compression: one cylinder low OK?

whereisrandall

whereisrandall

Pre-takeoff checklist
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Randall Williams
Hey gang,

I'm looking at getting a 1956 Cessna 172 with a 6 cylinder Lycoming 300. Compressions at last annual (February 2016) were 76-74-75-76-62-78. Any reason to be concerned about cylinder 5?

Thanks as always,
 
Well, it is a Continental and it sort of depends on where the cylinder is leaking compression. Can't really be much help here.
 
If it is not leaking past the exhaust valve, fly it. Might consider borescoping it, good general practice. Look for signs of uneven heating on exhaust valve face.
 
Budget for a new cylinder, if it's a good airplane the rest of the way around, you'll probably love an old straight tail.
 
Stewartb said:
One low cylinder isn't something anyone wants. Some guys rationalize that it's okay. If it was so okay they'd really love all 6 low. Figure to have that cylinder pulled and repaired. Better yet make you purchase offer contingent on the repair. Don't inherit somebody else's deferred maintenance.
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Why? Why is one lower cylinder bad? If you read the TCM SB regarding differential air leak checks it may be fine.
 
How much has it flown since the annual? Has the compression been tested again since? Sometimes a "soft" cylinder is just a bit of carbon which will typically resolve itself over a few hours. If it has been flown several hours and is still low, I'd want to know why. Either budget in pulling the cylinder and having it overhauled or replaced, or better yet have the seller have it corrected beforehand.
 
whereisrandall said:
Hey gang,

I'm looking at getting a 1956 Cessna 172 with a 6 cylinder Lycoming 300. Compressions at last annual (February 2016) were 76-74-75-76-62-78. Any reason to be concerned about cylinder 5?

Thanks as always,
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How many hours on the cylinder? where is the leak? was the compression check done hot or cold?
 
When I bought my 235, one cylinder was reading 58/80.

Flew it home and trained in it, 40hrs later it was 72/80.

Sometimes it works out.
 
Compression tests are notoriously inaccurate. I honestly can't believe how many people rely on them for a go/no-go decision on a purchase. If you have something low instead of passing on it have the cylinder boroscoped, or pull it and really inspect it. That's if you are REALLY concerned about it.

Anything under 60 requires inspection IIRC, so a value as high as what you said wouldn't even concern me.

Under 40 and the plane is not airworthy IIRC, but I don't recall the source.
 
Stewartb said:
Because they had a stretch of years where they couldn't make a cylinder that lasted. Now they've bought ECI and will produce all their cylinders using the ECI plant and process. Interesting times at TCM.
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Got a reference for that, the only after market cylinders are available now are Superiors, and there is a 6 month wait on back orders.
The TCM cylinder we now see are not the ECI cylinders we were getting last year.
As far as where they are made is not important, it's the quality of the product.

Call and talk to the TCM parts desk in Mobile.Al.
 
Always been a fan of ECI

Agree 100% with Stewart, low compression on any cylinder ain't good, if it was you'd want low numbers on all cylinders, airworthy, sure, ideal, heck no.

Remember when you do a prebuy, it ain't just about airworthy, that's just expected and baseline, it's about STATED CONDITION, unless he said in his add that it had low compression on that cylinder, that's going to change the advertised or agreed upon price, unless after some flight time on his dime the number comes back up,
period.

Speaking of which, how much has the plane run in the last 6 months?

Sometimes if they sit for a little they'll get low compression which comes back up just fine after some flying.

Maybe see if the plane could get some hours on it and re check?

Otherwise, I'd include a cylinder in what you pay the guy for the plane. And ALWAYS borescope and cut filters before you pull out your checkbook for a plane.
 
The base limit is 60/80. With the Continental SB, it can go significantly lower.

As long as it's not going out of the exhaust valve, I wouldn't worry about it and I wouldn't change it unless I had to due to compression numbers or leakage past the exhaust valve.

Sure, everyone wants to have compressions in the high 70s on all cylinders, but the reality is that these things wear with time. The other reality is that the performance difference is negligible. On the Aztec I had compressions hovering around 60/80 for 400 hours on the left engine. Cylinders were junk. Pulled all 6 and replaced with brand new factory Lycoming cylinders. I noticed zero difference in performance.

Some people could say it's rationalizing, when I view it as normal variation and wear.
 
Things wear with time, but the price needs to reflected worn out components.

