Q: I did a compression check today on my Continental IO-520D engine. It’s a low time (300+ hours) factory overhauled engine, done in 2012. All cylinder compressions are excellent, the lowest being 77/80. But, a few of the cylinders under 80 psi are causing an air sound in the oil filler tube.
Can I have bad rings even when I get good compression numbers?
I’m puzzled by this and would value your input.
Rich Stone, Colorado
A: Rich, I’ll take a quick stab at this one with the information you provided.
I would like to know if the compression check was taken when the engine was cold or whether it was at normal operating temperature? From my experience, there can be a difference in the readings. I’ve found that compression checks taken with the engine at normal operating temperatures are more reliable.
The air sound you hear at the oil filler tube certainly would indicate ring wear, but it may be that the piston was not exactly at the top, which may have allowed some air to pass the rings.
My suggestion would be to conduct another compression check with a hot engine and carefully rock the prop back and forth to see if you can get a better reading.
I’m certainly not concerned at all at this point and feel a little more investigation is in order.
Let’s say there may be a small amount of ring wear and that raises the question as to the oil consumption. Has it increased at all? If not, then again, I wouldn’t be concerned with the results of the compression readings you’ve mentioned.
I would suggest you conduct another check within the next few operating hours just for peace of mind.
This is the type of situation that should be followed up on. Be sure to record your compression readings so you can compare them with the previous readings and observe any trends that might be taking place.
Again, there is no reason to be concerned at this point, and I commend you for sharing this with our readers.
Hopefully the comments helped answer the question of the compressions.
There are always some good comments on these articles.
My IO-520D has been doing a version of what you experienced on an irregular basis for 20 years. The excess air from 80 psi has to go someplace and the rings, based on their changing position overtime, is the path of least resistance with everything being normal.
Follow the SB, as suggested, fly it, recheck and you’ll probably see the compressions change slightly up and down over time. Normal…
As a side note, Phillips with CamGuard been good for my engine over the years with regard to compressions and oil analysis.
Rings rotate around as the engine runs. Not a lot, just little by little every operating hour. The ring gaps may have aligned on those cylinders that indicate below 80 psi. Not anything to worry about, check it again after a few hours of operation. Another thing, the cylinder differential tester should never read 80/80. The .040″ orifice inside the tester imitates the ring gaps on the piston. 78 or 79 would be the highest you could get on any cylinder. Check the calibration of the gauges on the tester. I agree also that oil consumption tells a lot of things to suspect. Fly as often as you can, get the engine up to temp. 1 hour minimum flights.
SB03-3 also explicitly says that if you aren’t happy with the compression test, bore scope the engine. If that all looks nominal, then fly the plane and try the compression test again.
I bought a borescope (the ablescope model Mike Busch recommends) from Amazon for under $300, and it is easily one of the most used tools in my box. If there is ever a question, I put eyes on the problem, rather than make an indirect guess.
TCM issued SB03-3 in 2003 that defines the compression tests.
My thoughts are that 77/80 is great compression. Some small leakage past the rings is much preferred to exhaust valve guide wear and leaks, and a possible burned valve.
Also 300 hrs in 8 years is very low use, so I’d be concerned about rust in the cylinders, between engine runs. I’d suggest using some Phillips Anti-Rust as an additive, 10% max., or Camguard.
If the oil use is in the 6 hrs or more per quart, I’d just continue the fly it.