Cylinder Wear

[adam1961]

How much cylinder wear do you tolerate on motors that are to be good runners. In the case on OMC twins, the factory spec is usually allows up to about .001 over the nominal bore size. I find that after a (light) hone on most of my motors, I end up around .0020-.0035 oversize.

Would you run these with fresh rings, if out of round is .0010 or less? Most of the common OMC classics don’t really justify the cost of a full re-build with a bore to .020 over, unless it is a full top to bottom resto…

What do you think?

[chris-p]

I too get about .002 it seems after a light hone. If the new rings end gap is within spec, I don’t see a problem with it. If the ring gap is too big, even with new rings, I find another block.

[wbeaton]

Me and the guys in my Thursday Night Motor Club allow up to .005 out of round. To be fair, we normally only rebuild 4 cyl Mercury’s. IMO .001 is too little wear to worry about.

Wayne
Upper Canada Chapter

uccaomci.com

[fleetwin]

What engine are you working on? I have never really heard of a bore spec tolerance of only .001", but it might be so on the smallest bores. Most oversize/out of round tolerances are about .003". But, I agree with you, most of the old OMC twins are very tough/tolerant to some wear and will run just fine. Try using one of the ball hones to minimize oversizing the cylinder wall while honing.

[r-c]

A lot of my OMC twins, I measure compression and if pretty even, I don’t worry much unless really low or one is 10 or 15 psi lower than the other, which is a factory spec too. You say it runs, so no problem. I pull the head and sometimes just take 320 wet and dry and clean the cylinder above the piston. Depends on rust and other conditions. Most bores are loose at the top anyhow and not the bottom, and the bottom end give you crankcase compression. That makes for hard starting if loose. Sometimes added oil solves that sealing problem. Also the added oil helps seal the rings. The rings expand from compression, not spring pressure, so the ring gap in a pleasure motor isn’t as important as a race one. A few thousands after a hone I don’t worry much. If you get towards 0.005" and more I begin to worry. You can cheat it by a number of tricks. One is to knurrel around the bottom of the skirt which will expand that area and help to seal for crankcase compression. It helps to hold oil too, therefore more sealing. The other is to have the piston coated with a Teflon coating that adds 0.002" per coating up to 0.006". Swain Tech in NY is a good place to go for the coating. If apart, new rings don’t hurt. We all have to think how many hours are we putting on these engines in a season? And they have lasted how many years as is?
I have a whole line of 1958 blue Evinrude’s and the only thing I had to do was water pump, ignition, carb and gear oil. Sometimes I think we over think these things.
Life at 4,500 rpm is a lot different than life of a race motor at 12,000 rpm.

[fleetwin]

I’m glad to hear I’m not the only one that gives old engines the 50 cent hone job with a little wet/dry/emery cloth. I usually just clean out any sanding dust with some brake clean and compressed air. This method, crude as it may seem, is a great way to break years of cylinder glaze and improve ring sealing. And, like RC says, most of these old engines never get enough hours put on them to be actually worn out enough to warrant a "rebuild". Needless to say, obvious major scores/scuffs from overheat/lack of oil, are another story.
So, again, please don’t pull this engine all apart once you have it freed up, unless the rings are hopelessly stuck/rusted. Once it is freed up, you may want to pull the head and have a look/clean up the cylinders a bit. You can always pull one/both of the intake bypass covers to get a better idea if the rings are stuck. Pulling the exhaust cover is a messy job that may include drilling out broken bolts, something to be avoided.

[The Boat House]

• This reply was modified 6 years ago by david-bartlett.
• This reply was modified 3 years, 9 months ago by The Boat House.

[chris-p]

I cannot speak for Adam, but for me I rebuild motors when they broke a rod, rings hopelessly stuck, bearing went, etc…. While I have them apart, I hone the cylinders to break the glaze. Check ring wear, and if end gap is more than .015, I re ring.

I wouldn’t say hone it out of spec, more likely that it ends up being about .002 after honing, and after years of wear

[The Boat House]

• This reply was modified 6 years ago by david-bartlett.
• This reply was modified 3 years, 9 months ago by The Boat House.

[adam1961]

Just to clarify.
I don’t always open and hone each bore. If the motor has good compression it gets left alone. That being said, I work on a lot of early Bigtwin/RDs. There are prone to worn bushings in the small end of the rod. I often have to tear down to change a rod (usually the top). While it’s apart, if the cylinder has a heavy glaze, I will hone it and install new rings.

I re-read a couple versions of the OMC service manuals. Some give the spec for the bore as 2.8750, others say it should be 2.8745-2.8755. I assume that the .001 range is the tolerance for a new bore leaving the factory??? I have not found any mention of a maximum allowable size before an overbore is required. Did most dealers assume that if the piston to wall clearance were acceptable, and ring gap within spec, that it was good to run?

In any case. I re-measured my bore. They have more barrel shape than out of round. The port area ranges from .0015 to .0030 oversize. Out of round at any point is .0010 or less. I will put new rings in it and close her up.

[Author]

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