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markscogg

Well Known Member
Just got a DIFFERENTIAL CYLINDER PRESSURE TESTER MODEL E2M with the .040 master orifice. This is to be used on a Lycoming.
When testing the master orifice, it only holds 44/80. Does that seem correct?
Thanks
 
Yep, that's correct. When in testing mode, the gauge reads the lowest allowable reading.
 
Big Bore Continentals

I can't speak to other applications, but I have a Bonanza with a Continental IO-550. Continental considers a cylinder "airworthy" if the cylinder compression is greater than the reading of the master orifice.

Don
 
Lycoming SI 1191A

The low number with the master orifice open only applies to Continental engines and in my opinion is not a good idea. Lycoming SI 1191A gives the procedures and recommendations for compression test. My experience agrees with the service instruction guidance. I typically don't see a 320/360 with a hot compression below 70/80, even with times well over TBO, unless something is malfunctioning, worn out, or broken. Good Luck, Russ
 
You could keep flying a plane if it tested 44/80? I thought 60/80 was the lowest allowed.
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Basically the answer to your question is yes.

Lycoming SI 1191A gives the Lycoming view. Continental SB03-3 gives their view. If a single cylinder is below 60/80 - something is going on and the engine is trying to tell you something. The Continental SB is a little more explicit about a borescope inspection (which is a really good idea).

My own feeling (admittedly and anecdotal sample of one) is that a compression check by itself is important, but not complete. Coupling the compression check with a visual inspection (as in the Continental SB) is a really good way to go.

My experience is that brand C and brand L engines have about the same failure percentages, but I agree with an IA who once observed to me that the difference is the brand L will usually get you home and the brand C is more of an "event."

So, for example, if I had a few years of all cylinders checking in the range of mid-70s and one year at the condition inspection one of them was in the range of 65, that's not so good. That it's over 60 is not a good reason to just sign off and keep flying. Similarly, if it were 58, it wouldn't mean that I'd just pull the cylinder (as per the Lycoming SI) solely on the basis of the compression test reading without doing some more analysis and inspection first (and the Continental SB is a really good source of info on how to approach this).

Dan
 
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DanBaier said:
...So, for example, if I had a few years of all cylinders checking in the range of mid-70s and one year at the condition inspection one of them was in the range of 65, that's not so good. That it's over 60 is not a good reason to just sign off and keep flying. Similarly, if it were 58, it wouldn't mean that I'd just pull the cylinder (as per the Lycoming SI) solely on the basis of the compression test reading without doing some more analysis and inspection first...
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Agreed. The compression test is similar to the "punch test" done on fabric aircraft in the respect that the test itself is not definitive -it is merely a piece of evidence. The compression test is further compromised as definitive by the fact that your "58 PSI problem child" of today might be back in the 70's after the next flight. While that's admittedly a big swing and is indicative of an acute (and possibly temporary) problem, looking at my logbooks shows that I routinely get a few PSI swing from year to year.
 
It's important to know where that air is leaking out of the cylinder.
If you get a low reading, take a listen at the exhaust pipe, air filter inlet and oil fill tube. The telltale hiss lets you know if you have a leaky exhaust valve, intake valve, or piston rings. This determines what you do next. Engines that have not been run for a while may give a poor compression check but return to good after being flown a few hours.
Although very rare, Intake valve problems scare me the most.
Every now & then the head or cylinder will be cracked, so a soot or oil stain may be observed on the outside as well. No way out, the cylinder comes off.
I advocate getting the full service life out of everything, especially your life.
 
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