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  1. #21

    Re: Gas engines rebuild cost?

    FWIW: Although they are good for finding an obvious problem like very low or no compression in a cylinder, I'm not a big fan of compression checks alone for engine health. From a useful information point of view, the compression check provides maybe 1/4 of the info you need and a LEAKDOWN test provides 3/4. Heck, you can pretty accurately tell an engine's condition with only a leakdown but you can't do that with only a compression check. Of course, the compression check should come first because if it shows one or more bad cyls, the engine has to come apart anyway and there's no reason to bother with the leakdown at that point.

    Compression test can be misleading because carbon buildup on the piston/combustion chamber/valves can cause a compression reading to be even higher than specs. This makes one think that the engine is in REALLY good shape when actually it is not. A leakdown test will give you an accurate picture of what's really going on.

    A search of "leakdown test" will find tons of articles describing the equipment/procedures. It takes a lot longer than a compression test and some additional equipment.
    Mike P
    San Miguel de Allende, Mexico; Kent Island MD; San Antonio TX
    1980 53MY "Brigadoon"

  2. #22

    Re: Gas engines rebuild cost?

    Mike if compression is even and not burning oil then can one assume rings are good? Leak down it a little tougher to do and equipment expensive or at least for a good set.

  3. #23

    Re: Gas engines rebuild cost?

    I've got my airplane in for its anual inspection right now,one of the main tests is the leakdown. They rate the cyls at a % based on 80 being 100% 80/60 is marginal . The test is good for finding leaking valve seats and cyl cracks since we're talking AL jugs. But the test is subject to the person doing it. You can move the prop back and forth and unseat the rings to make it look good or bad. Also a little oil in the cyls will bring the #up. And its all done cold. I think the cranking compression test tells more of the engine health but no one wants to stand next to a spinning prop.I think the leakdown is more of a diagnostic test to determine where the problem is coming from.
    "DON'T BELIEVE ANYTHING YOU READ OR HEAR AND ONLY HALF OF WHAT YOU SEE" - BEN FRANKLIN




    Endless Summer
    1967 50c 12/71n DDA 525hp
    ex Miss Betsy
    owners:
    Howard P. Miller 1967-1974
    Richard F Hull 1974-1976
    Robert J. & R.Scott Smith 1976-present

  4. #24

    Re: Gas engines rebuild cost?

    I would say that if the compression is within specs and there is no sign of oil burning then the ring/cyl wall condition is "good enough".

    Normally, "within specs" for a compression test is no more than 10% variation between the high and low cylinder. For a good engine builder that spec is way too much but it IS the standard for factory service work.
    Mike P
    San Miguel de Allende, Mexico; Kent Island MD; San Antonio TX
    1980 53MY "Brigadoon"

  5. #25

    Re: Gas engines rebuild cost?

    I think if the engine is running, making good power, not burning oil, and giving no problems, then one should keep running it. But hey, we all have opinions.

    Hours are better than sitting.

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