1972 Johnson 4HP Outboard Motor Service Manual, Page 41Get this manual
NOTE DO NOT scratch cylinder head gasket surface of cylinderUse surface plate to surface this
PISTONS Check the pistons for roundness, excessive shirt wear, and scoringThe piston skirts must fectly round and unscratched to prevent the entry of exhaust gases into the crankcase chamberCarefi.l11y remove carbon deposits from inside piston headInspect the ring grooves for carbon accumulation, excessive wear, 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 thorough1y removed without scratching otherwise damaging the groovestool for cleaning the ring grooves made from broken ring with sharpened edgemust taken not to damage the ring landsSee Figure 5-14Clean carbon from top of piston, taking not to alter piston deflectorCheck piston for size and roundness, using micrometerCorrect sizes given in Section 2