One bad compression cylinder does not a complete rebuild make?

[MTBoat]

On my 91 3.0 mercuiser with not great compression on all, but only 30lbs on number two.

We did a wet test and it only went up a couple lbs.

We pulled the valve cover and the valves seem seated ok, but, we'll get a tool to push em out and see how they seat.
We saw some small rust/pitting on the inside 40% of the circle of the very upper 1/2 inch of the cylander wall, on the problem cylinder.

Is that usually enough to actually drop the compression 70%?

We are just trying to imagine if there are ways to get a decent engine up and running without a full blown $1500 overhaul. Meaning, what if we do all our own labor, and hone the cylinder walls, put new rings on all, and do a valve job and new head gasket after throughout cleaning. Is it likely we might be all cylinders back to 130lbs or so, vs a brand new 145plus.
The point being, the boat is older and would be fine with just a decent running engine for the next 5-10 years for possibly a few hundred bucks and some sweat, and enjoy the boat.

Appreciate anyone that has past experiences that might shed some light on such concepts, I've seen it done with trucks with some good results, but, always a low cost gamble.

thanks much

 

[tpenfield]

Re: One bad compression cylinder does not a complete rebuild make?

Do a cylinder leak test and it will indicate where the cylinder is letting air out. Then you will have a better idea of what needs fixing before you start loosening bolts, etc.

 

[Fishermark]

Re: One bad compression cylinder does not a complete rebuild make?

We are just trying to imagine if there are ways to get a decent engine up and running without a full blown $1500 overhaul.

Like mentioned, do a leak down test and that will tell you a lot.

Even if you ended up boring it out and putting new pistons, etc you could do it for well under $1,500.

 

[ajax5240]

Re: One bad compression cylinder does not a complete rebuild make?

What is the trick for turning the engine on these 3.0's with that engine mount in the way and no hex head bolt on the balanced? You need to he at TDC for the leakage test, ingot it into position with the alternator nut and belt, but it would slip when I apply pressure to the cylinder.

Any tricks or do you need the special tool that would hold the balancer for removal?

Strap wrench?

 

[ajax5240]

Re: One bad compression cylinder does not a complete rebuild make?

And to the OP, even a dead cylinder will make 30psi unless it has a Hole in the piston or a really bent valve.

I would wager a guess that it's time to get the engine hoist out.

It takes water to make rust, and you don't get that much from the HC+o2+CO2 reaction to cause cylinder rust. I would bet here has been water in the oil before, and it lived there for a while and you probably have damaged rings from running on that kind of surface.

 

[Alumarine]

Re: One bad compression cylinder does not a complete rebuild make?

What is the trick for turning the engine on these 3.0's with that engine mount in the way and no hex head bolt on the balanced? You need to he at TDC for the leakage test, ingot it into position with the alternator nut and belt, but it would slip when I apply pressure to the cylinder.

Any tricks or do you need the special tool that would hold the balancer for removal?

Strap wrench?

I put tension on the drive belt and turn the alternator.