Monday, March 15, 2010

Stuck Piston Rings in Lycoming - What to do

Dear John, I have a TIO-540 in an Aero Commander and recently I had a stuck ring. I attached a photo of the piston/case and you can see a lot of buildup. The engines have about 650 since major and I change the oil (Aeroshell W100) every 25 hours. They run relatively cool and I usually run lean of peak at just under 65% power. I attached a recent oil analysis and everything is pretty normal except the dissolved solids are kind of high. We changed the cylinder but I am concerned that it will happen again. I have had no valve sticking issues.  Is there anything I can do to remove/reduce the deposits and clean the ring grooves? How do you feel about MMO? How about engine flush concentrates? Any advice that doesn’t involve removing all 12 cylinders would be much appreciated!

The dark staining below the ring is not normal and indicates gas past the piston ring belt. The oil control ring (lower ring) has black chunks between the rails. Top two ring faces have black staining indicating ring leakage.

You have combustion gas leaking past the ring belt - i.e. "low compression" due to faulty piston ring/cylinder(s) .  Everything else you mention is just a symptom. Hopefully, this is occurring in only this cylinder and will be corrected with the replacement cylinder. If the oil still turns black then the problem has not been corrected.

The problem has nothing to do with your oil type, change frequency, leaning. It is a mechanical problem.

MMO will not fix leaking piston rings. Instead it is an attempt to mask the problem until a more serious failure occurs.

Hi John,

Thank you very much for your advice. I do have very good compressions when doing a diff compression test. I get at least 73/80 and often better. But I think you are right that I am getting blowby past the rings which is turning the oil dark. I burn 1 quart every 4 hours approximately, and the oil consumption is less with higher power settings. Both engines are exactly the same and have the same wear pattern in the oil analysis. If I regularly use MMO do you think I can prolong things until I have the top ends done? What is the likely failure mode besides sticky rings?

The dynamics of ring leakage during engine operation are far different than when the engine is cold and turned off. A compression test does not always detect dynamic ring leakage. It's best to use a compression test as one tool of several to access engine condition; High oil consumption, dark oil color, higher than normal oil temperature, and any abnormal amount of oil out the engine breather, and oily spark plugs should also be considered as signs of piston ring problems. 

What can go wrong with continued operation:

I'm not good enough to do predictions. I would access the engine's airworthiness per Lycoming and FAA standards and let the chips fall where they may. 

Oil past the piston rings and in the combustion chamber lowers the octane rating (and possibly increases the engine's octane requirement) which can lead to detonation. This was the big issue in the Chevron misfuel debacle in California back in the 1980's? when Chevron had to purchase 1,000'nds of engines. A small amount of jet fuel mixed into the avgas did cause big problems. Below is a photograph of what happens when you mix a bit of oil into avgas.

Are you sure engine problem is engine wide? All of that combustion gas makes the whole engine look bad but it can be from one source. I think I would pull the bottom spark plugs and see which ones are oily. The ones that are dry and gray and pass a compression test are probably ok. The oily ones, well the rings are leaking and they need attending too.

I don't believe MMO has any usefulness in the aircraft industry. It does not cure a problem and at best masks it until a small problem becomes a big problem.


  1. Anonymous1:04 PM

    My guess is that he is running it too lean.

  2. Anonymous1:42 PM

    Good comment about MMO and rings. It worked well to free up the rings in a 40 year old boat engine, however, some people use it for everything. Mystery. Good luck to the writer though. Maddening experience to have.

  3. Anonymous8:45 AM

    Why are you running it lean of peak? That is probably your problem.