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750 cylinder block on 650

Discussion in 'XJ Modifications' started by desmotom, Jul 20, 2016.

  1. desmotom

    desmotom Active Member

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    wondering if its possible to use pistons and cylinder block from a 750 motor on a 650...not sure where the extra stroke comes from on the 750.
     
    desmotom, Jul 20, 2016
    #1
  2. k-moe

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

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    XJ650 bore: 63 mm, Stroke: 52.4mm

    XJ750 bore: 65.mm, Stroke: 56.4mm

    The stroke comes from the crank.

    The castings will swap, but I've never done that so I can't speak to whether or not you'll need to make changes to keep the compression ratio, or to keep moving parts from kissing.
     
    Last edited: Jul 20, 2016
    k-moe, Jul 20, 2016
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  3. MiGhost

    MiGhost Well-Known Member

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    The 4 mm difference in the stroke is handled by connecting rods, and/or piston pin placement.

    https://books.google.com/books?id=_...age&q=yamaha xj 650 piston pin height&f=false

    Cycle World Magazine June 1981 ppg44-50
    Cycle World Test: Yamaha 750 SECA


    The 750's pistons shorter overall measuring 56.1mm from top of dome to bottom of skirt, and shorter from the center of piston pin bore to the deck as well, measuring 25.7mm. The XJ650's pistons are 59.8mm tall overall, and have a deck height of 26.2mm. But the 750's pistons are actually taller from pin center to skirt bottom 27.8mm to the 650's 27.5mm.

    Piston dimensions were changed to suit the longer stroke and hold down reciprocating weight in spite of the larger bore. The 750's pistons are just .17 oz. heavier. 5.64oz. to a Maxim piston's 5.47oz. The 750's connecting rods are longer but otherwise dimensionally identical to the 650's rods.


    Piston Pin Height (measured from pin center to piston deck)
    650: 26.2mm
    750: 25.7mm (0.5mm shorter)

    The 750 connecting rod must be at least 2mm longer to clear the longer stroke length.

    Shorter 750 pin height with a shorter 650 rod would leave the piston significantly below deck causing extremely low compression. Without the correct specs it would be a wild guess as to the actual piston location.

    If you are trying to make the engine a 750. You would need to use the crank/rods along with the cylinders/pistons.

    ~Ghost
     
    MiGhost, Jul 25, 2016
    #3
    Stephen Downey, Jetfixer and k-moe like this.

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