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  1. #1
    Myself's Avatar
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    A word about 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. This makes 3 I've fixed, and a couple more that others have fixed after I told them this info. So before you throw that cheapo tester out, try changing out the schrader.


  2. #2
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    Quote Originally Posted by Myself View Post
    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. This makes 3 I've fixed, and a couple more that others have fixed after I told them this info. So before you throw that cheapo tester out, try changing out the schrader.
    The valves are usually color coded for sensitivity. Which valve are you considering to be correct and what is your source?

  3. #3
    Myself's Avatar
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    I've been using white ones. I bought a 3 pack at my local Napa last year.

  4. #4
    Team Bilford's Avatar
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    Quote Originally Posted by Myself View Post
    I've been using white ones. I bought a 3 pack at my local Napa last year.
    Good job. I was told to use the white ones several years ago. Red for tires, white for gauges.
    The best compression testers have the valve at the tip of the hose so that the length of the leader hose is not increasing the compression volume causing an artificially low reading.

  5. #5
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    Ideally, a compression tester would have an electronic probe and no hose to change the combustion chamber volume at all. I've looked and have not found one like this.

    Another alternative might be to put oil in the hose. Compression pressure would push the oil toward the gauge, but I would think that an air pocket would remain to keep oil from reaching the gauge itself. I have not tried this, so I'm not recommending it.

    EDIT: An electronic gauge that doesn't use a hose would have to give higher readings because the combustion chamber volume would be smaller than with a hose. I would venture to guess that the compression specs listed in a manufacturer's service manual are based upon using the specific gauge called out in the manual. For example, Kawasaki specifies a Kawasaki part number gauge, 57001-221, for their compression tests. The gauge sells for $135, and I'm sure it's the same gauge sold by other companies for less. OR, perhaps the compression numbers given in the specs are theoretical, i.e. based simply upon cylinder/combustion chamber volume dimensions. The plot thickens...
    Last edited by steve45; 11-28-2021 at 11:02 AM.

  6. #6
    Myself's Avatar
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    Quote Originally Posted by steve45 View Post
    The gauge sells for $135, and I'm sure it's the same gauge sold by other companies for less.
    OOoo, OOoo, pick me-pick me!!

    The gauge might be the same but Kawasaki ALREADY changed out to the white schrader LOL!

  7. #7
    Team Bilford's Avatar
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    Myself,
    The issue or concern that I have with most of the lower priced units like the HF is the use of adapters on the end of one basic hose. I am scared to death that the hose will unthread from the adapter leaving it in the spark plug hole. On an overhead cam engine with plug tubes, this would be a disaster. That's why I like the threaded fitting to be crimped onto the hose like those on the Snap On unit. What are your thoughts?

  8. #8
    Myself's Avatar
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    I haven't really worried about that, allthough, I HAVE had the adapter come off down inside a Seadoo head AND a Yamaha head. No big deal, they have a hex on them. I just stuck a socket on an extension down in there and unscrewed it, then used my long needle nose to grab it out. I have learned since then to put the adapter on tight then only snug the assembly down when testing so the adapter stays put on the hose.

  9. #9
    Team Bilford's Avatar
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    Quote Originally Posted by Myself View Post
    I haven't really worried about that, allthough, I HAVE had the adapter come off down inside a Seadoo head AND a Yamaha head. No big deal, they have a hex on them. I just stuck a socket on an extension down in there and unscrewed it, then used my long needle nose to grab it out. I have learned since then to put the adapter on tight then only snug the assembly down when testing so the adapter stays put on the hose.
    Lucky that you had room to get a socket around the hex. Dang, that must be one heck of a long needle nose.

    Even the mid priced units that have multiple hoses seem to never have the 12mm that we need without an adapter. I want the 12mm on a hose!

  10. #10
    Mod less, ride more! troyheb's Avatar
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    Nice post. Was having an issue with my Craftsman a few years ago, then DUH, it dawned on me. Changed the valve. Worked like it should after that.

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