Cylinder Compression/ Pre Buy

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papapatoto

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papapatoto
What do you guys think of this? Is there an issue with one being so low and the other so high? This is on a Mooney m20k with a TSIO360MB2B-L-SB
 

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I realize it’s a Continental, but I wouldn’t call those compressions “all good”.
 
RyanB said:
I realize it’s a Continental, but I wouldn’t call those compressions “all good”.
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How would you refer to them? I was thinking they looked a little bit wonky, isn’t it technically not airworthy with that much of a disparity between numbers?
 
How was the test performed? Continental has detailed guidance on cylinder leakage tests and determining good from bad. That's where I'd start.
 
When was last overhaul (date and hours ago)?
 
papapatoto said:
Is there an issue with one being so low and the other so high?
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Not enough info to make a call. For example, was this single check? Cold? Was any effort made to improve the numbers, and so on. Regardless, the person to ask this question to is the mechanic who will sign off your next annual as its his call what he'll accept as good vs the prebuy guy.
 
For the differential test to have any value it should be coaxed into best (better if possible) numbers by moving the prop. The leakage is then evaluated by listening to the area where the air escapes to help determine if is an exhaust valve (exhaust stack), intake valve (filtered airbox) or rings issue (oil breather or oil filler neck).

Absolute numbers for a differential test provide some indication of trouble, however they should be retested after a few more flights.
 
I'd call those numbers 'mediocre'. Really depends on where the leakage was coming from. Not a deal-breaker for me, but maybe something to use in negotiations because of the possibility of some cylinder work in the near future.
 
I would wonder why the inspecting mechanic called the numbers "good" with no additional commentary.
 
papapatoto said:
What should I expect him to be saying?
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Describing the results a bit more. I'd want to know where the air is going to understand if you have a problem or not. If you hear the leak in the breather tube and you aren't consuming excessive oil, you probably don't have a big problem. If you hear the air coming out of the exhaust pipe, you have a problem that needs fixing, so details matter. I wouldn't be terribly enthusiastic about this pre-buy report if it were me.
 
RyanB said:
Describing the results a bit more. I'd want to know where the air is going to understand if you have a problem or not. If you hear the leak in the breather tube and you aren't consuming excessive oil, you probably don't have a big problem. If you hear the air coming out of the exhaust pipe, you have a problem that needs fixing, so details matter. I wouldn't be terribly enthusiastic about this pre-buy report if it were me.
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Okay, thank you very much, I appreciate the information I am going to be calling him tomorrow
 
Mongoose Aviator said:
I would wonder why the inspecting mechanic called the numbers "good" with no additional commentary.
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Probably because they were simply higher than the minimum acceptable pressures which is "good" per TCM.
 
IIRC TCM says zero leakage at valves so I presume it’s blowing by the rings.
 
Magman said:
IIRC TCM says zero leakage at valves so I presume it’s blowing by the rings.
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With Continental’s M-0 service manual, this is not true. Leakage past the intake or exhaust valves less than the tested orifice limits is cause for borescope inspection, but is not necessarily reason for cylinder work.

Of course, following M-0 means having the leak down number from the master orifice, which isn’t in the report. Nor is a comment on borescope inspection, which I would want more than a compression test.
 
Lycosaurus said:
For the differential test to have any value it should be coaxed into best (better if possible) numbers by moving the prop. The leakage is then evaluated by listening to the area where the air escapes to help determine if is an exhaust valve (exhaust stack), intake valve (filtered airbox) or rings issue (oil breather or oil filler neck).


Absolute numbers for a differential test provide some indication of trouble, however they should be retested after a few more flights.
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How is the pre inspection buyer going to retest after a few more flights?
 
Clip4 said:
How is the pre inspection buyer going to retest after a few more flights?
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Sometimes ring gaps line up and cause low pressure, or carbon is found under the valve seat. Another flight can help. Go fly with the owner if you can. A short flight is better than nothing and have it retested.

In the OP case, he likely does not have the possibility to fly and retest; the alternative is to use the inspection result as a negotiating tool for making an offer (assuming he is still interested), expecting the worst case for repair costs.

The fact that the shop did not diagnose the origin of the leakage makes me wonder what else they skipped during the inspection.

What I described in my previous post is what should be done during a differential test. It is up to you to decide what to forego and accept the risks.
 
papapatoto said:
Do you know the minimum accepted pressures for this engine?
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No as that figure is determined before each test procedure and varies. However, I have seen minimum accepted values as low as the mid-40s.
 
All this debate and no quotes from the authoritative advice on the matter. Just guesses.

https://www.aircraftspruce.com/catalog/pdf/servicebulletin.pdf

It is Continental's Service bulletin SB03-3, since incorporated in their manual M-0, Section 6-4.11.2.4. They use the usual differential compression tester, but they want you to first use their calibrated orifice tool to find the minimum compression number for the conditions at the same time of same day you're doing the test. Density altitude and humidity both affect the readings. I often saw numbers around 43 psi, on a typical 70°F day at 1100 ASL, for instance. That's pretty forgiving, that number. I wouldn't want to fly with that, but Continental thinks it's OK. A Lycoming with numbers like that would be a smoking pile of junk.

