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I need replace my engine bottom crank case

Discussion in 'XJ Technical Chat' started by SqBobSpongePants, May 26, 2025.

  1. SqBobSpongePants

    SqBobSpongePants New Member

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    Hi all.

    I have an 1980 XJ650 4KO and I need to replace my engines bottom crankcase.
    Currently I can find a reasonable clean looking crankcase for a 1981 4H7. Does anyone know if the 2 cases are compatible?
    Thanks
     
    Last edited: May 26, 2025
  2. Franz

    Franz Well-Known Member

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    I don't think you can replace half the case as the crank main bearing areas we're line bored at the factory. We'll that's what l read on this forum.
     
  3. SqBobSpongePants

    SqBobSpongePants New Member

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    Shouldn't it be possible to replace the bearings too though?
     
  4. Minimutly

    Minimutly Well-Known Member

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    No, you have to keep top and bottom crankase halves together, as @Franz said, they are machined together.
    If you were to replace both, you will still need to check main bearing fitment.
    Is your lower case not repairable?
     
  5. Dan Gardner

    Dan Gardner Well-Known Member Premium Member

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    What is the condition of the crankshaft??

    I think we need more details and perhaps some photos
     
  6. SqBobSpongePants

    SqBobSpongePants New Member

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    Ok. So it's either repair or replace entire bottom end then. The crankcase engine mount has broken off (see photo).
    The engine was not run with the gaping hole there, so I'm relatively certain that no other damage was caused.
    I'm doubtful that the case can be repaired given that it a stress point?
    Thanks
    Steve
     

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  7. Franz

    Franz Well-Known Member

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    I read aluminium can be welded at high stress points but it would need to be Tig welded by someone with the expertise to do it properly.
     
    SqBobSpongePants likes this.
  8. SqBobSpongePants

    SqBobSpongePants New Member

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    There's a couple of places nearby that offer aluminium TIG welding services for engines. I'll call them.
    Ultimately, I'd really prefer not having to change the original engine number on this machine, so all this helps.
    Thanks for the advice all.
     
  9. Franz

    Franz Well-Known Member

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    @Minimutly knows about welding aluminium he might be able to offer you more advice.
     
  10. Dan Gardner

    Dan Gardner Well-Known Member Premium Member

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    Wow. That's some damage you don't see every day!
     
  11. bensalf

    bensalf Well-Known Member

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    i had exactly the same damage on my xj600 , it was atempted to repair by PO. with a 2 pack resin, obviously it leaked, so i split the bottom half case off and took it to a scooter repairers, who do this type of repair on Lambreter and Vespa scooter engine/shock mounts, the guy repaired it without any problems, he did say he would have to heat the whole half case up to enable the ally weld to take. good job done ,but it did cost me £130 GBP, but ive retained the original engine number
     
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  12. Minimutly

    Minimutly Well-Known Member

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    The welder will want just the bottom casing and the mount, I would recommend you get a propane torch and wire brush on both to remove the paint near the area to be welded, then clean everything scrupulously before presenting it to him. The weld repair is a 10 minute job, cleaning and prep potentially 2 hours... This is your best course of action.
    If you were desperate it could be tipped upside down, cleaned and welded in place, but I would not be recommending this unless you were stuck in the desert!
     
    Franz likes this.

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