Mercury Force 9.9, 15HP Outboards Service Manual, Page 131Get this manual

4 Measure the cyli nder bores with inside micrometer or dial indicator Measure 14" (6.35 mm) down from top Measure 14" (6.35 mm) up from exhaust portMeasure 316" (4.76 mm) down from intake port
DIAL INDICATOR
INSIDE MICROMETER
'Take measurements again at 90 degrees from first set
NOTE: The taper, or difference between the largest and smallest measurement, must not be more than 0.002" (0.05 mm)

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