But as RyanB says, it depends where the leakage is. If it's exhaust valves, watch out. If running the engine and checking again doesn't change anything, the engine needs top-end work.

And if the mechanic isn't rocking the prop to seat the rings (as they would be in actual operation) and thereby get the best numbers, the test is not accurate. It can be WAY off.

Continental's M-0 manual: https://pceonline.com/wp-content/uploads/2017/04/M-0standardpractice2017-01-15.pdf
 
Those numbers look acceptable when compared to the master orifice. On my 470's I have a couple in the 49-50 range when cold. The SB calls for you to run the engine for 1 hour before testing so those numbers would go up considerably.


papapatoto said:
How would you refer to them? I was thinking they looked a little bit wonky, isn’t it technically not airworthy with that much of a disparity between numbers?
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Disparity between cylinders is not cause for rejection.
 
I feel like compressions aren't really as absolute as we (pilots) think of them. I think that if you retest the same cylinder on the same day it can vary by several PSI. Testing cold vs. hot can make an even larger difference.

I've also been told many times that "anyone who says their Continental has compressions that are all in the 70s, is lying." But at our last annual, for the first time ever, our IO-550 came out all in the 70s. And it's got 2300 hours on it!
 
flyingcheesehead said:
I've also been told many times that "anyone who says their Continental has compressions that are all in the 70s, is lying." But at our last annual, for the first time ever, our IO-550 came out all in the 70s. And it's got 2300 hours on it!
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2300 on the original cylinders? No top end work? Hard to believe for a Continental.

But well-cared-for engines can indeed maintain their compressions well. The guys that go out to the airport and pull the airplane out and run it for ten or 15 minutes, then put it away, are the guys that see terrific corrosion in their engines and shortened life.

The Lycs in our flight-school airplanes were replaced at TBO, typically 2000 hours, and had compressions still in the high 70s. No top end work.
 
Dan Thomas said:
2300 on the original cylinders? No top end work? Hard to believe for a Continental.
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Original cylinders, yes. I think three of them have been worked on, including two at the previous annual though rumor among the local mechanics is that the shop owner needed a new boat/rv or something. I kinda regret letting him do both of them without asking more questions, as I think one of them may not have really needed anything.

But yeah, I have to say that my experience running the IO-550 (1) Regularly for the most part, (2) generally at 65%, and (3) lean of peak, has been a pretty positive one. Not planning to change anything except maybe to fly more!
 
That prebuy as is seems to be one that any amateur could do (cosmetics), many of which appear in the ad.

I'll ignore the engine stuff, other know more.

What is good about the plane? Did you get a better review of things then that letter? For me, I want to know about good PM being performed, corrosion inspections, age of belts and hoses. In other words, does it have good bones?
 
How much did you pay this guy? I might have to rethink my strategy and start doing PPI's, it looks like easy money.
 
This was asked over on MS.

The question there is, is there another list of things that were looked at, but were fine?

Like spar corrosion? Steel tube cage corrosion? Fuel leaks? etc?

And, based on my experience, on a Continental, if not flown a lot, pull the lifters and check for pitting and look at the cam.

But it is not unknown for TSIO360s to need a top overhaul at mid time.
 
Pinecone said:
This was asked over on MS.

The question there is, is there another list of things that were looked at, but were fine?

Like spar corrosion? Steel tube cage corrosion? Fuel leaks? etc?

And, based on my experience, on a Continental, if not flown a lot, pull the lifters and check for pitting and look at the cam.

But it is not unknown for TSIO360s to need a top overhaul at mid time.
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With the litany of crappy prebuy inspections and missed deficiencies discussed on the internet spanning years, why don't prospective aircraft purchasers generate a list of things they want inspected, instead of relying on someone else's idea of a prebuy? It would be supremely easy to do, considering the vast trove of information available to anyone with a WiFi connection.

A person doesn't have to be mechanically inclined or have special knowledge to do it. Engage in studying and investigation. Ask a few mechanics and other airplane owners. Find out what should be inspected and why. Demand specific answers. Calling Savvy Aviation and hiring them or another "specialist" does nothing but pass the buck. One still wouldn't know if important issues were ignored.

Sometimes I get the impression all a prospective owner can muster is a mumbled "Uh, can you do a prebuy on a Cherokee?" and that's the extent of guidance the A&P receives.
 
Agree with preceding.

All PB’s seem to start with “ How much for a pre-buy?”

Later this changes to “ Isn’t a Pre- Buy a Warranty?”


Dollars = Depth

No Fed standard here.
 
I own three compression testers by two manufacturers. All have a Master Orifice built in. It isn’t rocket science!

I’d be concerned with the low cylinder and would want to re-test after a good warmup to see if it’s consistent and determine where it’s leaking.
 
There is nothing like buying an airplane and owning an airplane to educate the owner about the pitfalls and gotchas of said experiences.

(maybe "expose the owner to..." rather than "educate the owner about..." would be more accurate... some people don't seem to learn quickly)
 
BTW, if you get AOPA's magazine, the latest issue has a good article by Mike Bush on compression testing.
 
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