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Piston interchange?

Discussion in 'XJ Technical Chat' started by Colin 85 700, Oct 17, 2019.

  1. Colin 85 700

    Colin 85 700 Active Member

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    Ok, now knowing ( thanks to you awesome people) the 700 is essentially a 650 bottom. End with 750 jugs, my question is this....
    Do 81-84 750 pistons fit the 85 700?
    They are both 65mm bore stock best as i can tell but every microfiche i can find skips 1984-1979 wtf?
    Both being 2 valve motors, unless the pin height is different should be the same
    Any help is appreciated
     
    Colin 85 700, Oct 17, 2019
    #1
  2. Simmy

    Simmy Well-Known Member

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    According to the parts fiche this piston is unique to the 700.
    Having said that there is always a way :rolleyes:

    When I grafted FZR1000 pistons and block to my 750X motor I assembled it with just #1 piston and the cylinder block.
    I greased the rings before assembly so they completely sealed this cylinder. Then I set the crank so #1 piston was top dead centre.
    With a graduated cylinder I filled the combustion chamber with WD40 to the bottom of the spark plug hole.
    Knowing how much fluid went in I was able to determine the resulting compression ratio.
    In my case I had a small amount to machine off the barrels to bring my compression ratio back close to 11:1 of the 750X.

    In the case of the 700, Yamaha would have had to lower the compression ratio somehow.
    If you put 65 mm pistons in the 650 motor and change nothing else your compression will go way up as you are cramming more volume into the combustion chamber.
    Yamaha could alter the pistons, the rods, the barrels, or the head, probably did all 4.
     
    Simmy, Oct 18, 2019
    #2
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    XJ550H, Oct 18, 2019
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  4. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The wrist pin height on the 700 pistons is not the same as on the 750 pistons. I can't think of what other differences there are ATM.
     
    k-moe, Oct 18, 2019
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