TM  10-3930-638-24&P 4-2.   ENGINE   MAINTENANCE   (cont) b.  Cylinder  Head  and  Valves  (cont). STEP LOCATION ITEM ACTION REMARKS CLEANING  (cont) WARNING Wear safety glasses when drying parts with compressed air. Fail- ure to do so could cause serious injury to eyes and possible blind- ness. If you hurt your eyes or if a foreign object is blown into your eyes, seek medical attention immediately. 4 Cylinder head a. Overall Clean Use  cleaning  solvent  P-D-680.  Dry (8) with  moisture  free  compressed  air or clean cloth b. Valve ports Clean Use fine rotary wire brush c. Machined surface Clean Use medium grit emery cloth carefully areas to remove all gasket material, carbon 5 and  rust All other parts Clean Use cleaning solvent P-D-680. Dry with  moisture  free  compressed  air INSPECTION  AND  REPAIR 6 Valve  spring  (3) Inspect Install in spring tester. Check that force to compress spring to 1.521 inch is 110 to 118 pounds; force to compress spring to 1.875 inch is 53 to 59 pounds. Replace spring if force is less than these values 7 Cylinder  head a.  Bottom  surface Check Lay  straight  edge  diagonally  from  corner (8) warpage to corner and try to get a 0.006 inch feeler gage under straight edge. Check at several  points.  Reposition  straight  edge at  opposite  corners  and  repeat  checks. If warpage is greater than 0.006 inch resurface  or  replace  cylinder  head NOTE If  cylinder  head  is  resurfaced,  maintain  a  minimum  thickness  of 3.968 inches. b. Valve guides Check inside Use bore gage and check in three places (9 and 10) diameter as shown below. Measure bore gage with micrometer. If greater than 0.3449 inch at any point replace valve guides 4-11