TM 10-3930-669-34 3-6. CYLINDER HEAD REPAIR (CONT). NOTE • Use the following procedure to replace valve seats and valve guides. • Mandrels come in two sizes; one to fit exhaust valve seat and the other to fit intake valve seat. (11)  Place  mandrel  on  valve  seat  (13)  and  insert guide  through  mandrel  into  valve  guide  (15). Tighten guide so that mandrel is firmly in place. CAUTION Do not drill into cylinder head. Damage to equipment may result. (12)  Insert  drill  into  mandrel  and  drill  through  valve seat (13) at two positions 180 degrees apart. (13)  Remove guide and mandrel. (14)  Carefully  remove  and  discard  drilled  seat  (13) from cylinder head (1). (15)  Measure  diameter  of  valve  seat  bore  (16)  in cylinder head (1). NOTE Intake valve seat has a larger outside diameter than exhaust valve seat. (16)  Measure  outside  diameter  of  new  valve  seat (13).      Subtract   diameter   of   bore   (16)   from diameter of  seat.    Difference  must  not  be  less than 0.003 inch (0.076 mm). CAUTION Heating  cylinder  head  for  longer  than  30  minutes  or  at  a  higher  temperature  than 430°F (220° C) may cause head to warp. (17)   Heat  cylinder  head  (1)  in  oven  uniformly  to  a  temperature  of  430°  F  (220°  C).    Do  not  heat  for  more  than  30 minutes. 3-46