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  1. #1

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    compression testing

    How do you guys check compression on a stripped down engine. I have a couple used engines with no housing for mounting a pull starter. I'd like to test the compression before going any further, but I have two questions. 1. If I use a starter motor to turn it over, will higher rpm's result in high/misleading numbers? 2. If the engine has not run in some time, the rings may not have the right amount of oil on them to create a seal. I could lube the cylinder walls, but would that also cause a wrong reading?

  2. #2
    Moderator w8ye's Avatar
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    compression testing

    Compression reading by itself is only a relative reference mainly because of the factors you mention.

    You would have to make as many factors the same as each other and compare too engines (A new one and a old one of the same type) for it to have a concrete comparison.

    People that mess with them all the time develop a pretty good feel for an engines worth by putting a little oil in the cylinder and pulling it through compression several times. I try it fast and slow so I can feel the "leak - down" as well as the over all compression. While I'm at it, I feel for the piston - rod fit as well as the main bearings. I don't bother with the compression gauge itself.

    Good luck,

    Jim
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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  3. #3

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    compression testing

    Ditto.

    However, I wouldn't put oil in the cylinder. This would result in a higher reading, unless of course you need oil to turn the engine over, then why would you test it anyway. To see what I mean, try it both ways on the same engine. If there is no change in compression, and your sure the cylinder was dry before hand, then the engine is no good. The engine is worn enough so that the viscosity of the oil will not seal off the wear. Not sure if I worded that correctly. Anyway you get my point.

    All non-RC engines are tested without oil, even two cycle outboard engines and motorcycles. If someone puts oil in the cylinder(s), before testing, of something you are going to buy, they are trying to put one over on you.

    Perry
    \"Lying about your planes and engines to your wife is not a sin ........ I think\".

  4. #4

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    compression testing

    Thanks, pretty much what I have thought. Just a point about the oil; I wasn't thinking a heavy oil. That would surely raise the compression reading. I was thinking there is a certain amount of oil in the fuel mix and if an engine is tested after having recently run, it could have more oil in the cylinder than one that has been sitting on a shelf for a few years.

  5. #5
    Moderator w8ye's Avatar
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    compression testing

    Oil might be 'OK' if you always use the same kind of oil.

    Judging an engines condition by feel and not necessarily by a compression Gage, is sort of a 'practice' that you build up the technique by 'feeling' of every engine you come across. Books and spec sheets cannot give you 'feel' so they have to rely on specifications.

    Enjoy,

    Jim
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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    compression

    I would just take spark plug out and pour in a ounce of the fuel mix you are going to run the engine with. Crank it over several times to get mix all through engine parts. Put compression gage in place....crank engine like you would to start it. That should give you pretty accurate reading. Put plug in and check compression by hand propping engine. Remember oil /fuel mix you just used is the same mix you are going to use to start engine. The use of any oil with high viscosity will give false compression readings. Good luck Capt,n
    Imagination is far more important than knowledge. Albert Einstein.

    http://www.lambertsrc.com/

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    RE: compression

    I do not know how this laptop got me here....still a good subject ......
    Imagination is far more important than knowledge. Albert Einstein.

    http://www.lambertsrc.com/

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    RE: compression testing

    I presume gas two strokes are the topic of discussion. If you think about it a pull starter with a vigorous pull will flip the motor over quite fast so I wouldn’t worry about using an electric starter. If it has anything over 90 lbs compression it is run able. It will only get better as the engine becomes oiled up from running and the rings reseat themselves. Compression checks should be done with the throttle wide open to allow unrestricted air flow.

    More important than compression is crank case sealing. If there are leaks the engine will become hard starting or unrunable. Using snoop or a soapy water solution, roll the engine over slowly dripping the snoop on the crankcase seals and joints. The odd small bubble is nothing to be concerned about but obvious bubbles or the snoop being sucked inside the engine is a definite warning sign. At this point minimum would be seal replacement most likely bearing and seal replacement.

    If there is compression and no obvious air leaks from the seals or crankcase joints go for it the engine should run. OOPPS my bad meant 90 lbs above.

    Dennis

  9. #9
    Moderator w8ye's Avatar
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    RE: compression testing

    With chainsaws, you screw in the compression gauge to the spark plug hole, Then pull the chainsaw through compression strokes until the gauge quits rising. This is usually a half dozen pulls.

    Most good chain saws will pull 150 pounds. If they get below 120 pounds they are hard to start.
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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    Original AMA #31261

  10. #10
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    RE: compression testing

    I often repair used yard machines, and of course, convert engines for airplanes. When I convert them for airplanes I just automatically use a new Frank Bowman ring because I want max power, easy starting, etc., and what's 11 bucks for a new ring in the grand scheme of things??

    As for the yard machines, I cheat a little. I shoot some WD-40 into the spark plug hole, and then turn them over several times. Then I check the compression. In my experience, anything below 90 psi is probably not worth fixing. Anything below 60 or 70 lbs. probably won't even start. 100 or over still has life in it. This has proved out over numerous years of repairing and selling the machines.

    The compression reading will vary according to the original compression ratio of the engine, the exhaust port timing, the amount of carbon build up on the piston and head, and of course ring wear. I figure that my WD-40 treatment leaves more or less the same amount of oil to help seal the rings as a normally just run engine would have for the compression test. The injection of premix (oil/gas) and then turning it over numerous times before taking the reading has merit as well.

    As mentioned, be sure to block the carburetor wide open when checking the compression.

    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

  11. #11
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    RE: compression testing

    Other than what's been said here, the only thing I do differently to distinguish from an engine in decent shape versus trash heap is I pull the head off and turn the engine over swiftly. I stick my nose over the cylinder and note the 'pop' and air (fuel charge) flow from the transfer ports. If you get a good pop and can feel a little wisp of air coming from the transfers, then it should run at the very least 'O.K'. If it doesn't pop good and the air flow is minimal, it's at the very least in need of a new ring(s) and light honing.

    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.



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