1977 Evinrude 4HP Outboards Service Manual, PN 5303, Page 50Get this manual
C] NOTE DO NOT scratch cylinder head gasket surface of cylinderUse surface plate to surface this area
PISTONS Check the pistons for roundness, excessive shirt wear, and scoringThe piston skirts must be perfectly round and unscratched to prevent the entry of exhaust gases into the crankcase chamberCarefully remove carbon depOSits from inside piston headInspect the ring grooves for carbon accumulation, excessive wearor damage to the ring groovesCarefully scrape carbon from the ring grooves, making certain that carbon clinging to the bottom and Sides of the grooves has been thoroughly removed without scratching or otherwise damaging the groovesA tool for cleaning the ring grooves can be made from broken ring with sharpened edgeCare must be taken not to damage the ring landsSee Figure 5-14Clean carbon from top of piston, taking care not to alter piston deflectorCheck piston for size and roundnessusing micrometerCorrect sizes are given in Section 2If out of round more than 0.0025" (0.064 mm), replace piston