TM 10-3930-671-24 Valve Position in Head "A  poor  valve  grinding  job  cannot  be  corrected  by valve lapping." For  example,  after  the  valve  has  been  hand  lapped,  it would look like this when cold.  The thin line visible here shows  where  the  valve  lapped  into  its  seat.    Keep  in mind that the engine is now at rest and cold. This  illustration  shows  the  same  valve  only  now  it  has reached  normal  operating  temperature.    Notice  that  the lapped  area  of  the  valve  and  the  seat  no  longer  match each other.  This is perfectly normal due to expansion of the   valve.      Now   you   can   see   that   the   effects   of   the lapping  job  are  lost  completely.    So  remember...do  not lap valves in after grinding. 5. Coat the valve stem with a light film of engine oil. VALVE SPRINGS 1. Check   all   valve   springs   on   a   spring   tester   to make   sure   they   meet   specifications   regarding weight and length. Valve Spring Tester Springs, when compressed to the "valve open" or “valve closed" length, must fall within the specifications  shown on the Limits and Clearance chart when new, and must not show more than 10% loss to re-use. Valve Assembly F-172