1985 mercurial 3.0 130hp piston

[brian3127]

Engine was knocking last time out. So me and my son pulled the engine out of the boat. Started to tear it down to day.
We have owned this boat for 2 years
This is piston #3
Question.
what would cause this?
Engine had plenty of oil. Oil pressure on the gauge was above 40lbs.
water temp was at about 170 to 180

piston 2 part of the skirt broke off

 

[Scott Danforth]

that is what happens when you have detonation from improper timing and or running lean. that rattle you were hearing was your motor eating itself.

additionally your temp is way too high 140F for a carbureted motor and raw water cooling, 160F for a carbureted motor and heat exchanged cooling.

 

[brian3127]

that is what happens when you have detonation from improper timing and or running lean. that rattle you were hearing was your motor eating itself.

additionally your temp is way too high 140F for a carbureted motor and raw water cooling, 160F for a carbureted motor and heat exchanged cooling.

Thanks scott
We are rebuilding it. So we are putting all new parts. Especially a new thermostat and water pump. Surprisingly the cylinder where that piston was is actually smooth.