Were rebuilding a merc 300, I don't know the year around 50-60s maybe. The pistons we decided have made a new shape on the top. Its bowed down making a divot on the top of all 4 pistons, were thinking get the welder and weld a gob on top and put it in the lathe and make it flat again. Does anyone have pistons or any ideas? What should the ring gap be?