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?