TM 10-3930-671-24 Grinding Valve Seat Indicating Valve Seat VALVES 1. Inspect valves for condition and replace any that are   "necked",   cracked   or   burned,   also   any   of which  valve  stems  are  bent  or  worn  more  than .002"    0.05mm    over    the    maximum    allowable limits.  Reface or replace all valves. IF   AREA   "A"   IS   LESS   THAN   50%   OF   ORIGINAL, DISCARD VALVE Allowable Head Thickness of Refaced Valves 2. All valves having less than 50% margin thickness    (outer    edge    of    valve    head)    after refacing has been completed must be  replaced. To  check  this  dimension,  compare  the  refaced valve with a new valve. Checking Valve Face in "V" Blocks 3. Check all refaced or new valves in V-blocks with indicator  to  determine  if  the  contact  face  is  true with   the   stem   within   .002"   (0.05mm).      If   not, repeat the refacing operation. 4. After  the  valves  and  seats  have  been  refaced and reground, coat the seat lightly with Prussian blue and drop the valve into position, oscillating it slightly  to  transfer  the  blue  pattern  to  the  valve face.  This should show a contact width of 1/16’ to   3/32"   (1.62   to   3.2mm)   and   should   fall   well within the width of the valve face, leaving at least 1/64"   (0.4mm)   on   either   side   where   the   blue does   not   show.      If   the   contact   is   over   3/32" (3.2mm)   wide,   the   seat   in   the   head   may   be narrowed  by  using  a  150  stone  to  reduce  the outside diameter  or  using  a  6(0  or  750  stone  to increase the inside diameter. Checking for Proper Valve Seating Never allow valves to set down Inside the seat. After the narrowed-down seat is brought within specifications,  the  seat  should  be  retouched  lightly  with the original stone to remove burrs or feathered edge. F-171