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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 (STX-RC) 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.

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Is the crankset the same? Some cranksets "sink deeper" in the bottom bracket, so if you have a different crankset, that might be an issue. Besides, even the same crankset sinks differently around different bottom brackets, due to spindle taper. If that is the case, do you think it's possible to tighten the bolt a bit more? That would bring the crankset closer to the front derailer. –  heltonbiker May 14 '13 at 20:02
sheldonbrown.com/bbsize.html –  mattnz May 14 '13 at 22:43
Same crank, yes. I took the crank off the old BB, put the new BB in, and then installed the same crank onto the new BB. I don't think I could tighten it any more without stripping the bolt. I tightened it quite a bit. –  unsavory May 14 '13 at 22:49
Yes, the two external measurements of a BB are spindle length and spindle offset. (There are also cartridge body/bearing race and cup dimensions, of course.) You can have BB with the same spindle length and different offsets. Some BB cartridges (such as Phil Woods) have two movable cups and can be adjusted left/right by a few mm, but Shimanos generally have a fixed cup on one end and cannot be adjusted. –  Daniel R Hicks May 15 '13 at 0:45
I discovered that the lip on the new BB is keeping it from being screwed in far enough, as the old one had the cup on the drive side and sat flush with the frame. I'm going to try grinding the lip off so I can screw the BB into the frame further. –  unsavory May 15 '13 at 18:21
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1 Answer

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Despite the the spindle length is 113, but as you observed, the offset is different - so the cranks are sitting in different positions. Not all Shimano BB are made symmetrical - and UN55 BC 1.37 x 24 113mm. means its a 1.37*24tpi thread, with a 113mm shaft - not mention on the offset of the shaft. Based on all this, evidence is that you have an imcompatable BB.

Presuming you have correctly refitted the crank (so it is seated properly) - What to do about it - best would be to go to a 110, or even 107 mm BB - this will move the crank to the centre of the bike and fix the problem, also providing better alignment.

Alternatively, are you certain the derailleur won't move out further. Best thing to do it unhook the cable outer from the frame (or shifter) so the cable is certainly free and not stopping the shifter moving to it's outer most position, and release the outer limit adjustment screw on the derailleur. If the derailuer then moves out far enough, you should be good to set up, as it's "just" adjustment of cable and limit stops. If not, you will need a different BB.

It is not possible to install the BB backwards - the right hand side has a left hand thread.......

Edit: In response to OPs comment..... The problem is then to do with the BB getting to the correct chainline - MTB sets should be about 47.5-50mm. If you cannot find the correct sized BB, and a 110mm is too short for the left crank, you have two other options - try a different left crank, or (Perhaps for the really desperate only) I have seen this web article about grinding off the lip on the fixed right hand side, allowing you the thread it into the frame further, thereby to give the ability to adjust the chain line. (I would get a local engineering shop to use a lathe.....)

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Thanks for your comments. Part of the issue with a shorter spindle is that the left side crank is already almost touching the the rear frame as it comes around. Any closer and it would either touch the frame, or my feet would be too close to it as I was peddling. Yes, I'm sure the derailleur won't move out further, as I first checked this with the cable removed. –  unsavory May 15 '13 at 18:18
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