1990 Johnson Evinrude "ES" 40 thru 55, P/N 507872, Page 145Get this manual
If any cylinder is outside these tolerances, it must be bored oversizeA 0.030 in(0,762 mm) oversize piston is available for this engineTo determine oversize bore add 0.030 in(0,762 mm) to the standard boreRefer to Engine Specifications, Section 1It is permissible to have one or more oversize pistons in an engine E.J in(3,2 mm) above the bottom edge of the piston skirtThe micrometer should be in line with the upper piston ring dowel pin The major diameter should be no smaller than 3.1831 in(80,85 mm) for standard piston and 3.2131 in(81,61 mm) for an oversize piston
1sl 9Use micrometer to measure the piston major diameter Vs
10Measure the piston in several more locations lS in(3,2 mm) above the bottom edge of the piston skirtNo measurement should be larger than the major diameter and no measurement should be more than 0.004 in(0,10 mm) smaller than the major diameter
f the grooves on the sides of the piston are worn smooth in an area similar to that shown, replace the piston71 11Place each ring, separately, in its respective boreUse piston to square the ring in the cylinderUse feeler gauge to measure the ring end gap The ring gap (0,17-0,43 mm)must be within 0.007-0.017 in