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Soft Cylinder 48/80: Safe to Fly?

Dave H.

Member
My O-320 has had a soft cylinder that has gotten worse. No. 3 is 48/80. The others are 60's & 70's. During the compression test the air was heard in the breather and dipstick tube, not the exhaust, so we think the valves are OK.
I'm asking for an opinion from the forum as to weather this engine is safe to fly. I want to do a long trip that will total about 12 hours. Am I just looking at high oil usage and a greasy belly, or can this piston actually fall apart on me?

Thanks,
Dave
 
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Dave, it is hard to say exactly what will happen in 12 hours under full load. It appears part of the problem has been some rust on the walls. But that is all moot now.

Low compression will elevate blow-by. The energy will cause that cylinder to be hotter and the piston hotter. A hot piston will tend to gall - leaving aluminum on the wall in the travel zone of the rings. Progression could take several turns, seizure of the ring band and separation of the piston crown.

When will it fail - can not say, but is highly likely to fail if left long enough.

If it fails in 12 or 20 hrs, is it worth the risk? That is the real question. It won't take any more time or money to repair now or later. Maybe less now, the barrel maybe can be rebuilt now, maybe not later.

Removing the barrel and piston sending for overhaul is not a big deal, so why not just fix it?
 
I was once told that oil on top the piston can propagate detonation, if the rings are that far gone it could transfer a lot of oil up there. How do the plugs look, what if you foul them in this cylinder on the trip?
 
No Foul

Bret,

The plugs are dry and this engine never fouls plugs.

There is a lot of oil in the photos. I should have mentioned that the engine was run (lean) on the ground for total of about an hour on the ground before the scope. 1/2 hour before the compression test, and another half hour today to give me a warm environment to work.

-Dave
 
Big for me

BillL,

Thanks for your insight. Pulling a jug for overhaul may not be a big deal for you, but I don't think I have the confidence to do it myself. I've rebuilt auto engines but that is different.

Thanks,
-Dave
 
Background

Some background on my O-320: 562 hours time since assembly. No papers. The logbook just says "...built up from individual components by the [airframe] builder". British Columbia, Canada in 2000.

It got flown a lot until 2007, then was hangared for 6 years, being run occasionally to heat the oil. I'm sure this down period was hard on the engine. I acquired the airplane in 2013 and it had a soft cylinder by then. I've put nearly 200 hours on it and am getting better at understanding its quirks. The 48/80 at this annual was not welcome news.

Best,
Dave
 
Here's the thing Dave - 48/80 is well below the value where Lycoming says you should look into the cause. they advise running it for an hour or two, then checking again. Not 12 hours on a cross-country.

If the cylinder has a problem and decides to start making metal, then you can trash the entire engine, and be in for an overhaul instead of a $1,000 jug. That's a heck of a risk just to make a trip. And, of course, maintenance or dealing with a break down a long way from home is expensive, troublesome, and can lead to a LOT of expenses.

You've got a single engine, and its already got a problem that you don't understand. You've got not backup plan.

To me - that's just not worth it. I frequently hop on Southwest because of weather - I'd sure as heck do it if I couldn't be sure of my engine.
 
Pull the jug. 60 is the minimum and that's a lot of oil on top of the piston. I had that happen to me last year, luckily close to home field. Cht ran away and the engine was shaking tremendously. I was able the reduce power and land. Ended up pulling the jug and found oil on top of the piston. And my compression was 78.
 
Engine issues

Compression that low and leaking around the rings is enough reason to remove that cylinder for a look inside now. No one can say how long it might go without causing problems. You may be able to save some $$$ by removing/reinstalling the cowling, exhaust, intake & baffles for the person or shop who will do the work. I've done recovery operations to fix planes away from home (some times a long way) and it's always easier, simpler & cheaper to do it locally.

Don Broussard
 
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Pull the jug. 60 is the minimum and that's a lot of oil on top of the piston. I had that happen to me last year, luckily close to home field. Cht ran away and the engine was shaking tremendously. I was able the reduce power and land. Ended up pulling the jug and found oil on top of the piston. And my compression was 78.

