Mercury Force 9.9, 15HP Outboards Service Manual, Page 130Get this manual
1Check cylinder head gasket surface for signs of water channeling NOTE: Water channeling is an erosion of the cylinder head material at the gasket seal areaIt can occur when an outboard motor has been run with loose or blown head gasket
2Check flatness of cylinder head Lay cyli nder head on piece of plate glassMeasure at different poi nts between cyli nder head and glass with fee ler gaugeIf cyli nder head is warped 0.012 or less, it shou ld be resurfaced If warped more than 0.012, replace