Rocker Arms and Pushrods

Whether  the  causes  of  broken  valve  heads  are mechanical  deformation  or  metal  fatigue,  you  must  take every precaution to prevent their occurrence. If a valve head breaks loose, be sure to make a thorough inspection of all associated parts before you replace the valve. 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. CAM FOLLOWERS AND LASH ADJUSTERS Regardless of the type of cam follower, wear is the most common trouble. Worn rollers will usually develop holes or pit marks in the roller surfaces. The mushroom type  may  develop  a  shallow  channel  when  the  cam Figure 3-16.—Adjusting valve clearance. 3-12


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