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  1. #1
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    New question here. Replaced Shimano Bottom Bracket, now front derailleur won't shift to largest gear.

    OK, so in my quest to get my old Trek 7000 ZX MTB running again after a long retirement, I am having an issue with replacing the bottom bracket.

    The bike came with a Shimano LP25 that is marked BC 1.37 x 24. The spindle has LL 113 stamped into it.

    So I ordered a Shimano UN55 BC 1.37 x 24 113mm.

    They should be exactly the same size right? Well the new one seemed to be offset to the right (drive side) by about 1/8 inch, even though the spindles are the same length.

    Now my front derailleur won't shift to the largest outside sprocket even when the adjustment screw is all the way out. What can I do to correct this issue? I thought the only thing that mattered when replacing a bottom bracket was the thread size (68mm) and the spindle length.

    Another strange issue is that the orginal LP25 has the threads on the L side, while the new UN55 has the threads on the R side. I could have sworn the threads were on the drive side when I removed the bottom bracket, but now I'm not so sure given the markings on it. Unless the bottom bracket was originally installed backwards? Would that explain the 1/8 inch offset discrepancy?

    Thanks for any help. I'm really confused at this point.

  2. #2
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    Common bottom bracket widths are 68 and 73 mm. I'll bet you've got a 73 BB in a 68 shell.
    The left and right sides are threaded differently so you can't have it swapped left for right.

  3. #3
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    Could you expand on that? How could I have a 73 BB in a 68 shell? Is that something that comes on a brand new bike that you buy off the shelf?

  4. #4
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    Quote Originally Posted by unsavory View Post
    Could you expand on that? How could I have a 73 BB in a 68 shell? Is that something that comes on a brand new bike that you buy off the shelf?
    To the last part of your OP, the Shimano LP bottom brackets were configured differently than modern models. They unscrewed from the non-drive side, as depicted here:

    http://bernd.sluka.de/Fahrrad/Shiman...-LP25_1995.gif

    Yours is fine. The modern UN models will screw in on the other side, but won't affect anything.


    The problem you are having is interesting. Have you checked if both are 68mm? Can you take a picture of both? Can you visually see that they are both the same length, and that the threads line up?

    Is the crank seated fully on the new spindle?

  5. #5
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    Thanks for the help everyone.

    I took a couple of photos this morning as suggested by jt. I'm pretty sure how I need to go about getting this to work at this point, but someone let me know if this sounds like a bad idea please.

    First the pics:
    Replaced Shimano Bottom Bracket, now front derailleur won't shift to largest gear.-shimano1.jpg
    Replaced Shimano Bottom Bracket, now front derailleur won't shift to largest gear.-shimano2.jpg

    • Threads match.
    • Spindles are both 113mm.
    • Spindle heads are identical.


    After looking at them for a bit, it came to me. The lip on the new Shimano BB is keeping it from screwing into the frame far enough. On the old one, the cup screws in from the drive side and sits flush with the frame with nothing protruding. On the new one, the lip keeps it from screwing in all the way, which is why the crank sits about 1/8th of an inch or more out from where it should.

    So my idea, is to shave the lip off the new BB. Does this sound like a bad idea? My thinking is that any modern Shimano BB is going to have that same lip, so I'm probably not going to find one with the same spindle length that gives me the same chainline.

  6. #6
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    Measure the width of the bottom bracket shell on the frame. It will be either 68 or 73 mm wide.

  7. #7
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    Did you try to adjust the derailleur? A new BB can often mean that the cranks can often sit at a slighly different position in relation to derailleur, particularly of the taper on the old BB was worn.

    Tim

  8. #8
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    Your old BB is 73 mm. Your new one is 68 mm. Slash had it backward. The distance from the end of the spindle to the shell of the BB is greater on 68 mm bottom brackets than on 73 mm, which is why the crank sits out further. This is so that frames with the narrower BB width would have the same chainline as frames with 73 mm BB. Basically, more spindle pokes out on the drive side of the 68 mm BB.

    Just looking at your pics, the old one looks longer too. Slap a 73mm BB in there and you'll be fine. Don't shave off the lip of the new bottom bracket.

    I'm assuming this is your bike, and confirms the BB shell width.

    1997 Trek 7000 - BikePedia

  9. #9
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    JT you are right! I can't believe it, as I've spent days on the internet trying to figure out why this bottom bracket wasn't fitting correctly.

    For some reason I thought the 68/73mm measurements were referring to the hole size in the BB shell. I'm American so am used to inches, not mm. =)

    Thank you all for helping me uncover this mystery. All I want is for my poor bike to be back on the trails again.

    So last night I already ground the lip off my 68mm BB, and after doing so, it slipped right in and everything lined right up. So should I still look into buying a 73mm BB and replacing this one? Or do you think this one will work fine?

    Replaced Shimano Bottom Bracket, now front derailleur won't shift to largest gear.-shimanoground.jpg

  10. #10
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    Quote Originally Posted by unsavory View Post
    So last night I already ground the lip off my 68mm BB, and after doing so, it slipped right in and everything lined right up. So should I still look into buying a 73mm BB and replacing this one? Or do you think this one will work fine?
    Glad to help! So I think the lip serves to secure the BB laterally in the shell. Without the lip, the bottom bracket doesn't have as much purchase on the shell and can rotate under stress. Usually the combination of the lip against the shell and the non-drive side threads sandwich the bb so it doesn't move.

    You may not have any problems, or you may find that the bottom bracket moves. Or is not perfectly aligned, causing you chainline issues. Your safest bet would be to just get a new one, but you can always use your current setup until it gives you problems.

    Good luck!

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