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78 yz400 piston question

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  • 78 yz400 piston question

    Alright, I am looking at putting in a new piston and boring out the cylinder. I have everything apart but I am getting a little confused.

    The bore currently measures between 85.28mm and 85.42mm checked at eight points along its length. Standard bore is 85mm.

    The piston that I took out is a .25 over (its stamped on the rings and the top of the piston)

    Only issue is that when I measure the piston the largest reading I can get is 84.83mm?

    What's up with that? Can a piston actually wear down that far? Or did they accidentally stamp the wrong number on a standard piston?

    With these measurements my piston clearance is .45mm which is huge compared to the .05mm it should be.

    What do you guys think?

  • #2
    I would suspect that the PO or whoever worked on it may have substituted a piston from another bike that was 'close enough'... but that's just a guess.
    Fast, Cheap, Reliable... Pick any two

    '78E original owner - resto project
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