1994 Johnson/Evinrude "ER" 9.9 thru 30 outboards Service Manual, Page 183Get this manual
1431 9Measure the piston diameter in at least two places to verify piston conditionMake your measurements 18 to 1f4 in(3,2 to 6,4 mm) above the bottom edge of the piston skirtThese 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 dimension See the chart