1990 Johnson Evinrude "ES" 9.9 thru 30 Service Manual, P/N 507871, Page 145Get this manual
6Check 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 the difference between the two measurements is greater than 0.002 in(0,05 mm), the piston is out of round or worn and must be replaced7Install piston rings on each piston Spread each ring with ring expander just enough to slip it over the head of the piston and down into place8Coat the pistons and cylinder bores with oil f311
9Place the piston in OMC Ring Compressor PI 326590 (for standard size piston)Make certain that the rings are correctly positioned in their grooves with respect to the dowel pinsDamaged pistons and broken rings may result from imperfect alignment of the ring gap and the dowel pins
I pinS piston rings must be correctly located on their dowel to prevent permanent powerhead damage.of