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Thread: Low compressions in (2) cylinders..

  1. #1
    grbamford's Avatar
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    Default Low compressions in (2) cylinders..

    5731D is for annual this week. Looks like I have two cylinders in the mid to low 60s and have been trending down the last couple of annuals. Any advice on what the next step ought to be?

    Greg

  2. #2
    Troy Hamon's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Low 60s in a Lycoming is not good, but it is airworthy. How much have you been flying? I would fly it hard, 2450 rpm, for 10 hours, then see where it is. Engines don't like sitting, so if it is not flying much, that can be a problem.

    My 2 cents.

  3. #3
    andya's Avatar
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    Default Re: Low compressions in (2) cylinders..

    It would be helpful to know why it was loosing compression, intake or exhaust valves, or rings.
    That would help deciding which way to go.
    "Progress is our most important problem"

  4. #4

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    Default Re: Low compressions in (2) cylinders..

    I'm cautious about reading too much into a bleed down test (what's called a compression test when it really isn't). The numbers can vary a lot, over the course of a few hours. The numbers can also vary a lot based on how the test is performed.

    For example I can show you numbers down around 60-62 in a cylinder, then move the propeller tip 1/2" or less one way or the other to find the sweet spot and have numbers in the mid 70s.

    I personally put a lot more stock in what the cylinder looks like with a bore scope. There should be no rust on the cylinder walls and an even cross hatch pattern, there should be no scuff marks from the aluminum piston rod pin, the exhaust valves should have a nice bullseye shaped pattern on them, and you should not be able to see any deposits on the valve stems and valve seat, when the exhaust valve is fully open.

    When I bought my 7KCAB it had been both infrequently flown (every 2-3 weeks, not bad but not used weekly) and flown as a trainer where it was left full rich all the time, on the ground and in the air. I'd asked for a bore scope as part of the pre buy, and it either wasn't done or his standards were a lot lower than mine as the #2 and #4 exhaust valves had an asymmetric heat pattern on them, as well as deposits on portions of the valve seats.

    Before I did my own bore scope inspection, I'd flown it for awhile leaning 100 degree ROP in flight and just short of stumbling on the ground, rather than full rich all the time. When I finally inspected it, it the deposits had sharp edges on the ends, and it appeared they were flaking off the valve seats, so I figured I'd fly it at higher power power settings (2400-2500 rpm rather than 2150-2200 rpm) and while still leaning 100 degree ROP and check it again at the next oil change. I had a rainy day after 18 tach hours and looked at it again. The exhaust valve and seat in the number 4 cylinder looked perfect, and the exhaust valve and seat in the number 2 cylinder had only a hint of color on a very small section of the edge of the exhaust valve and the deposits on the seat are almost gone. That's significant improvement in just 18 hours, and trending away from the warped and eventually potentially swallowed valve that would have otherwise happened had it continued to be run fill rich.

    -----

    Your mileage may vary but the advice someone gave above to fly it at 2450 rpm or so for 10 hours and then recheck it is sound advice. Running it at 65% power with higher EGTs and CHTs using a reasonably lean mixture may help if you're collecting lead deposits on the exhaust valve seats.

    The O-320 A and E series 150 hp engines have 7.0 to 1 compression pistons and were designed to use 80/87. While 100 LL has about half as much lead in it as the old 100 octane green av gas, it still has four times the lead content of 80/87. As a result, running an O-320 A or E series engine at overly rich mixtures, especially at low power settings where the combustion temperatures are low, can cause the lead in the combustion gases to condense on the valve seats, valve stems and spark plugs (the lead condenses out of those gaseous compounds at around 1300 degrees).

    Leaning aggressively on the ground helps reduce the build up at lowe power, and leaning aggressively (just short of stumbling while running the engine at 1200-1500 rpm for a minute prior to shut down can also help scavenge some the lead.

    Keeping the combustion temps up in flight, and keeping the cylinder head temps around 350-375 degrees will make a big difference. (I should note here that with winter coming on the air entering the engine is denser and you'll be developing more power at the same rpm, so CHTs running in the 300-350 range are still fine - and getting higher CHTs might not happen again until spring.)

    The O-320 B and D series 160 hp engines have 8.5 to 1 compression pistons and were designed for 91/96 octane av gas. They are more lead tolerant, but also develop higher combustion temps and CHTs at similar RPMs, so they tend to have less of an issue with lead deposits when burning 100LL.

  5. #5
    Administrator Steve Pierce's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Where is the air coming from. Listen at the exhaust pipe, carburetor air box and the oil filler tube. Inspect the valves with a borescope and see what the valves look like.
    https://www.aopa.org/training-and-sa...e/valve-safety
    Mostly what I see is worn exhaust valve guides. The compressions start creeping down. The valve is wallowing around in the guide and doesn't seal well on the valve seat thus leak exhaust gasses. Pictures of my wobble test fixture hee on how to check those valve guides.
    https://www.shortwingpipers.org/foru...t-Oh-CHIT-quot

  6. #6
    Gilbert Pierce's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Pull the rocker cover on those two cylinders. If the inside of the cover on the exhaust side and the exhaust rocker is black you have leaking exhaust guides. The intake side will be noticeable cleaner in this situation.

  7. #7
    Marc Davis's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Savvy aviation has a lot of info on his youtube channel about this issue and how to read the tea leaves.
    Short story:
    Pay much less attention to leak down tests.
    Take the best reading you can get from the leak down test (not the worst).
    Do regular bore scope inspections of the valves faces and seats.

    I just replaces a jug that was low on compression and burning oil. The scope told the hole story, bad intake valve guide caused oil consumption. The back of the intake valve was very cruddy on that cylinder. The oil burning lead to the rings getting coked up and stuck. Removing the cylinder confirmed all this. The new (low time) cylinder is running great and oil consumption is low again.

    https://www.youtube.com/channel/UCc-...zvGsAfMzH2lIcA

  8. #8
    Gilbert Pierce's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Here is a bore scope example of what Marc explained above. I had the same issue a couple of years ago. The picture was taken in the open intake port.
    Attached Images Attached Images

  9. #9
    Marc Davis's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Here is a picture of the back of my intake valve looking through the sparkplug hole.

    2019-09-22_15-20-04-71.jpg

  10. #10
    Marc Davis's Avatar
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    Default Re: Low compressions in (2) cylinders..

    Here is a picture of a good exhaust valve. Note the symmetric bulls eye pattern showing even heating. Asymmetry would indicate impending valve failure. Savvy explains this in detail.

    2019-09-22_14-44-31-64.jpg

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