C90 low compression test on one cylinder

[8466]

I have a c90-12F and during my annual they checked and found #2 cyl with 58 compression which is the limit but still acceptable. All the other cyl are in the 70’s. They suggested I fly another 10 hours and check it again. Since last annual the #2 cyl was 75. Any suggestions?

[a64pilot]

I’m running into the same problem with both odd number 100ish hour Millemiums. I ran the engine hard for about 10 hours and number 1 when hot tested about 78, and #3 has dropped to high 40’s from mid 60’s. All used to test high 70’s I mean 78 to 80.
I’m a C-85 but I don’t think it matters much.
My plugs run clean and don’t foul so I know there is little oil consumption, motor makes good power, passes mag check and static run etc.

First be sure that during your compression check that the air loss is from the rings and not the valves or even possibly a cylinder. You’ll hear the air from the carb, exhaust or blow by tube. Valves or cylinder leakage is a no go. You can often “stake’ a valve to knock off carbon and make it reseat.

Anyway I am certainly no Conti expert by any means, but I’ve heard in the past that Conti rings are low tension, meaning that they fit loosely in the cylinder and it’s the force of ignition pressures that blows the rings outward and that’s what forms the seal, and 80 PSI simply may not be enough to blow the rings out.
That is a rather modern design element so it surprises me, modern engines do it to reduce friction, same as why they have such short pistons now.

All of that is to try to say that as long as it makes good power, doesn’t foul plugs with oil and runs smooth, what’s to fix?

[8342]

All of that is to try to say that as long as it makes good power, doesn’t foul plugs with oil and runs smooth, what’s to fix?

I agree.
Unless you can determine exactly what, if anything, may be amiss, run it.

Rick

[V529]

As Stated above, first determine where your leak is coming from. 58/80 is not the end of the world and is ok per the Continental SB that covers compression checks.

That being said..........IF you have a leaking exhaust valve and IF the valve is burnt.......(borescope the inside of the cylinder and you will see a crescent moon discoloration on the edge of the exhaust valve) then you need to remove the cylinder and get the valve replaced and seated.

Intake valve leaks and cylinder ring leaks are not quite as serious.

[8466]

Thank you guys. Appreciate the information. It pretty much concurs with my A & P.

[a64pilot]

Link to applicable Conti SB. I have the tester with the master orifice, and it places my min acceptable compression to be just above 40PSI, which is wildly different from the accepted 60 PSI. Of course due largely to the ability of the compressor to supply air under pressure that low number will change plus air density etc.
15 CFM at 125 PSI is a larger compressor than many have.
https://www.aircraftspruce.com/catalog/ ... lletin.pdf

Sounds like you have a good A&P

[8434]

I went out flying yesterday and during my preflight I always swing the prop through each cylinder (mags off, no fuel, clear of prop). I noticed very little resistance on one cylinder. I did an extended runup and found nothing then I flew it hard for an hour. Today I can't tell which cylinder was the weaker one.

All this to say, compression tests are good "trends". A bad reading may be a one off, simply a piece of carbon on a valve seat.

[a64pilot]

Big difference between an automobile compression test where you screw in a gauge in the spark plug hole and see how much compression there is , and an aircraft compression check, which is really a leak down test, we are mislabeling it calling it a compression check, is that with a leak down test you can determine the source of the leak by listening for where the air is escaping from.
Many keep talking about valves, well according to the SB, valve leakage isn’t allowed, the only significant leakage that’s allowable is from the rings, so if you hear air escaping from anywhere except the blow by tube, that’s a no go.

[6555]

I also have the same issue but with several cylinders . I have not been flying my 140 much and had 1 great cylinder a couple in th 50s and one in the mid 40s. I am thinking of selling my 140 and it was running fine and the borescope was good on all 4. I haven't flow much , 15 hours last year, but I am thinking I will get crushed on a sale with low comps. It's been a great running machine and my A&P say go run it hard and we will check again.

Its a 1400 hour engine and I may have to do a top OH, I have been getting good oil analysis reports so I figure the bottom end will easily make TBO of 1800 and figured I could go to 2000 if the reports were good. I change oil/filter ever 25 hours...any thoughts?

[6643]

any thoughts?

Take it out and run it at 80% power for an hour then retest it.