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Xj750 to 900 parts fitment information?

Discussion in 'XJ Technical Chat' started by Thomas Agar, Jul 31, 2020.

  1. XJ550H

    XJ550H Well-Known Member Premium Member

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    Another thought is that if Yamaha could have gotten away with thinner cylinder sleeves to start with they would have done that save themselves some money I'm sure they're engineers have reasons for this current stock sleeve thickness in the maximum replaceable piston sizes. Anything can be done it's just how long it's going to last. Sometimes I do like to trust Yamahas engineers were correct and what they did.
     
    Last edited: Aug 4, 2020
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    Last edited: Aug 4, 2020
  3. Minimutly

    Minimutly Well-Known Member

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    If you take that to its logical conclusion they'd still be selling the xj....

    However - what your saying is not wrong - but really, what has it to do with the op's question? He isn't Yamaha, doesn't have a 891, and isn't going to be selling hundreds of thousands of them. In the real world you would simply measure up the wall thickness, subtract 30 thou and make the judgement. Pontificating about the skills of Yamaha engineers of yesteryear doesn't really help?
     
    Last edited: Aug 5, 2020
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    yess he already has some sort of big bore kit in the 750 jugs. which is ok as it is his. he wants to drag all day up to and past red line. big question is how often he wants to rebuild replace jugs while persuing the fastest run. his motor/trans is already beat to shi$ . well he find an 891 in the US I doubt it, he will have to find someone willing to ship one from europe good luck with that and very expensive.
    he would be better off getting a US 900 motor and expanding on that. with his wish of stronger parts. have some pistons made for over size.


    I have found 2 sources for big bore kits oddly cheap and one guy waiting for his dad to come up with a price on his nos bigbore kit and cam shafts on FB.
    I get the desire to chase the lowest 1/4 mile, but quite frankly I do not think the op has the skill set to design/upgrade maintain a machine like that .
    his posts infer this.

    maybe i read to much into his post
     
    Last edited: Aug 5, 2020
  5. Thomas Agar

    Thomas Agar New Member

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    @XJ550H this bike wasnt/isnt being built to pursue the fastest run. Its bracket racing, consistency over speed. My goal as originally stated is for it to be as strong as it can so it can repeat the years of racing its already done. Not only that but this is a backup/training bike, better believe i want it as strong as possible when my gal or nephew decide to race. If i can achieve better flow while adding the strength of the 900 head(or bigger valves with the 750 head), great. Will it actually be revved passed stock redline, that will depend on the valvetrain and also if there is any power further up the rev range when it gets dynoed. I appreciate your faith btw. When it comes down to my skillset, my main 2009 Buell 1125r drag bike is way more complicated the any xj and ive raced/worked/maintained it for years.

    When it comes to the juggs, id have to call the sellers of each the 853 and 891 juggs for measurements to see if they would even fit in the 750 case. Then determine which has the best sleeve choice for the pistons i have. Id love to measure the set i have now, but unfortunately its still at the machine shop
     
  6. Minimutly

    Minimutly Well-Known Member

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    Yes, you need to start measuring the jugs - personally I'd be surprised if they fit the 750 casings, otherwise they would have sufficient "meat" to take the 891 pistons - plus some overbore allowance (thats assuming Yamaha ever made oversize 891 pistons)...
    Let us all know?
     
  7. Jerrod Robinson1086

    Jerrod Robinson1086 Member

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    I have a Similar idea, but I'm not sure it will Work. I want to Bore My 750 Maxim to 67mm and run 900 Piston's with the 56.4mm stroke. If My calculations are right that should be 795.39cc's "800". But instead of installing large Valves in the 750 Head it appears the 900 Head may bolt right in. I could be wrong about that though, if anyone knows better please let Me know
     
  8. Jerrod Robinson1086

    Jerrod Robinson1086 Member

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    Also does anyone know where I can get a set of 67mm High Compression Piston's? I've found a few Candidate's I think will Work, but I can't find anything designed for the XJ. I've heard there used to be Kit's, but their hard to find now.
     
  9. Jerrod Robinson1086

    Jerrod Robinson1086 Member

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    This is my number 1 Contender so far. I think it's for an old Honda 3 Wheeler. I can't see what would stop Me from Running 4 of them in an XJ, but I will admit I'm in a little over My Head with this Project. It's not really a Project, just an idea at this stage...
     

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