1998 Johnson Evinrude "EC" 90, 115 SPL Service Repair Manual, P/N 520209, Page 146Get this manual
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10 Use micrometer to check piston conditionMake at least two measurements, 90 from each otherMeasure the piston diameter 1J4 in(6,4 mm) above the bottom edgeRefer to Engine Specifications, Section 1,