Mercury Mariner Outboards 2.2 / 2.5 / 3.0 Service Shop Manual, Page 44Get this manual
DETERMINING PISTON CLEARANCE 1Measure the outside diameter of the piston at the piston skirt and at right angle to the piston pin Obtain maximum piston diameter2Measure cylinder bore and obtain minimum bore diameter3Subtract the maximum piston measurement from the minimum cylinder bore measurementThis equals your piston clearance Minimum bore measurementMaximum piston measurement
= Piston clearance
Piston Clearance .002 into .005 in (.06mm to .15mm) If piston clearance exceeds the specification, replace cylinder block or replace piston so that clearance is within the specification Measure side clearance of piston ring grooves as fol lows: Use feeler gauge and check clearance between piston ring and groove as shownClearance should be within specification Clearance between piston ring and ring groove .0003 in to .0010 in (.01 mm to .05mm)