TM 10-3930-671-24 This  may  be  checked  on  a  fixture  with  the  piston  pin assembled  in  the  rod  before  assembling  the  piston;  but regardless of this preliminary check the completed piston and rod assembly must be rechecked and there must not be more than .002" (0.05mm) twist or out of squareness checked over a spread of approximately 4" (100mm).  If excessive,  replace  the  rod.    Pistons  are  cam  and  taper ground, and this must be taken  into  consideration  when checking alignment of the assembly,  since  the  diameter in line with the piston pin would be less at the top of the skirt than at the bottom. PISTON RINGS Check the piston rings in the cylinder for gap. Checking Ring Gap To  do  this,   insert   a   piston   in   the   cylinder   bore   in   an inverted position and then insert each ring one at a time about 2" (50mm) down in the bore.  If the ring does not have  sufficient  end  gap  clearance,  file  one  end  of  the ring until sufficient clearance is obtained.  Check the gap between   the   ends   of   the   ring   with   a   feeler   gauge   in accordance  with  specifications  shown  in  the  Limits  and Clearance   Chart.      Prior   to   installing   the   rings   on   the piston,  check  each  ring  in  its  respective  groove  to  the limits  specified  in  the  manual.    If  ring-to-land  clearance exceeds  maximum  serviceable  limits,  pistons  must  be replaced. Checking Ring Clearance in Groove RECOMMENDED  METHOD  OF  INSTALLING  PISTON RINGS 1. Prior to ring assembly check the ring grooves for nicks  and  burrs.    This  is  done  by  rotating  each unassembled  ring  around  its  groove  to  be  sure of free action. Installing Rings with Ring Expander Tool Oil Ring 2. The oil ring should be installed first on the piston, from the top side so skirt will not be scratched. F-177