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  1. #1
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    XT 770 vs Dura Ace 7900 BB

    My XT BB needs to be replaced and I have a DA7900 on hand. Looking at them both I can see little difference. Anyone see a problem with using this Road BB on a mtb? Are the spindles the same size on Shimano road cranks and mtb cranks? The cups thread right into the frame but I didnt want to force my XT cranks into the BB if the ID is smaller on the road BB bearings. Looks like the only difference is the part numbers.
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  2. #2
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    The road BB's have a slightly narrower drive side bearing as I recall, plus they are designed for a 68mm BB shell where as the mtb BB's are designed for a 73mm BB shell and spaced out for a 68mm shell. Therefore the road crank BB's have a shorter spindle than the mtb crank spindle.

    So if you could space out the road BB correctly so that it equals the same width as the mtn BB then it should work.
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  3. #3
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    road vs xt

    Not 100% sure on DA7900, but Shimano road BB has 1/4 inch shorter plastic sleeve and maybe 2~3 fewer threads on the cups.

    Road sleeve:
    Photobucket

    XT sleeve:
    Photobucket

    Left is XT cup and right is road:
    Photobucket

  4. #4
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    Thanks for the replies but it looks like I may need to replace the crankset. The NDS bearing is totally locked up and it scored the spindle pretty bad. The DS bearing is fine and didnt even leave a mark on the spindle.

    I have one race left this season and then I was going to tear the bike down for service anyway now it looks like I will have to get it done in a hurry.
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  5. #5
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    It seems very strange that the hardened steel spindle has been worn by the cup's plastic inner ring. I would check the diameter with a Vernier. Almost sure the spindle is ok

  6. #6
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    Quote Originally Posted by Vindiu View Post
    It seems very strange that the hardened steel spindle has been worn by the cup's plastic inner ring. I would check the diameter with a Vernier. Almost sure the spindle is ok
    Strange indeed, have you ever noticed that the plastic liner on cables will wear down the frame where it rubs ... the spindle is pitted and gouged past the point that I would use it again
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  7. #7
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    The mountain cups are narrower by the width of one of those plastic spacers (2.5mm iirc). If you have at least one of the spacers behind your cup you can remove it and run a road bb if you need to. On the topic of your crankset it's fine. Clean the 'race' up with some scotch bright and call it a day. The plastic cups are there to help cover mis-alignment and imperfections in the system. This mostly means the BB faces being out of phase. However it will also cover up the less than perfect race you have on your crankset. Keep in mind that that race doesn't actually contact the bearing itself and it's surface condition isn't any where nearly as important as you would expect in a cup/cone bearing. Unless your worried about it shearing off I'd run it with confidence.

  8. #8
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    Quote Originally Posted by Zoke2 View Post
    Strange indeed, have you ever noticed that the plastic liner on cables will wear down the frame where it rubs ... the spindle is pitted and gouged past the point that I would use it again
    PVC liners can wear aluminum, for sure, but we're talking about hardened CrMo here. Not even a gauge or Vernier is needed there. As long as a fresh cup inserts there without radial play, the spindle is fine. As customfab says, that's not a race but a fit to tolerance diameter (h6 fit as far as I remember).

  9. #9
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    Thanks for the input. I will give it a try and clean up the spindle and replace the bearings.
    I used to ride to Win ... Now I ride to Grin

    While my guitar gently weeps, my bike sits there mocking me

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