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Xj750 to 900 parts fitment information?

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

  1. Thomas Agar

    Thomas Agar New Member

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    So I figured id make a new thread instead of cluttering up the drag bike.

    So can you make a 750 a 900? The engine is already torn apart so i could frankinstin soemthing as the drag motor is kind of already anyways. Im not interested in the 853cc version but the 891cc version.

    My engine already has 1983 xj900 31a rods in it with a 68.5mm highcompression piston.

    Can you get the 900xj valves to fit by sending it to a machine swap? If not can i use a 900 head on a 750? I know the cams will be different. Are the cranks as well?

    Id have to find new pistons as the valve reliefs i dont believe will accommodate the 36/30mm valves

    Whats the maximum 750 bore? 68.5 is the 891 dimensions.

    Or if anyone has a 891 xj900 engine they want to get rid of let me know. Im having trouble sourcing one stateside.

    Trying to make the heart of the engine as stout as i can within reason. As it is, ill be sourcing a transmission from the 900 for the 750 case along with the swingarm and shaft (i know its a bit longer). Im just going to put the strongest stuff i can find to hold up to drag hits over and over

    Thomas
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    Sure why not we had somebody turn a 650 and to an 883.
    Why not find out what motor you actually have by using the short vin?
    A 900 motor will fit in a 750 Frame. On the cylinders between exhaust pipe 2 and 3 you'll see the size of the motor stamped .
    Your motor does not have 900 crank covers but the 650 and 750 crank covers will fit the 900 motor.
    Why look for a 900 motor when you may already have one?
    And there was no reason to start a new thread. It's usually easier to keep all the posts in one thread
     
  3. Thomas Agar

    Thomas Agar New Member

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    I did use your link and posted my findings in the drag thread yesterday i believe.

    Why an actual 900 when i already have the displacement of a 900? I have a 750 built with the bore and stroke of a 900 yes. But its not as strong as an actual 900

    Here is a couple things i dont have being a 750
    Bigger valves
    6 cam towers per cam instead of 4.
    The transmission is stronger.

    Thats the purpose of this thread, to figure out if i can adapt what i have to make it that or get the correct engine from the get go
     
  4. Minimutly

    Minimutly Active Member

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    Bigger valves? Does anyone know how much bigger the 900s valves are?
    Stronger gearbox/gears. Is this just about material or treatment? Does anyone know for real.
    Cam towers - don't see any real gains here, unless the 750 cam bearings had issues?

    Oh, if indeed you do find that you haveto use your fitted pistons, you can have them recut.
     
  5. XJ550H

    XJ550H Well-Known Member Premium Member

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    next step is to look at the jugs and confirm it is a 750 jug that has been turned into a 900. stamped ito jugs
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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    750
    750 seca valves.PNG
    900

    900valve1.PNG
    9002.PNG
     
  7. Thomas Agar

    Thomas Agar New Member

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    @xj550 Went over to the machine shop and confirmed this morning. 5g2 is on the jugs and head. Confirmed the size of the valves as well as being the 750 valves.

    @Minimulty as xj550 has posted. 750 has 33mm Intake valves and 28mm exhaust while the 900 has 36/33
    ~ive only read in one place so far that the turbo 650 and 900 transmissions are stronger.
    ~the benefit of the cam towers is just additional strenght when im making pass after pass going to redline or higher depending on what i do with the valvetrain
    ~the worry i have about the valve reliefs is there isnt much meat left before hitting ringland/ring groove area

    When building a race engine not designed to be the fasted engine at the track but something you want to last years, you should invest/put time into things that will strengthen the engine. Just my opinion

    Thomas
     
  8. Thomas Agar

    Thomas Agar New Member

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    Just try to keep in mind that this is a drag race engine guys, even if the xj's werent really drag/supersport bikes
     
  9. Minimutly

    Minimutly Active Member

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    Nothing wrong with trying to beef up a weakness, or adding valve area to get more power. I'd be looking for a low miles 900 engine, cheap enough over here. If you're going to explore the top end of the rev counter I would suggest gas flowing the head diy, but carefully.
     
  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    where did you find 5g2 stamped/cast into head and jugs?
     
  11. Thomas Agar

    Thomas Agar New Member

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    Xj550 I love going back and forth with you but can we keep posts related to the questions asked please. I started a new thread so the information requested could help someone else out as I haven't been able to find the answers from the searching I did.

    [​IMG]

    [​IMG]
     

    Attached Files:

  12. XJ550H

    XJ550H Well-Known Member Premium Member

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    it is called an exchange of information. you posted about numbers and I asked where they were found. nothing wrong with a little quid pro quo.
    we help you with answers , you will be asked questions.

    my understanding about the 891 motor is it is the one produced for the rest of the world and not the US. we got the 853 in 1983 then in 84 the 891 was produced.
    US did not get any 84 900s.
     
    Franz and k-moe like this.
  13. Minimutly

    Minimutly Active Member

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    I wonder why they made an 853 then an 891?
    What was wrong with the early engine?
     
  14. Thomas Agar

    Thomas Agar New Member

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    Yea that's why I don't have much faith in finding a 891 here. I'm sure it can be done, but a bit rare I'm sure
     
  15. k-moe

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

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    Nothing wrong with the earlier engine. The "more power" game was on in full force during the 80's, and every tiny increase in HP sold more bikes.

    I'm somewhat thankful that the 'bold new colors/graphics" game took over in the 90's. There were fewer changes to parts each year.
     
  16. Thomas Agar

    Thomas Agar New Member

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    Alright gents, i have to set a deadline just to keep stuff pushing for the bike. If no one comes forward with the answers the the original questions by the end of the week. Ill take one for the team and start ordering stuff from the uk. Starting with the head, cams and tranny. Id buy the jugs as well, but if i have a chance at making my pistons fit, ill need to get the 853 jugs and have them bored to my pistons. Followed by the 891 crank, as they dont cost much and im down to spend a couple bucks to see if the two cranks are different for the xj community.
     
  17. Thomas Agar

    Thomas Agar New Member

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    Let me correct my information as I was looking at the 900 camshafts on ebay noticed this.

    The 750 head has 3 camshaft journals per camshaft not 4
    The 900 head has 4 camshaft journals per camshaft not 6

    The extra towers I was pointing out arent journals at all, just a support for the valve cover bolts to go into.

    Thomas
     
  18. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    This might be risky; the 891 pistons are 1.50mm larger than the 853 pistons; that's more than the allowable oversize for boring cylinder jugs...........
     
  19. Minimutly

    Minimutly Active Member

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    More than allowable or more than made available? I would guess given how well these things last there would be little call for a plus 60 thou piston. The important dimension would be the thickness of the cast liner below the casting.
    It doesn't sound not doable, but then I've not eyeballed one.
     
  20. XJ550H

    XJ550H Well-Known Member Premium Member

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    When the maximum OEM piston size is used and then it needs to be replaced the cylinders get resleved.
    I would think going to high-compression motor infants sleep walls could lead to fractures.
    Occasionally I hear talk about 883 big bore kits for the 750 recently saw somebody offering one on Facebook.
    Possibly in the racing world we can constantly rebuild change cylinders making runs then rebuilding cylinders on a regular basis would be a reasonable thing but not something someone would want to do for a street bike
     
    Last edited: Aug 5, 2020

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