80HP Mercury Low Compression, Burned Piston Rings

[Ranger118]

S/N: 7134777

120 PSI on cylinder #1, #2, #4
only 10 PSI on cylinder #3

After open the powerhead, we can see #3 piston burned and the rings stuck on it, part of the ring missed.

DSC07517s.jpg

[Ranger118]

It's hard to remove the rings from piston.
It looks like they melt and stuck together.

What should we do next?
Should we replace the whole thing or the rings only?
How to to do a piston groove reshaping?

[jimmbo]

That piston is toast. Looking at it, I will assume the cylinder is scored. Have that cylinder bored oversize and get a piston, deglaze the others and re-ring those pistons. Don't forget to follow the break in proceedure

[Ranger118]

Thanks for the advice.

Can we use this to replace the 6664A3 piston?
http://www.iboats.com/2-Ring-Standar...view_id.173992

[CharlieB]

Clean that carb!!!!!!!!!!!!!! Or you will melt the next piston also.

Anytime you freshen a motor, it is wise to kit all carbs, fuel pump, and water pump. Together, you are reducing the probability of things could go wrong and quickly destroy all that you have carefully rebuilt.

[orbanp]

You did not say what is the condition of the cylinder, but chances are that it needs a rebore. In that case, as jimmbo mentioned, you would need an oversize piston, and the link shows a standard size piston.
Take your block to a machine shop who does two-stroke work, and they would be able to say if the cylinder can be saved.

Peter