I have a Cessna with an 0470L. When doing a compression check I rotate the prop to TDC. On two of my cylinders the compression remains low. One is at around 40 / 80.
When I move the prop back and forth, I can hear a faint click. The compression jumps up into the 70s. I think it is a sticking ring but it could be a valve. There is a lot of what I think is carbon buildup on the top of the piston and the cylinder walls. If it is a sticking ring, what are my options. There is air coming out of the breather which would indicate rings. Thanks Flyer

Some people say run Marvel in the gas for 5 or 10 hours and check it again. I have heard people swear by Marvel and some say its snake oil. Who knows, it may work freeing up the rings, but then again it may not.

Is this an engine that you've been flying behind, or has it been sitting?
It's amazing how much the compression numbers can come up on a been-sitting engine just by running it for a bit.

I know a guy who's revived a number of old ain't-run-in-years engines.
On a couple that didn't want to come up, he ran the piston up to TDC, filled it up with mystery oil, & let her soak.
It generally improved things.

I would run it for another hour and see if it still reads the same. I have seen cylinder compressions vary wildly on big bore continentals.

I made the mistake of testing them cold once and I thought that my engine was toast. Only one above 70, and some completely blowing by. Ran it for an hour and all were high 70s. A low cylinder compression can reveal if there is a problem, but also suggest a problem where there is not one.

Also, the old 60/80 benchmark for compression tests isn't necessarily the last word nowadays.
Here's an article by Mike Busch (yeah, I know!) that discusses this--
bottom line is, google up & read Continental service bulletin SB3-03.

https://www.avweb.com/ownership/the-sav ... n-context/

It's likely wear in the ring grooves and rocking it allows the ring to seat, that's the click you're hearing. If the oil burn is good and the cylinders look ok through the borescope, then don't be concerned. Nothing unusual about a low reading initially, then improving when the prop is rocked back and forth a little

Mike Busch is a self righteous muppet, but I do agree with him that too many cylinders get pulled unnecessarily. If the oil burn is good and it looks fine through the borescope then don't get too concerned with the leakrates, just use it as a diagnostic tool to find any real issues. I challenge anyone to notice a performance difference with a 20/80 cylinder vs a 78/80, it would have to be verging on catastrophic failure if it didn't make rated power

Having said that, I'm changing out a cylinder on my 402 today due to a burned valve

I don't condone anything outside manufacturers ICA's and of course I'm not accounting for that niggling back of the mind concern that something is not quite right that many owners get when there's an issue like this. But after 30 years maintaining and flying all manner of aircraft, I know the chances are about zero that a low leakrate will cause an engine failure

I suspect that onefifty is right, widening of the ring grove which is allowing the ring tilt which results in low compression.

If it were my engine I’d put a upper cylinder additive in the fuel and run the engine hard for a few hours then recheck the compression. If it meets the specs laid out in SB3-03 then I’d keep running it. If it does not then I’d pull the cylinder.

Also as onefify said, it takes an almost dead cylinder to notice a difference in performance. The pistons in my engine all failed at the ring lands which widened the top groove and collapsed the second groove. I didn’t check the compression before pulling the cylinders but I can’t image there was much there. The only performance indication I had noticed was that the engine was down about 30rpm during a WOT static run up.

An alternate test is to use an automotive compression tester. With those, you attach the tester to the subject cylinder, then motor the engine through with the starter. Takes two people, but it actually tests the engine's ability to MAKE compression, whereas the aircraft compression tester only tests the engine's ability to HOLD compression.

Two very different tasks, and what we really care about is the engine making compression. I had a Lycoming O-320 that ran hot, passed aircraft compression test on all four with flying colors. Called Lyc Tech Reps, who said go get an automotive tester and try that, report back. Three cylinders weren't making good compression. Turned out entire engine was pretty much junk.

But, it could pass a "compression test".

Borescope would have shown some of the problem with that engine, but not all. At least one ring was broken on some cylinders. And, it passed a compression test.......seriously.

MTV

I'm sure you're checking it with the engine warm.

You can "spike" the valves to make sure nothing is hanging up in them. Your mechanic will know how. Or should.

I had one cylinder go to zero on my recent annual, which was crazy because it was running perfect as I flew it an hour to Kenai and to the hangar. One of the rockers weren't square and side loaded a valve. Had the engine shop reseat the rockers, grind the valves, and new rings installed. A little over $400 and good to go.

As stated by others rings will move and your compression can and will move with them. If it's in TCM spec I'd run it.