Hey there I'm was givin a 16ft fiberform boat with a 115 hp merc on it... I was told that it's a 1971 but can't confirm thy yet. The engine seems to run fine... Rough idle but revs up fine. I pulled the top plug on this inline 6 cyl. And there is a small plate of metal covering the inside of the spark plug hole. It is not the top of the piston the cylinder has compression but very little because of the metal piece... The othe five cyl below dint not have this. I'm stumped and the only I an think of is that it is there to create crankcase pressure... Any help would be appreciated thanks. I'll post a picture later on