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Thread: 65u engine ?

  1. #1

    65u engine ?

    So I'm working on a 1998 yamaha exciter 135 jet boat.

    It has a 65u engine. I'm rebuilding it at the moment. I pulled the cylinders off and I see the bore is at 84mm. The sleeves look thin. One place I read 81mm is stock bore then another place I read 84 is.


    The piston says 65u on top and I do not see any over size markings.

    Thanks guys.


  2. #2
    Echo's Avatar
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    Stock=84mm 65u with no other numbers indicates stock pistons. Boreing to 1.5 over, or 85.5mm is considered safe. Some guys push it to 2mm over, but some have experienced overheating at 2mm over.

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  4. #3
    Thank you. I measured the bottom of the sleeve and it was somewhere around 3mm. 1.5 over the sleeve would only be 1.5mm thick. Crazy.

  5. #4
    Echo's Avatar
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    I dont recall ever really looking at the bottom all that hard, I just always check the actual bore size if the pistons are questionable.

  6. #5
    Echo's Avatar
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    Also 1.5 over would be .75 cut off of the assumed 3mm, taking 1.5 off all the way around would result In a 3mm diameter increase.

  7. #6
    Yes, your correct.

  8. #7
    ZachAttack's Avatar
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    I had 85.5mm in my 65u in a ski and ran fine. No overheat issues at all. I used pro x pistons in that build.

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