It's not a hard, fast rule based on 60 psi - this is what Lycoming says -

b. If the pressure reading for all cylinders is equal and above 70 psi; the engine is satisfactory; less then 65 psi indicates wear has occurred and subsequent compression checks should be made at 100 hour intervals to determine rate and amount of wear. If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered.

https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf

However, in this case a 40 psi reading is probably "beyond consideration". :D

The other big caution is to make sure you (or the A&P) are using a tester with a 0.040 orifice as stated by Lycoming, a "Bonanza" tester with a 0.060 orifice will give a false, and very optimistic, result.
 
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Bret,

The plugs are dry and this engine never fouls plugs.

There is a lot of oil in the photos. I should have mentioned that the engine was run (lean) on the ground for total of about an hour on the ground before the scope. 1/2 hour before the compression test, and another half hour today to give me a warm environment to work.

-Dave

If it were me, I would fly it for an hour and recheck. If still below 60/80, I would fix it before further flight.
Good Luck,
Mahlon
 
Anything Mahlon recommends I would stand behind. From the sounds of it this has been going downhill for some time, which means an hour of flight probably won't help much, but it is worth a try.
 
Pull it !

I've seen enough. Pull the cylinder before more potential damage can occur.
Cut the filter, look for metal, pull the bottom screen for the bug chunks, and look at the cam, lifters.....now. If I saw this it would be off right now and looking at everything inside I could see with just one jug off.

But that's just me.
John
 
Low cylinder

After reading all the facts, the unknown engine condition and the degrading of the cylinders over short hrs, I'd pull the engine and have it majored before its to late.
To many factors going on here.
 
Do your self a favor and at least pull that cyl.now you will save money in the long run also inspect condition of engine while removed . If it were me I would consider additional work giving history of engine
 
Not that hard

Seriously, you have already been given a lot of great advice, and I don't mean to pile on in any way. The question you have to ask is if it is potentially worth risking the airplane, and perhaps life and limb, for the price of a cylinder?
I know it can be intimidating to replace a cylinder, but these engines are really simple, and you will realize that once you get into it. A cylinder can be swapped in a day or 2, even by a novice, and you will look back on it and realize you are less stressed and probably even enjoyed the learning experience. :)

Vic
 
BillL,

Thanks for your insight. Pulling a jug for overhaul may not be a big deal for you, but I don't think I have the confidence to do it myself. I've rebuilt auto engines but that is different.
You don't have the pull the cylinder yourself. Hire an experienced A&P to do the work, with you as his assistant.
 
Is the airplane airworthy?

I am not an A&P and have very little experience with engine overhauls.

A lot of good advice has been provided here already.

However, as an airline pilot I often view my GA flying through the lens of "could this affect my license." The FAR's (91.7) are pretty clear about the requirement of the PIC to make the determination of the airworthiness of an aircraft.

Just the fact that you have come to this forum indicates to me you have concerns and/or questions about the airworthiness of this particular airplane. So as an outsider looking in, I don't think you would have much in the way of a defense of flying this airplane in it's current condition. I know I wouldn't, but I am generally very risk averse. That has served me pretty well so far in over 40 years of flying and 31+ years at the airlines.

Please don't let your desire to fly this trip turn into another GA incident/accident. There are enough of those already.
 
It could be worse....now is the perfect time to do repairs in the hangar, its not snowing and -10, and its not 100 in the shade...unless your in Phoenix :rolleyes:
 
Good articles!

Two articles by Mike Busch that might be germane:

https://www.savvyaviation.com/wp-co...s_eaa/EAA_2014-06_cylinder-work-be-afraid.pdf

https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-08_teardown.pdf

Probably wouldn't cost all that much to have Savvy Analysis review the issue.

And here's a webinar he put on about the subject:

https://www.youtube.com/watch?v=3-3pkRLqoGw&list=PLYRORvQ4_I4YBu8BxvEA6XdJRJ_kGA0Ld&index=15

Edited to add a link to another Busch webinar "How Healthy is Your Engine?":

https://www.youtube.com/watch?v=AwGifm36mKY
 
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Cylinder

Puts some Marvel Mystery Oil in the oil and gas and fly it a couple hours around the airport and then check it again.Works every time.
Bob
 
The great debate!

Its pretty simple, cut the filter open and have a look at it and the sump screen, if they are clean borescope it. If that's negative fly a pattern or two and recheck. If it's still down it's not gonna improve and you need to pull it. Flying it without knowing why it's down is asking for it.
Tim Andres
 
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