Compression on fresh overhaul

[rturbett]

What is a normal expectation for compression results on a freshly overhauled IO-360-A1B6D  (under 50 hours)

Is there any any kind of scale that compares engine hours and expected reduction in compression over time?

 

Thanks,

Rob

 

 

 

[Ibra]

78-79-78-79 on overhaul, it went down to 74-75-73-74 after one year and 120h on it 

Edited September 26, 2023 by Ibra

[EricJ]

It might be a little random until the rings seat fully.

[rturbett]

good to know- a log book I reviewed showed 74's at about 30 hours.   I thought that was low for so  little time.

What is an accepted time frame for rings to seat?

[Andy95W]

A big part is whether the compression test was done hot or cold.  72-75 is fairly normal for a cold engine, for hot I’d rather see 76-79.

At 30 hours, ring seating should not be an issue if the engine was broken in properly.

Edited September 26, 2023 by Andy95W

[Pinecone]

Rings should seat in 10 - 20 hours at the most.

Was it broken in properly (High Power settings)?

[PT20J]

Since we installed my factory rebuilt engine during an annual, we had to measure the compression. We did this cold before we ran it - it had been run in Lycoming's test cell before delivery, of course - and all cylinders were 78/80 or 79/80. At 400 hours it measures between 74/80 and 78/80. There is a lot of variation in the measurements depending on how the rings align and the operator technique. I can get higher readings by a couple of psi than my A&P by jogging the prop back and forth or sometimes spinning it around a few turns and then remeasuring. My A&P can't be bothered and doesn't care. I only do it out of curiosity about how variable it is. Personally, unless you hear a lot of air escaping through the breather (rings) or the exhaust (valve) and/or the compression is really low, I wouldn't worry about it. At the museum, when we got low compression on a cylinder (usually a radial engine, but I don't think that matters) we would just run it up to temp and check it again. Often it was much improved.

Skip

[EricJ]

41 minutes ago, PT20J said:

There is a lot of variation in the measurements depending on how the rings align and the operator technique. I can get higher readings by a couple of psi than my A&P by jogging the prop back and forth or sometimes spinning it around a few turns and then remeasuring. My A&P can't be bothered and doesn't care. I only do it out of curiosity about how variable it is.

^^This.   If you're willing to futz around with it a little bit sometimes you can make it come out however you want, which is why there are a lot of opinions that these types of leakdown compression tests aren't a good measure of engine health, which I agree with.   If you always do it yourself and are consistent about how you do it, long-term trend can be useful but even a short-term anomaly is often not a big deal.

41 minutes ago, PT20J said:

Personally, unless you hear a lot of air escaping through the breather (rings) or the exhaust (valve) and/or the compression is really low, I wouldn't worry about it. At the museum, when we got low compression on a cylinder (usually a radial engine, but I don't think that matters) we would just run it up to temp and check it again. Often it was much improved.

Skip

To me this is the main utility of a compression leak-down test, to get some idea of where the air is going if there is a significant leak, so that you can analyze what's going on and/or make a plan about how to deal with it.   Sometimes that's just getting an idea of what to monitor over time.    Following up a really bad leak with a borescope inspection is generally a good idea, but on a brand-new overhaul I think it'll be more about just keeping an eye on trends for break-in.

[A64Pilot]

To a great extent it depends on the equipment used to check compression.

My set is the one you use for Continentals, it has an orifice that you open up and induce a calibrated leak, then with it leaking you adjust the pressure until you get a reading of 80, so as long ad your cylinder isn’t leaking more than that calibrated leak, you will get a compression of 80.

Some testers have no calibrated leak, so the only way you can get a true 80 reading is with zero leak, and that’s not likely at all.

74 at 30 hours is low, but what are you going to do? If it’s not burning excessive oil and makes good power then it’s fine.

You may want to listen to see where the leak is at, if it’s exhaust valves then that may need addressing.

[Fly Boomer]

7 hours ago, rturbett said:

What is a normal expectation for compression results on a freshly overhauled IO-360-A1B6D  (under 50 hours)

Is there any any kind of scale that compares engine hours and expected reduction in compression over time?

 

Thanks,

Rob

 

 

 

Lots of good answers above, but Mike Busch would say something along the lines of "Doesn't matter -- inspect the valve heads, valve faces and seats, cylinder walls, etc. using your Vividia VA-400 V5 borescope.  That will tell you if you have a problem."