ARMY TM 9-2815-256-24 AIR FORCE TO 38G1-96-2 MARINE CORPS TM 2815-24/5 (5) Combustion  face  surface  finish  (surface  grind  only)  should  be  75AA  minimum  to  130AA  maximum. Maximum wave depth should be 0.0005 inch (0.012 mm). j. Clean and measure valve guides as follows: (1) Use a plastic brush to clean valve guides before inspection or repair. NOTE A few drops of light oil or kerosene will help to fully clean the guide. (2) Using a telescopic gage, measure valve guides for wear. (3)  ID  of  valve  guide  bore  in  a  new  cylinder head is 0.312 to 0.313 inch (7.912 to 7.938 mm). (4) New guide-to-valve stem clearance is 0.002 to 0.004 inch (0.05 to 0.10 mm). (5) Maximum permissible clearance is 0.006 inch (0.015 mm). (6) If valve guide oil clearance is more than maximum allowable, but not more than 0.008 inch (0.20 mm), valve guides can be knurled.  Refer to step k. (7) If valve guide oil clearance is over 0.008 inch (0.20 mm), intake and exhaust valves with oversize stems are available.  Valve guides can be knurled and then reamed to fit oversize valve stems.  Oversize valve stems are available for service in 0.015 inch (0.38 mm) or 0.030 inch (0.76 mm) sizes. k. Knurl, thread, and ream valve guides as follows: CAUTION Always  knurl  valve  guides  before  threading.    Hand  ream  after  threading  to  assure  proper  valve guide-to-stem clearance. (1) Use knurling tool set to knurl valve guides.  Use tool set exactly as directed by the manufacturer. (2) After  knurling,  thread  full  circumference  of  valve  guides  for  entire  length  of  bore  using  a  5/16-24NF modified internal thread tap with a major OD of 0.317 to 0.320 inch (8.05 to 8.09 mm) (3) After threading, hand ream valve guide to finished size to provide specified stem-to-guide clearance.  A 0.3105 inch (7.887 mm) OD pin must pass freely through entire length of bore after reaming. I. Clean and inspect valve seats as follows: (1) Use an electric hand drill with end brush to remove all carbon on valve seats. (2) Check seats for cracks, pits, or excessive wear. (3) Check entire combustion face for rust, scoring, pitting, or cracks. CAUTION Grind  valve  seats  to  obtain  correct  valve  recess  and  valve  head-to-seat  contact  angle.    Refer  to paragraph 3-34.2.  for recess inspection.  Be sure valve guide bores are clean and serviceable (refer to step j) before grinding valve seats to maintain valve guide bore-to-seat runout.  Always maintain sure  footing  and  a  balanced  position.    Give  the  work  your  undivided  attention,  otherwise  engine performance will be poor. NOTE Always keep work area clean when grinding valve seats. m. Using valve seat grinding set, grind valve seats as follows: 3-152