1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

XJ900 58L or 4BB1 891.4 cc

Discussion in 'XJ Technical Chat' started by Simmy, Jun 11, 2021.

  1. Simmy

    Simmy Well-Known Member

    Messages:
    2,261
    Likes Received:
    1,126
    Trophy Points:
    113
    Location:
    Waterloo Ontario Canada
    Addressing this to those here who have an XJ900 85 onwards with the real 900 motor.
    The 891.4 cc with the 68.5 mm bore.
    Looking for the height of the cylinder block.
    I've began building a 900 and am considering using the 1985+ 68.5 bore cylinder block.
    There are always some from Germany on eBay
    The height is a factor as I don't have much room to spare.
    I don't expect you'll have the motor apart but with digital calipers you should be able to get a decent measurement.
    The 650 Turbo block is almost 77mm.
    The pic below is the XJ900 31A (853) approx 77.4mm
    cylinder block height.jpg
     
  2. Franz

    Franz Well-Known Member

    Messages:
    3,646
    Likes Received:
    1,037
    Trophy Points:
    113
    Location:
    Scotland UK.
    I have 78mm 4BB engine but it's not a digital gauge l have.
     
  3. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    What's the story with these 900s? Why make an 850, call it a 900, then make a 890 cc engine? Same bore longer stroke - or did they change both? I think My 900 is an early one.
     
  4. Franz

    Franz Well-Known Member

    Messages:
    3,646
    Likes Received:
    1,037
    Trophy Points:
    113
    Location:
    Scotland UK.
    The first bikes with the handlebar mounted fairing had handling problems according to the road testers. Increasing the engine size was one of the things Yamaha did when they fitted the frame mounted fairing. That's what l read in the Haynes manual.

    Bore
    31A model 67.0mm

    All other models 68.5mm

    Stroke 60.5mm same for all engines.
     
    Last edited: Jun 11, 2021
  5. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    I guess they needed more horses, and had room in the casing so the old addage about not being able to beat cubic inches came into play. So if my old 900 engine has knacked bores I could find a later set of jugs.
     
  6. Franz

    Franz Well-Known Member

    Messages:
    3,646
    Likes Received:
    1,037
    Trophy Points:
    113
    Location:
    Scotland UK.
    Yes might be an option Minimutly if you can get a good set with the pistons and rings.
     
  7. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Did they bore the 850 jugs or cast new ones I wonder - depends how much metal there was around the 850 ones I expect?
     
  8. Franz

    Franz Well-Known Member

    Messages:
    3,646
    Likes Received:
    1,037
    Trophy Points:
    113
    Location:
    Scotland UK.
    Don't know better just getting 900 ones for your bike.
     
  9. Simmy

    Simmy Well-Known Member

    Messages:
    2,261
    Likes Received:
    1,126
    Trophy Points:
    113
    Location:
    Waterloo Ontario Canada
    that’s what I’m trying to determine, I assume the 891.4 block would have to be taller to keep the compression ratio at 9.6:1.
    With @Franz measurement of 78 for the 891 and what I’ve measured my 853 at 77.4 they are indeed different heights.
     
  10. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Hmm, I wouldn't make my mind up with just two samples. And, if the bore is bigger, the area above the piston will be bigger, and the volume at tdc as well, so maybe those extra ccs might not raise the cr by much (and that's assuming the piston heights are the same). You really need two open on the bench to be sure. What about measuring liner outer diameters?
     

Share This Page