85 hp force compression trouble

[daveski1964]

I have 120lbs compression in top two cylinders and only 95 in bottom cylinder,,my question is this mean I need a total rebuild or could I fix the bottom cylinder by ring job etc?????,please help

 

[pnwboat]

There are a couple of causes for low compression. Bad head gasket, bad/broken rings, rings seized in the ring groove from carbon build up. To eliminate the carbon build up possibility, you could try pouring some Mercury Power Tune into the bottom cylinder through the spark plug hole and let it sit overnight and see if that helps. Next step would be to replace the head gasket. Assuming that you have to re-build it without determining what the cause for the low compression may not be the best choice.

Also try another compression gauge just to make sure you are getting accurate readings. I have and old mechanical gauge made by a well known reputable manufacturer that I used to use for 20 years or so. A few years ago I bought a calibrated electronic gauge. Found out my old mechanical gauge was reading about 20 - 30 lbs low.

 

[Frank Acampora]

It is possible that the head gasket is bad. It is also possible that the bottom piston has broken the top ring. Remove the head and look carefully at the head gasket. Then, examine the top of the piston to see if there is any obvious damage from a broken ring. If the piston is damaged, you will need to replace it and the rings. If the cylinder is not scored, there will be no need to rebuild the engine.


These engines are remarkably tolerant as far as repair. It is possible to overbore only one cylinder and replace only one piston. The engine won't even know it.

 

[daveski1964]

yes MR Frank you were right on. .I took head off and you could see about 1/8 inch of top ring broken. I would call the cylinder having a very very littile scratch .running finger nail across will not catch ,just can tell it there.. how would you go forward?.can I just rebuild that cylinder hone new piston new rings?

 

[Jiggz]

Yes, as mentioned by Frank, use the Wiseco pistons but before ordering you need to verify the old piston's size or even better hone the cylinder and then measure it to find out if it is still within the standard piston measurement or already over bored. A stamped "1" on the piston crown indicates it is a standard piston size. I'm surmising since the cylinder was not really that roughed out, that even with slight honing (basically de-glazing) and the old piston is still standard, you can still use the standard piston size. With Wiseco, you will need to order their wrist bearing kit. if you have not done this before, I recommend you search the forum for piston replacement for procedures.

 

[daveski1964]

I just want to say thanks to all you experience boaters.we would be lost without you guys...THANKS AGAIN