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1981 XJ750 Seca Drag Bike Looking For Information & Opinions

Discussion in 'XJ Technical Chat' started by Thomas Agar, Jun 24, 2018.

  1. Minimutly

    Minimutly Well-Known Member

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    well please forgive me, I am no expert on the innards of an xj engine, but the information I have is that the 900s had the rod bolts upside down, the 650 and 750 didn't. But if you're sure...
    Bore area times stroke (in cms) times 4...
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    correct on the 900 bolts in "upside down " for the clearance

    seca rods have 5g200?? numbers on rod. his are likely aftermarket
     
    Last edited: Jul 26, 2020
  3. Thomas Agar

    Thomas Agar New Member

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    @Minimulty, let me apologise. I need to take any information I get from what's seen and verify it, because at this point nothing has been what it's supposed to be. Anyways I think you may be right. When I looked at my bore and stroke measurements, i assumed I measured off. I calculated just a second ago and they read ~67x60. That would be a 900 if my information serves me right. Plus the part numbers on the rods start with 31a which is the same the 900 rods start with (the ones I found on eBay).

    Part number on the side of the road is 31a0001
     
  4. k-moe

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

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  5. Thomas Agar

    Thomas Agar New Member

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    I'll have to do that, seems like I'll need to get a new transmission anyways as the spare is for a 750 and I read that the 900 trans is stronger. I'll definitely do the undercut gears though. Thanks for the recommendation
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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  7. Thomas Agar

    Thomas Agar New Member

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    What ive found out so far

    The engine code reads as a 81 750. First could digits is 5G2 which points to the 5G200 of the 750

    Bore & Stroke of a 900 67x60

    Head of what i believe is a 750 as it only has 4 cam towers per cam instead of 6 cam towers of the 900

    Rods im 90 percent sure are from a XJ900 with 31A0001 stamped on the side

    Still havent found a photo of any pistons that have the same size home as the ones i pulled out. They are yamaha brand though as per stamped underneath.

    Head and Jugs are at the machine shop to find the cc of the piston dome and help me out with a compression ratio

    Thanks for the help so far everyone
     
  8. Thomas Agar

    Thomas Agar New Member

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    Forget everything i said in this. I must have read the gauge wrong. I miked the base circle and then measured the lift. Looks like the intake life is .380 exhaust is .360

    The cam i mentioned wasnt even for the xj750. Must have been late when i measured and researched.

    valves are standard size 33/28
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    What is the short vin on the motor

    If you click on link I supplied above you can put it in that link and find out what your motor is. That would be a lot better can measuring up parts etc and guessing
     
  10. Thomas Agar

    Thomas Agar New Member

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    If you read my message second to last, it says the code reads a 81 750 seca. I did use your link :)
     

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