This adjustment or inspection was a continuous maintenance  item until hydraulic lifters were used.  Some aircraft engines incorporate hydraulic tappets that  automatically keep the valve clearance at zero, eliminating  the necessity for any valve clearance adjustment mechanism.  A typical hydraulic tappet (zero-lash valve lifter) is shown  in  Figure 1-30 . When the engine valve is closed, the face of the tappet body  (cam follower) is on the base circle or back of the cam.  [Figure 1-30]  The light plunger spring lifts the hydraulic  plunger so that its outer end contacts the push rod socket,  exerting a light pressure against it, thus eliminating any  clearance in the valve linkage. As the plunger moves outward,  the ball check valve moves off its seat. Oil from the supply  chamber, which is directly connected with the engine  lubrication system, flows in and fills the pressure chamber. As  the camshaft rotates, the cam pushes the tappet body and the  hydraulic lifter cylinder outward. This action forces the ball  check valve onto its seat; thus, the body of oil trapped in the  pressure chamber acts as a cushion. During the interval when  the engine valve is off its seat, a predetermined leakage occurs  between plunger and cylinder bore, which compensates for  any expansion or contraction in the valve train. Immediately  after the engine valve closes, the amount of oil required to  fill the pressure chamber flows in from the supply chamber,  preparing for another cycle of operation. The function of the tappet assembly is to convert the rotational  movement of the cam lobe into reciprocating motion and to  transmit this motion to the push rod, rocker arm, and then  to the valve tip, opening the valve at the proper time. The  purpose of the tappet spring is to take up the clearance  between the rocker arm and the valve tip to reduce the shock  load when the valve is opened. A hole is drilled through the  tappet to allow engine oil to flow to the hollow push rods to  lubricate the rocker assemblies. Solid lifters or cam followers generally require the valve  clearance to be adjusted manually by adjusting a screw and  lock nut. Valve clearance is needed to assure that the valve  has enough clearance in the valve train to close completely.  Hydraulic valve lifters are normally adjusted at the time of  overhaul. They are assembled dry (no lubrication), clearances  checked, and adjustments are usually made by using push  rods of different lengths. A minimum and maximum valve  clearance is established. Any measurement between these  extremes is acceptable, but approximately half way between  the extremes is desired. Hydraulic valve lifters require less  maintenance, are better lubricated, and operate more quietly  than the screw adjustment type. The push rod, tubular in form, transmits the lifting force from  the valve tappet to the rocker arm. A hardened-steel ball is  pressed over or into each end of the tube. One ball end fits into  the socket of the rocker arm. In some instances, the balls are  on the tappet and rocker arm, and the sockets are on the push  rod. The tubular form is employed because of its lightness and  strength. It permits the engine lubricating oil under pressure  to pass through the hollow rod and the drilled ball ends to  lubricate the ball ends, rocker-arm bearing, and valve-stem  guide. The push rod is enclosed in a tubular housing that  extends from the crankcase to the cylinder head, referred to  as push rod tubes. 1-21 Push Rod Solid Lifters/Tappets Hydraulic Valve Tappets/Lifters Figure 1-29. Cam load on lifter body. Figure 1-28. Cam drive mechanism opposed-type aircraft engine.  Timing gear Crankshaft gear Crankshaft Camshaft lobe Cam gear is twice the  size of the crankshaft  gear and operates at  1/2 speed Camshaft