Compressions on an O-470 - Backcountry Pilot (2024)

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Wed May 07, 2014 11:43 am

Compressions on an O-470

72,65,72,75,72,75 is the compressions.. any concerns on the one at 65? I honestly do not know enough about how to use compression readings. This is on an O-470

thanks

29singlespeed offline
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Wed May 07, 2014 12:51 pm

Re: Compressions on an O-470

How many hours on engine. Do you fly it much. I flew my last O-470L with all six in high 50's and low 60's and did not worry. My current engine has 300 hrs and all in low 70's. Just fly the crap out of it.

Tim

qmdv offline

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Wed May 07, 2014 1:21 pm

Re: Compressions on an O-470

Hot or cold measurement? I would not lose any sleep over a 65. Fly for five hours and measure them all again. I'd be surprised if you got the same measurement twice on two cylinders, and chances are the low one will come up... What's the oil consumption like?

A good read on cylinder compressions:

http://www.avweb.com/news/savvyaviator/ ... directed=1

"Never allow a cylinder to be pulled on the basis of a single compression test -- unless the gauge reads zero or something close enough to zero that you're pretty sure you have a hole in the piston or a chunk missing from the exhaust valve. If the compression is simply weak (as opposed to non-existent), fly the airplane for another three to five hours and then recheck the compression again. There's at least a fair chance that the cylinder will pass muster on the re-test, and that you'll be spared the expense and hassle of an unnecessary cylinder replacement."

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Wed May 07, 2014 2:05 pm

Re: Compressions on an O-470

Differential compression tests are pretty poor indicators of engine condition. It's possible to have a perfectly good cylinder with a low reading, and to have a really bad cylinder with a pretty good reading.

Lycoming recommends using an automotive compression tester, which is a better test of the actual function of valves, rings, cylinder condition, etc.

I sure wouldn't worry about one cylinder at 65 on one test.

MTV

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Wed May 07, 2014 3:25 pm

Re: Compressions on an O-470

There are a lot of good articles out there. My compression is highly variable. One might linger in the 50's, and then be solidly in the 60's from then on out. Another might be at 78 and go dead a year later. I've come to believe that compression numbers are a pretty poor way to gauge cylinder health alone. They float up and down, depending on hot or cold, and still make rated HP unless they get down to some very low number.

I use the test to check exhaust leakage or cylinder leakage for obvious problems and tracking down oil consumption somewhat. I use a cheap video borescope to look at the valves and cylinders.

My mechanic says he had oil issues or a hot cylinder one time on his plane, and the compression check turned out fine. He pulled the cylinder anyway, and the rings and spacers fell out in pieces. The walls showed more evidence than the compression check.

When I get a reading below 60, I check again in 5 hours or so and have gotten better numbers generally on the re-test (until I had to actually top the thing finally).

Good luck.

lesuther offline
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Wed May 07, 2014 3:31 pm

Re: Compressions on an O-470

I had an io470 in a Debonair which had compressions fro 55 to 65 for ten years, used a bit of oil 1 qrt in 4 or 5 hours, ran like a clock. Continental have a service buletin somewhere allowing comps to get down to the mid 40's provided some other factors are good.

ashysouthaus offline

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Wed May 07, 2014 7:40 pm

Re: Compressions on an O-470

Hey pal - The advice above is good. Put several hours on it and those compressions will likely improve.

On a Cont. the compressions should be used only as trend analysis.

Are you still looking at buying the bird?

jaudette offline

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Wed May 07, 2014 10:07 pm

Re: Compressions on an O-470

Thanks all. The numbers were from the last annual, 700SMOH ~30STOH .. so not much worry. I read the AVWEB article. Thanks for all the information.

jaudette - getting closer!

29singlespeed offline
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Thu May 08, 2014 7:48 am

Re: Compressions on an O-470

Here is the Continental service bulletin that deals with how to do a cylinder Inspection.

http://www.tcmlink.com/pdf2/sb03-3.pdf

TangoFox offline

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