Old Air Compressor Rebuild (and general piston engines) - Oil On Cylinder Walls?

Question

So I have recently acquired (for free) a circa 1973 Speedaire 20 gallon compressor with no history. After a short while of use the check valve failed and when I removed it I noticed it and the air manifold it was threaded into was all gunked up with oil. It actually wasn't that bad given the 40 years of age, but enough to destroy the plunger in the check valve and make things in there a bit messy. It is my understanding that the piston rings were letting some oil by, leading to the build up.

I have since started cleaning out the gunk, ordered a new check valve, drained the oil, and removed the head from the compressor. Everything was in surprisingly good shape except for some matching oil gunk in the head.

Here is what I noticed: As I rotate the flywheel and the piston falls, I notice a small film of oil left on the cylinder walls dripping down. The cylinder walls appear fine by eye but I don't have any real measurement equipment.

And my question is: Is this amount of oil normal for piston air compressors? Is it normal for any piston / ring combo to leave that much oil behind on the down stroke? I've never personally had the head off of an automotive engine but I can't imagine those rings letting visible quantities of oil past.

The reason for my question is: I'll most likely purchase a rebuild kit anyway, but I'd like to know if the gunk I see is small amounts over 40 years (if this is normal), or the same amount over lesser time since the rings or whatever have gone bad.

Bonus question: Is there a standard way to test piston rings?

Answer 1

The main difference between an air compressor and an internal combustion engine is the compressor doesn't have the heat/detonation phase which an engine does. This actually helps keep it cleaner than what you'd think. (So people don't think I'm crazy, an internal combustion engine and a compressor are both just air movers ... they just accomplish it a little differently).

Here are a set of piston rings from an Ingersoll-Rand compressor:

This is quite different from a set of rings from an internal combustion engine. Most engines have two compression rings and an oil control pair (which consists of two oil control rings and a spacer which all three reside in a single ring groove). The compressor set (and I don't know if this is typical, but appears as though it could be), has one compression ring, an oil scraper ring, and two oil wiper (oil control) ring sets. It seems to me Ingersoll-Rand is trying to keep oil out of the cylinder.

My take on this is, if you are actually seeing an oil film left on the cylinder wall after the piston down stroke, the oil control rings are not doing their job. It should be almost spotless after it sweeps downward.

One thought remains:

• The rings on this compressor are worn out and need replaced. You would need to go through the same process of deglazing or glaze busting the cylinder walls with a cylinder hone. Then you'd need to file fit the rings to the proper gap to ensure they don't seize in the bores during use. Then there is going to be a break-in period. I don't have the specifics on any of these for a compressor, but I'm sure they exist.