1989 Johnson Evinrude "CE" 9.9 thru 30 Service Repair Manual, P/N 507754, Page 147Get this manual
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6 Check piston with micrometer to determine if piston has been distorted during assemblyMeasure the piston at two places, 90 from each other, Va to in(3,2 to 6,4 mm) above bottom edge of the skirtIf t