"Flying out" a low cylinder compression?

alfadog

alfadog

Final Approach
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alfadog
So please correct me if I am wrong, but I have heard that if you have a low cylinder where you haven't seen that historically, that one thing you might want to try is just flying the airplane for 10 hours and checking it again.

I'm getting 20 psi on one cylinder of an O-235. Last time I checked it, and that was not so long ago, it was up in the seventies with the others. I've checked it a couple of times, then some high speed taxis and one tour of the pattern, and have had the camera inside. Everything looks fine inside. It's leaking out the exhaust valve. I've tried staking the valve also, so to speak. No joy. It's stuck at 20. Do you think flying the airplane for a few hours is a good idea or should I just pull it?
 
I did exactly that, in the last 2 weeks. It's fine now, but I am watching it.
 
SI 1191A says if it's below 60psi removal of the cylinder should be considered...."considered" I'm no A&P but if it's leaking that much past the exhaust I would be a little worried...not alot worried.

You tried staking it..how does it look in the scope? Symmetrical burn pattern on the valve face? When moving the prop the valve doesn't wobble trying to seat? How does the valve seat look, a bunch of carbon deposits? If all that looks good maybe try lapping it and see what that does??

But honestly if the value isn't showing an uneven burn pattern id fly the fool out of it hard for 10 more hours and test it again right after the last flight.
 
alfadog said:
So please correct me if I am wrong, but I have heard that if you have a low cylinder where you haven't seen that historically, that one thing you might want to try is just flying the airplane for 10 hours and checking it again.

I'm getting 20 psi on one cylinder of an O-235. Last time I checked it, and that was not so long ago, it was up in the seventies with the others. I've checked it a couple of times, then some high speed taxis and one tour of the pattern, and have had the camera inside. Everything looks fine inside. It's leaking out the exhaust valve. I've tried staking the valve also, so to speak. No joy. It's stuck at 20. Do you think flying the airplane for a few hours is a good idea or should I just pull it?
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There's a slight chance of detritus holding the valve off of the seat (assuming that it has proper clearance in the first place!) There's also a chance that it's slightly warped, and if so, it will not get better.
 
Did the plane sit for a good while?
I’d be ok with it in that case

Or was it a all of a sudden type thing?
 
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It’s an o-235... how is the valve lash on the questionable cylinder?

There can be a few conditions where you might be able to run the engine and subsequently have the leakage check improve, but overall I’ve found cylinders to rarely “self heal” despite trying it many times.
 
I would stick a borescope in and take a look at the valve. You might find it it's cracked or it just might need to be lapped. Either way, until you get a scope on it and look at it you really don't know for sure. If it's cracked and you take it flying you are asking for much bigger problems.

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Does anyone read the OP before replying?

I've had my mechanic advise me to fly it hard and other times to pull the cylinder. His advice turned out correct each time. What does your mechanic say?
 
Maybe, but, yeah I wouldn’t just go flying it more without investigating.

Where is the air going when you pressurize it? It’s either cracked and leaking out the cylinder, leaking into the case (breather tube will be dumping air), or leaking out the exhaust.

If you’re not leaking out the exhaust or into the case then you’ve got yourself a cracked cylinder and flying it more will not fix it and may be catastrophic.

If it were mine, I’d want to know where the air was going before I considered flying it.
 
What does "I've tried staking the valve also, so to speak" mean?

20/80 is very low. Continental (I think) ran a 4 cylinder without any rings and it made full power, but then, it didn't have an exhaust valve leak.

What I'd be afraid of is making the problem worse by burning the exhaust valve.

Got a photo of the exhaust valve face? Did you borescope with the exhaust valve wide open and get a look at the bevel?
 
20 PSI coming from the exhaust valve? Time for full stop and investigate. Valve lash, wobble check, push valve into cylinder for a detailed look at the seat and face.

Maybe it is a piece of gunk that may work itself out, or it may lead to a burned valve and more serious things. I wouldn't fly it like that, but it also isn't my plane. 50 or 60, sure, but not at 20.
 
Answers to a couple of questions. Yes, the airplane's been sitting since maybe June. It's been started and taxied but not been in the air since then. Staking the valve means to hit the exhaust valve stem with a mallet and piece of wood say while keeping pressure on the cylinder to knock anything loose out. And yes, the leak is coming out the exhaust pipe. What else, borescope looks good like the picture above all the way around.
 
When was the last valve lash check done? Wobble test? What does the seat look like?

I wouldn't run out and replace the cylinder just yet, but it is talking.
 
Valve lash is to spec. Didn't do a wobble or look at the seat particularly. I'll do that after he flies the airplane a bit though, to be honest, if it doesn't come up then, the jug is coming off.
 
alfadog said:
View attachment 78568
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It looks like it's leaking around the bottom part of the picture.
Before you tear it apart, I would try lapping in the valve. It might surprise you and come right back.
I think it's worth the hour or so that it would take to do.

Sent from my Pixel 2 XL using Tapatalk
 
Sure would be nice to know how many hours are on that cylinder before giving advice guys.
 
I've flown about 30 hours on a cylinder like that, removed it the next year, it was cracked in multiple places, from the spark plug holes and between valve seats was an O360A1F6D
 
flyingbrit said:
Ah yes, the old "Mexican valve job".
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I've tried it twice. Both times it turned out to be a waste of my time, as the real issue was a crack in the valve area of the cylinder.
 
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