve heads aremechanical deformation or metal fatigue, you must takeevery precaution to prevent their occurrence. If a valvehead breaks loose, be sure to make a thorough inspectionof all associated parts before you replace the valve.ROCKER ARMS AND PUSHRODSThe principal trouble that rocker arms and pushrodsmay have is WEAR, which may occur in bushings, oron the pads, end fittings, or tappet adjusting screws.Worn rocker arm bushings are usually caused bylubricating oil problems. A bushing with excessive wearmust be replaced. When installing a new bushing, youusually need to use a reamer for the final fit.Wear at the points of contact on a rocker arm isgenerally in the form of pitted, deformed, or scoredsurfaces. Wear on the rocker arm pads and end fittingsis greatly accelerated if lubrication is insufficient or ifthere is excessive tappet clearance. Pushrods are usuallypositioned to the cam followers and rocker arms by endfittings. The pads are the rocker arm ends that bear thevalve stem or valve stem cap. When the tappet clearanceis excessive, the rods shift around, greatly increasing therate of wear of both the rocker arm and the rod contactsurfaces. Worn fittings necessitate the replacement ofparts. Continued use of a poor fitting and worn pushrodis likely to result in further damage to the engine,especially if the rod should come loose.Worn tappet adjusting screws and locknuts usuallymake maintaining proper clearances and keeping thelocknuts tight very difficult. Wear of the adjustingscrews is usually caused by loose locknuts, which allowthe adjusting screw to work up and down on the threadseach time the valve is opened and closed. To prevent thiswear, tighten the locknuts after each adjustment andcheck the tightness at frequent intervals.If the threads are worn, replace the entire rockerarm. Do NOT attempt to repair the threads or to use anew tappet adjusting screw except in cases ofemergency.The adjustment of the rocker arm assembly consistschiefly of adjusting the tappets for proper runningclearance. The valve clearance for both intake andexhaust valves should be readjusted after overhaul. Theprocedure for adjusting the rocker arm tappets of atypical 4-stroke cycle engine is as follows:1. Rotate the crankshaft and move the piston whosetappets you plan to adjust to top dead center of thecompression stroke.2. Loosen the locknut (jam nut) on the tappetscrew, and insert a screwdriver in the slot of the screw.3. Insert a feeler gauge of the proper thicknessbetween the tappet bearing and the end of the valve stem.4. Tighten the tappet screw (fig. 3-16) until thefeeler gauge will just slide freely between the bearingand the valve stem.5. lighten the jam nut and check the clearance. Thejam nut has a tendency to increase the clearance whentightened; therefore, ALWAYS check the clearance afteryou tighten the jam nut.The procedure just outlined is a preliminary, or coldengine check. Check and readjust the clearance, ifnecessary, after the engine has been in operation for ashort time and has reached the normal operatingtemperature. The manufacturer’s technical manual willgive the recommended valve clearances for a specificmake and model of engine and will indicate whether theclearances given apply to cold or hot engines.CAM FOLLOWERS AND LASHADJUSTERSRegardless of the type of cam follower, wear is themost common trouble. Worn rollers will usually developholes or pit marks in the roller surfaces. The mushroomtype may develop a shallow channel when the camFigure 3-16.—Adjusting valve clearance.3-12
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