Re: Mercury 115 hp in line 6
There is no cyl head on this motor. You'll have to remove the powerhead, 'split' the crankcase, and the entire rotating mass (pistons, rods, crankshaft) comes out in one big Lump.
Add to the list of possible causes; overheating, carboned-up rings, too much spark advance, poor quality gas, overpropping the engine.
HTH & G'luck.........ed