Is 60/80 an on-condition compression?

[E1453]

I've been always thought that, when a cilinder compression falls below 60/80, I should summarily replace it.

However, yesterday, as usual, I was surprised when I read the Mike Busch's article about compression checks. He says that the minimum acceptable compression, according to TCM, should be around 50/80, as long as the pressure loss is not thru the valves.

The article:

http://jtatwins.com/Compression/putting_compressions_in_context_.htm

What limit do you use to cilinder compression?

[QDMQDMQDM]

Doing my annual we just had a compression of 60 on one cylinder down from 76 the year before. Took the cylinder off and it was simply due to the grooves in the piston rings wriggling themselves into alignment...

QDM

[A and C]

The answer is not a simple one ! First you have to look at the history of the engine if it is used about 25 hours a year then a reading of 60/80 will never get better and so it is time to pull the cylinder and inspect it , as QDM points out the answer may only be in re-positioning the rings but running a hone down the bore and a new set of rings is more likely to be the answer if the low comp is acompanyed by evidence of burning oil.

If the aircraft has a high rate of use then re-check within 5-7 hours , I have seen aircraft that have been layed up for the winter boost the compression by 10 psi just by doing 7 hours over a weekend.

As the compression drops an increase in oil consumption will usualy be seen , I shudder to think how much oil a small Lycoming would use with a compression of 50/80 and I guess that this is the opinion of Lycoming as they recomend a bottom limit of 60/80.

As the link states no leakage from the valves can be allowed but I have found that "staking" the valve works about 50% of the time.

If you have a Continental engine then I would read the SI about ten times and then follow the advice as the factory will have much more information about the product line but most of the big engines are high utalisation and what is good for a big engine that sees an inspection once a month is not going to be good if you own a small engine that flys less than 50 hours a year.

At present my personal aircraft that flys about 75-90 hours a year gives about 25% more problems per flight hour than my "working" aircraft that do about 300-400 hours a year.

The question on my mind is how many of the "experts" who write about these things on the internet actualy have to put pen to paper in a tech log and take responsability for thear opinions ?.

[ShyTorque]

Interesting stuff. I knew that bigger capacity engines had a lower reading but I wasn't aware that it could be as low as the figures quoted here. I have a modified small capacity car engine that has a compression pressure of 225 psi.

A & C,

What does the term "staking" a valve mean? Thanks.

:O Ah, I think I see why now, so the test is done differently, by pumping compressed air into the cylinder using the 80 psi as a datum and effectively doing a leakage check. On a car engine you crank it over on the starter and see what you get, comparing the differences between cylinders that way.

And "staking" means giving it a bit of persuasion to seat properly. Surely though, as a valve pounds up and down due to normal engine operation, does this really work to improve the seal with the seats?

Just curious.

[dirkdj]

I would wait to do any hardware work yet. Just fly it another 5-10 hours or so, LOP preferably, then do another compression check.

My friends IO520BA had low compression when he bought it used, then the next compression check was higher and better just because it got rid of all the ROP crud.

Obviously LOP won't get rid of a real mechanical problem like a burnt valve. No leaks via the valves should be allowed.