1985 Mercury Outboard V-300 V-3.4L Shop Service Manual, Page 124Get this manual
POWERHEAD REASSEMBLY Before nSlal ling new piston ringscheck gap bet weell ring ends by placi ng each ring in it respective cylinderthen pushing ring abo ut 0;" (13m m) in to cyli nder usi ng piston to assu re proper posit io n
Check end gap of each pi ston ring with feeler gauge (a), End gap must he wi thin .0 18" 10 .025" (O.45mm toO.64rnm )If end gap is grea ter hec ther nc ' piston rings in cylinder bore until ri ng! (wit hi tolerance) arc found If end gap is less than tolerancea nd another ring wit hill tolerance cannot be foundil rna)' be necessa ry 10 fi le so me material from piston ri ng end (at gap)I it is necessa ry to file piston ri ng, be sure that all rough edges re removed after filing