1990 Johnson Evinrude "ES" 9.9 thru 30 Service Manual, P/N 507871, Page 169Get this manual
L!!J verify piston conditionMake your measurements Va to
9Measure the piston diameter in at least two places to 1;4 in(3,2 to 6,4 mm) above the bottom edge of the piston skirt
These engines have pistons of several different manufacturersUse the chart below to visually identify the piston you are measuring and select the appropriate dimensions The major diameter ofthe piston is measured 90 from the wrist pin hole centerlineSee the chart The minor diameter of the piston is measured in line with the wrist pin hole centerlineThe minor diameter must be smaller than the major diameter by the cam dimensionSee the chart