TM 10-3930-671-24 Valve Clearance Check 1. Check  and  adjust  valve  clearances  in  the  same sequence as engine firing order. This makes the job  simpler  when  rotating  the  engine  crankshaft to  move  the  valves  into  he  correct  position  for checking clearance. 2. No. 1 cylinder is at fan-end of engine. 3. Check  and  adjust  valve  clearance  with  engine stopped (a) In   sequence   of   firing   order   starting   with No. 1 cylinder, rotate the engine clockwise (looking   at   fan-end)   until   the   respective cylinder piston is at top dead center (TDC) on the compression stroke. (b) When  each  cylinder  piston  is  at  TDC  on compression     stroke, both intake and exhaust  valves  are  closed  and  maximum clearance exists between rocker arms and valve stems. (c) The  engine  crankshaft  can  be  rotated  by using the fan belt or crank pulley to move each cylinder piston to the TDC position. (d) The correct TDC position for each cylinder can be determined by:  a) placing a finger over  the  injector  hole  on  the  compression stroke,   or   b)   checking   clearance   at   the rocker    arms    and    valve    stems    of    the subject cylinder combined with the position of  the  rocker  arms  on  alternate  cylinders (see "Valve Rocking Procedure" below). Valve Rocking Procedure When   No.   1   is   at   TDC,   the   valves   are   ’rocking’   (the period  between  the  opening  of  the  intake  valve  and  the closing  of  the  exhaust  valve)  on  No.  4  cylinder.  No.  4 exhaust  valve  is  nearly  fully  up  (closing)  and  the  intake valve  is  just  starting  its  downward  stroke.  At  this  point, No. 1 intake and exhaust valves and rocker arms should be   in   the   up   (free)   position.      Follow   the   firing   order sequence to determine the "rocking’ position of valves on alternate cylinders from the cylinder being adjusted. 4. Check   the   clearance   between   the   intake   and exhaust valve stems  and  the  rocker  arms  using a flat thickness (feeler) gauge. NOTE It is recommended to use a go/no go feeler gauge. 5. The    valves    are    adjusted    correctly    when    the thickness   gauge   moves   in   the   clearance   gap with a small amount of friction (slight drag). 6. If the thickness gauge moves freely and without a  slight  drag  on  it,  valve  clearance  is  too  great (excessive).     Valve     clearance     adjustment     is required. 7. If  the  feeler  gauge  cannot  be  inserted  between the valve stem and rocker  arm,  valve  clearance is too small. Valve clearance adjustment required. Valve Clearance Adjustment 1. To   adjust   valve   tappet   clearance,   put   feeler gauge  between  rocker  arm  and  valve  stem  and turn adjusting screw until there is a small amount of friction on the gauge while moving it. 2. Tighten  the  locknut  while  holding  the  adjusting screw in position. 3. Check valve clearance again after tightening the locknut. Readjust, as necessary. F-80