1971 Evinrude 40HP outboards Service Manual, Page 47Get this manual
grinding an angle on the edge, and breaking the lower sharp edge to prevent damage to lower ring landSee Figure 5-17Check piston for size and roundness, uSing micrometerSee Figure 5-18Piston skirt must be perfectly round and not scratchedCorrect sizes are given in Section 2Before installing new piston rings, check gap between ends of ring by placing ring in its respective cylinder bore, then pushing the ring down in the bore slightly with the bottom of the piston to square it upDiscard and replace with new ring if gap is excessiveSee Figure 5-19Check each ring in its respective ring groove for evidence of tightness or binding by rOlling the ring around the piston ring grooveSee Figure 5-20Check for groove side clearance with feeler gageSee Figure 5-21Correct gap and side clearances are given in Section 2Figure 5-19Checking Ring Gap in Cylinder