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  1. #1
    Sly Fox DKFox's Avatar
    Join Date
    Feb 2009
    Huntsville, Alabama

    Question Can I use larger rings to get compression back in this case?


    I've got 2 1996 GTI's. They are identical except one is a single carb setup and the other is a dual carb setup. I rebuilt the top end of both 720 engines at the same time. Used Pro-X pistons and broke them in the exact same way. This was all 4 months ago and each has about 10 hours ride time on them now. The dual carb setup has turned out great. Still has 145 psi both cylinders and it runs like a champ. Pistons still look brand new. The single carb setup ski has slowly lost pressure. Started out at 120 psi mag side... and 115 psi pto side. (Why not higher I don't know.) I knew this wasn't ideal but I didn't tear it back down. The ski has never had the "get up and go" the dual carb ski has and it has slowly gotten worse. I checked compression again last night. Now its 120 psi mag side and down to 90 psi pto side. Both pistons still look great! The cylinder walls still look great.

    Here is my question:

    I hired out the boring of these cylinders to a trusted machine shop. They do this stuff all the time. However, did they just goof up? Can I correct it by just replacing the rings with .1 mm larger rings in it... or do I have to go through the whole boring and buying new pistons and rings again? I would go ahead and do both cylinders while at it to try and get closer to 145 psi like my other ski. The compression loss is not due to loose bolts or torn gaskets. I've already checked all that.

    Suggestions are welcomed.

  2. #2

    Join Date
    May 2006
    Yorba Linda
    Simple answer, NO.

    that is way too low of compression for a fresh top end, I would tear it down to find why it was not right in the first place, it could be doing more damage as it is running.

    What was the ring end gap when new?

    Was there a top and bottom to the ring?

    Were they installed correctly?

  3. #3
    I agree with 2strok....was the ring or rings installed correctly? Semi-keystone rings are sometimes hard to tell which side is up, especially the bigger bores. Some have no marks. If piston clearance is right, then it doesn't need boring again. And the correct rings should be used.

  4. #4
    Banned User
    Join Date
    Sep 2009
    Id say pull the top end and find out the clearance between the piston and cylinder, if its too much Id say the shop ows you some over size pistons.. If you paid good money and got a receipt they should fix the problem no issue.. if you went in and offered the kid that cleans the machines 20 bucks to throw them on the machine.. no receipt needed.., then your shit outa luck.
    Regardless you need to pull it apart to check rings and clearance.

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