A word about cheap compression testers
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  1. #1
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    A word about cheap compression testers

    A valuable tool in any mechanics toolbox is a compression tester. Many times on the forums we ask for compression readings to help somebody verify that they don't maybe have a blown engine. Many times people will come on saying they have 90psi on all cylinders. We know this reading is low but they admit they bought a Harbor Freight or Autozone compression tester. I just fixed another Harbor Freight compression tester this evening simply by putting in the correct schrader valve.

    THAT'S RIGHT.....CHANGE OUT THE SCHRADER!!

    The correct one is available online or from Advance Auto Parts---->https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds

    This gauge now reads almost exactly as my trusty old Penske gauge. So before you throw that cheapo tester out, try changing out the schrader.
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  2. #2
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    Re: A word about cheap compression testers

    Thanks for the tip. I have a nice old tester that you have to read as you crank because that valve leaks down. I will have to get one of those.
    2, 1998 TS 770 L, restored and running great as of July 2019 (NO THANKS to the person at SBT, who put the WRONG crankshaft bearing into the rebuilt engine I got from them)

  3. #3
    Top Dog JC-SuperJet's Avatar
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    Re: A word about cheap compression testers

    Good Tip, if I say so Myself...

  4. #4
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    Re: A word about cheap compression testers

    Good info on compression testing and gauges here from GroupK: https://groupk.com/index.php/compres...s-measurement/
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  5. #5
    PWCToday Newbie thudd3rski's Avatar
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    Re: A word about cheap compression testers

    I tried an hf gauge and it read way low...can you recommend a decent compression tester for use once in a while?

  6. #6
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    Re: A word about cheap compression testers

    Quote Originally Posted by thudd3rski View Post
    I tried an hf gauge and it read way low...can you recommend a decent compression tester for use once in a while?
    just curious... Why not try the tip the OP mentioned to start this thread? How do you know it read way low?

    I am really not trying to be snarky here, but the most common use of a compression tester is to compare the values between cylinders. Variation is not good. For this purpose, most any gauge works fine. If they are much different when you test, you have a problem. If you used your HF gauge to test your cylinders at some point, and they were the same, then the next time you tested, they are different, then you have a new problem.

    IF you are an engine builder or serious tuner, then an exact PSI reading is more important to you. But when you get to the point where you are milling heads, changing deck height, or head gasket thickness to reach a certain PSI, you are way past even needing to ask "what is a good compression tester?"


    You need to READ the link posted by the Linkman!!
    Last edited by hemmjo; 11-29-2021 at 07:10 AM.
    2, 1998 TS 770 L, restored and running great as of July 2019 (NO THANKS to the person at SBT, who put the WRONG crankshaft bearing into the rebuilt engine I got from them)

  7. #7
    PWCToday Newbie thudd3rski's Avatar
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    Re: A word about cheap compression testers

    to start...i did read the link posted by linkman...but thanks

    i verified the hf readings back to back with a snap on compression tester...harbor freight tester 60 psi...snap on was 150 psi. the cylinders measure within 5% of each other. looks like the issue with the hf tester is the hose that comes with it...it has no valves in it, so the volume of the hose and fittings is added to the relatively small space actually being tested. according to the link posted by linkman, this can result in a 25-30psi error. my experience with the hf gauge was even farther off than that. had it been within that delta i couldve lived with it

    i am not an engine builder or serious tuner...but would like a decent gauge to test compression before i buy a used vehicle/pwc, especially one that wont start because it has been sitting for a couple of years full of bad gas. being in co...our compression reading are low anyways, add a gauge that reads over 50psi low and you start wondering if there is a gaping hole in the cylinder somewhere

    now hf sells more than one tester...maybe the "nicer" one has better accessories and the error can be resolved by replacing the valve...just guessing here
    Last edited by thudd3rski; 11-30-2021 at 12:30 AM.

  8. #8
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: A word about cheap compression testers

    Maybe, try replacing the schrader valve. There are MANY different variations of the schrader valve. Get a NEW.......CORRECT valve specifically for a compression tester. All of mine have white colored plastic on the valve.
    Last edited by Myself; 11-30-2021 at 10:34 AM.
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    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

  9. #9
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    Re: A word about cheap compression testers

    The pressure it takes to OPEN the schrader valve is always mechanically subtracted from the gauge reading.

    You can plumb a compression tester to an air hose with a pressure regulator and a gauge on the "cylinder side" of the tester. As you increase pressure with the regulator, the gauge on the regulator side will always read higher than the tester gauge, unless you are decreasing regulator pressure. (assuming both gauges are accurate) The difference is the pressure it takes to open the schrader.

    Then as you mentioned, there is the volume in the tester hose, so the location of the valve is important also.

    As referenced in the linked article, there are so many factors affecting compression pressure. Once more, unless you are a serious builder or tuner, the absolute pressure it not nearly as important as the precision of the RELATIVE pressures between cylinders. The change in pressures on the same engine over time is a good way to keep track of the health of an engine. BUT, keep in mind that getting a gauge that suddenly reads higher, does not magically make your engine more healthy.
    2, 1998 TS 770 L, restored and running great as of July 2019 (NO THANKS to the person at SBT, who put the WRONG crankshaft bearing into the rebuilt engine I got from them)

  10. #10
    Top Dog JC-SuperJet's Avatar
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    Re: A word about cheap compression testers

    Uhmmm...don't know where on the Internet I read this, but

    CHANGE OUT THE SCHRADER!!

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