question about cylinder wall scratches?

[coletrain777]

So I have a couple of questions, but first I will give you the rundown:

I have a 1965 Merc 390 Z code engine that has 47K original miles on it. It is a Z code engine that has 10:1 compression, so needless to say I really don't want to rebuild it with lower compression cast pistons. It had been sitting in the car unstarted since 1988, but the car turned over when I brought it home. I got if fired and running but it ran rough, so I ran a compression test on every cylinder. All cylinders had between 140 and 155 psi except cylinder 4 (it had less than 20 psi). So I thought it would either be a blown head gasket on that cylinder, or a stuck exhaust valve in the head. So I just went out and pulled the head and everything looks normal except the cylinder walls all have noticible scratches on them. The scratches are very noticeable to the naked eye, and you can feel them when you run your fingertip over them. They are not deep enough to catch your fingernail though. The scratches on the wall of the cylinder with 150+ psi are just as noticeable as the cylinder with less than 20 psi. So here's my questions:

1. I know in an ideal world that any noticeable scratches or scarring on the cylinder wall would warrant a rebore and new pistons, but how deep/noticeable do the scratches have to be in order to really hurt compression and cause oil burning? The reason I ask is I wan't to keep this high compression 390, and I sure don't want to go to the cost of a total rebuild.

2. How much good will a dingle hone do to freshen up a cylinder wall, will it just restore cross hatch? How far can you go with it?

3. It doesn't look like the exhaust valve is stuck open, and the gasket didn't look blown (???), what else could it be?


Sorry for the seemingly dumb questions, I just want some opinions before I do anything else.

[fordman]

you could try to hone it out. who knows. of course you probably know your clearance between the piston ringsd and cylinder wall is the most important thing of keeping it with good compression. so you will have to keep that in mind while honing it and replacing that stuff.

[DuckRyder]

Stuck ring maybe...

[70_F100]

[coletrain777]

Would a stuck ring give that low of a compression reading? It would seem like it should be a little higher than that even with a stuck ring (but i have never had one so what do I know).

[70_F100]

If it's stuck, there's no pressure on the cylinder wall to help the piston build compression.

Have you tried a "wet" compression check?

That should give you a better indicator.

[DuckRyder]

Yes, and if the cylinders all have essentually the same scratches odds are that isn't what is causing the low compression on one cylinder.

[coletrain777]

With the other cylinder walls having in my mind semi-significant scratches but holding over 145 psi on all of them, does this engine sound like a good candidate to at least pull apart, re-ring and maybe bearings, and put back together for a cruiser? I don't want to put money for gaskets and rings into the thing to find out that it is going to be a chain smoker at stoplights!

[coletrain777]

Also, what do you think the compression test numbers would be on a 10:1 engine when it was fairly fresh? Do my numbers of 145 to 155 sounds like it's doing pretty well?

[70_F100]

Those are some pretty good numbers.

What about some pics? We might be able to give you better advice...

[coletrain777]

I can shoot some pics tomorrow but I don't think you will be able to see much. The scratches look like maybe they were from slightly stuck piston/rings from when I fired the engine for the first time in 20 years. They aren't extremely deep, but deeper than I would like to see. I am hoping that I can just toss it back together and they will "polish" away in the next 20k miles