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ARMY TM 9-2815-255-24 AIR FORCE TO 38G1-95-2 MARINE CORPS TM 2815-24/4 (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  5/1624NF 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. 3-143


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