Is a tiny bit of escaping noise OK with properly sealing rings?

Q: I was reading your article “What’s contributing to such high high oil consumption?” I am having that issue on a Lycoming 390 that recently had all four cylinders honed and rings replaced.

My question stems from your comment that if the rings are seating properly you won’t hear any air escaping via the oil filler tube during a compression test. I performed one and got 78/79/78/79, but in all four I could hear very slight air escaping if I listened right next to the filler opening.

My thinking is it’s a little bit normal because the air is following the ring gaps through the cylinder and getting into the case. Is just a tiny bit of escaping noise ok with properly sealing rings?

TOM

A: Tom, with the information you provided, I can only say that it sounds reasonable that the slight air escaping is normal.

However, that’s without knowing the entire history of your engine, such as total time since the cylinders were honed and why that step was taken.

Of course, your break-in procedure would be extremely important, but that subject is well covered in several Lycoming publications and I hope you followed those instructions during break-in.

Also, the type of oil used for break-in and the oil that you are presently using may be a contributing factor.

Have you noticed any excessive oil coming from the engine breather tube, which could be an indication of excessive blow-by?

Your actual oil level may also be a factor. If your engine has an 8-quart oil sump capacity, you may find that filling the sump to that capacity may cause the engine to blow some portion of that out the breather and your engine may operate better with, say, 7.5 quarts of oil. 

Depending on the total number of hours currently on the engine, maybe you just haven’t reached the point where the rings have completely seated. If that is the case, I’d suggest you always use full power for takeoff and cruise at nothing less than 75% power until the oil consumption stabilizes.

The worst thing you can do is to baby the engine.

Tom Replies

Thank you Paul. I have only used mineral oil and have run it hard for the 39 hours since honing. The engine has 90 hours since new.

Honing and new rings were necessary due to corrosion after installation, but before flying. Cylinders 1 and 3 look great on the borescope, as do the plugs. The exhaust pipe is a nice dry tan and minimal oil on the bottom.

Cylinders 2 and 4 have dry top plugs, but some oil on the bottoms. There is a bit of pooling of oil in both 2 and 4 seen during the borescope. Cross hatching looks great, with no signs of glazing or scuffing.

I usually start a flight with 6.0 to 6.5 quarts and run it for two to three hours at 75% at 2,000-4,000 MSL.

I’ve spoken to Lycoming and they believe the cylinders need more break-in time. They said to run for 100 hours at 75% on mineral oil and then report back to them, so that is the plan. 

Paul weighs in

Thanks for the update and, at this point, I think you should just continue to fly it as Lycoming recommended.

I might add that since you are flying it at 2,000-4,000 MSL, you might consider varying the power setting from 75% to 85% for 20 minutes or so, then move back to the previous power setting. The only caution here is not to lean the mixture if you are operating above the 75% power point.

Everything else you mentioned sounds fine, so let’s continue to fly and see if you can’t get beyond this.