OMC Big Twin Cylinder Bores

[chris-p]

A fellow member brought over a 1956 Johnson 30hp for me to go over for him. He wanted to see if it was worth putting some money into.

Compression test revealed that the top cylinder had low compression, about 80, the bottom was good, at about 105.

So I pulled everything apart to further diagnose.

The head, rear pan, exhaust covers, and bypass covers came off WAY too easy, so Im assuming a previous owner already went through this. Got a bad feeling when these exhaust bolts are finger tight!

Actually, everything looked pretty good. Actually, the TOP cylinder looked better than the bottom. The top "looked" perfect. The bottom had a little scoring in the bore, and the piston skirts had been overheated and were scored, even though they were the better cylinder. Only 1 ring on the bottom cylinder was partially stuck where it was scored.

Head gasket didn’t have any tears or rips.

Head is pretty true.

So measured the bores.

The bore should be 2.8750" in diameter.
The TOP bore is anywhere from 2.8760 to 2.8850
The BOTTOM bore is anywhere from 2.8750 to 2.8760

So that top bore is definitely more worn, the one with low compression. Is there any tricks here, like installing a bigger ring on it that may eat up that bit more wear? Or is boring it out the only option here? I like to save what I can, but he wont want to pay me to have to rebuild it that far.

Any thoughts on this?

Moral of the story is, don’t always believe what looks better, is better!

[frankr]

So what do you plan to do about the .010" taper? If you fit the rings to the big end they will be too tight at the other end. I suppose with a lot of work and patience you could straighten it up a bit.

I guess it is your call. Personally I’d hone it real lightly and slap some new standard rings in it, and replace that scored piston and run it.

[Mumbles]

While measuring the bore, turn your bore gauge 90 degrees and measure at both top and bottom of the cylinder where the rings travel. The rings, not the pistons, are what wears the bore and comparing the measurements at 90 degrees to each other will tell you how much the cylinder is out of round plus give you the amount of taper. New rings in a used cylinder must be fitted at the tightest place, which is usually at the bottom of their travel. It is generally agreed that you can use +0.010 oversize rings in a given bore if fitted properly. In other words, you can use +0.030 OS rings in a cylinder which has been bored 0.020 OS. It might take a bit longer for them to seat in but it can be done.

[fleetwin]

I’m confused, looks like the cylinder is only off by .001" according to the specs/measurements you posted….But Frank mentions a .010" taper, am I missing something here? .001" oversize is surely nothing to worry about, but .010" oversize is a big problem….
If the .010" taper/oversize is in fact the case, then I am guessing somebody really went to town on that cylinder with a coarse rigid hone.

[frankr]

He sez top bore is as much as 2.885. Subtract standard bore of 2.875 and you get .010.

[fleetwin]

OK, I see that measurement now…
.010" is quite a bit out of round…But, he mentions this is at the top of the cylinder. I’m wondering if this measurement is actually in the ring travel area, or just above where combustion takes place. I wouldn’t worry about it too much if this out of round condition is only above the area of ring travel. The rest of the cylinder seems only off by .001" which is very little wear. I guess this is a case where a quick/light hone job with a nice rigid hone might help show just where the actual wear is….
I guess I would re-measure, and perhaps check the ring end gaps using the old rings. I’m with Frank, new standard rings and a quick hone job is probably all that is needed unless you find more excessive out of round/oversize conditions in the area of ring travel….

[chris-p]

No these measurements are in the ring travel area. I rotated the bore gauge in about 8 different spots to get a variety of readings. I usually measure them horizontally, then vertically, then on a 45 each way to get a few different readings. I do this at the top of the ring travel, and at the bottom. Not saying that’s right, but how i’ve always done it, to determine taper and as close to accurate as I can.

Correct, the bottom bore is only .001 out, which is fine. Its the top, which is worn .010 that i’m worried about, as that cylinder had over 20 psi less than the other on the compression gauge.

The owner doesn’t want to spend the money to rebuild it anyways, so ill just transplant another powerhead on it. Ill likely still rebuild this myself. I hate to toss blocks. I have a bunch of rebuilt ones on my shelf that are too expensive to sell now, but are ready to go anyways!

I have had members come over and offer me 50 bucks for a fully rebuilt powerhead. Yeah, Ill just keep it thanks 😉

[chris-p]

Ive always thought that the amount of wear allowed before you condemn a block, or bore it over, should be some what dependant on the HP.

Im personally, more worried about .005 over on a 3hp, say than a 40hp. (relative right)

How much wear do you guys allow, before you decide it needs to get popped up a level? Limit this to the 50s 60s OMCS for now.

[frankr]

I hope you plan to pull the exhaust cover and look for pin holes. Water ingestion may be the culprit for the wear on just one cylinder.

[chris-p]

I did Frank, could not find any on the exhaust. Im thinking lower crank seal perhaps failed. It is a ’56.

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