Otherwise I'd be expected to sell my plane with all six above 75/80 for the same price as a plane with on the edge numbers, ain't going to happen
 
Even with static leaks out the exhaust valve I'd still not be quick to jump on engine surgery so fast. I'd prolly wait till I saw other concerning signs. Like the beginnings of a burnt valve.

Once the valve shows signs of burning ....different story. A burnt valve will fail within 30-60hours of use. It then needs to come off.

I know of an engine running quite fine with several cylinders in the 40's and 50's and no burnt valves.
 
Checkout_my_Six said:
Even with static leaks out the exhaust valve I'd still not be quick to jump on engine surgery so fast. I'd prolly wait till I saw other concerning signs. Like the beginnings of a burnt valve.

Once the valve shows signs of burning ....different story. A burnt valve will fail within 30-60hours of use. It then needs to come off.

I know of an engine running quite fine with several cylinders in the 40's and 50's and no burnt valves.
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Would you pay the same for that engine as one with high compression?

My understanding is the OP is looking to buy, I'd ether want to see those numbers change or factor a new jug into the sale, wouldn't you?
 
Stewartb said:
My reference would be the parts manager at TCM that I spoke with. That was a month or two ago and they were still figuring out the PMA. At that point they could sell approved Lycoming cylinders but not Continental cylinders. I just looked on the web and the only big bore TCM cylinders they have to offer are nickel bores. Sounds like the parts manager was right. I've never been concerned with the 300 series so maybe they have old stock for those. Still an interesting shift for TCM.
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When you are the manufacturer and you have bought a company you do not need PMA to produce parts for the engine you own the production rights to. They could make the ECI cylinder today and sell it as their own, but they are not.
I understood from the parts desk that they will not produce any 50 CC cylinders until they sell the crappy stuff in their stock rooms.
 
Ted DuPuis said:
The base limit is 60/80. With the Continental SB, it can go significantly lower.

As long as it's not going out of the exhaust valve, I wouldn't worry about it and I wouldn't change it unless I had to due to compression numbers or leakage past the exhaust valve.

Sure, everyone wants to have compressions in the high 70s on all cylinders, but the reality is that these things wear with time. The other reality is that the performance difference is negligible. On the Aztec I had compressions hovering around 60/80 for 400 hours on the left engine. Cylinders were junk. Pulled all 6 and replaced with brand new factory Lycoming cylinders. I noticed zero difference in performance.

Some people could say it's rationalizing, when I view it as normal variation and wear.
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When you understand dynamic compression you'll understand why it doesn't make a hoot when a 1 cylinder is 10-15 pounds under the rest.
 
Whatever you say... because you said it. I have no interest in arguing.
 
Ted used to work as an engineer with both continental and lycoming. He used to figure out what it took to destroy engines for a living. Maybe you could learn something from him if you cared to listen.
 
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Ted is an ok guy. I met him once..... I think. :D
 
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Thanks gang - very helpful. I'm going to dig in more and see what more I learn re: hours flown and cutting filters.
 
timwinters said:
Ted used to work as an engineer with both continental and lycoming. He used to figure out what it took to destroy engines for a living. Maybe you could learn something from him if you cared to listen.
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No doubt Ted is a great engineer, but the 0-300 hasn't been supported by the factory in some years. plus I'll wager he is not maintaining any of them either.
You'll just have to get past the filter incident, because it simply makes every one believe you are sympathetic to dead beat customers and question your motive to do so.
 
SCCutler said:
If it is not leaking past the exhaust valve, fly it. Might consider borescoping it, good general practice. Look for signs of uneven heating on exhaust valve face.
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we usually run the engine .. fly if possible then quickly do another compression check .. don't be surprised if it comes back up. Continentals don't like cold compression checks Lycomings don't care
 
Tom-D said:
When you understand dynamic compression you'll understand why it doesn't make a hoot when a 1 cylinder is 10-15 pounds under the rest.
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I'd venture a guess I know a thing or two about compression, the numbers, and what actually happens inside the combustion chamber.

I'm also not sure whether you're trying to disagree with me or not, because the summary of my post was "Don't worry about it."

You are correct I don't service these things daily. But, I've spent a few thousand hours running these engines on dynos, as can be proven by my hearing loss. Really shoulda been wearing my Zulus in the lab, not just in the plane.
 
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