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I'm restoring a 1994 Mongoose Switchback (which I've had since college so I'm unreasonably attached to it). When I was removing the bottom bracket, a Shimano BB-UN 53, I cracked the plastic non-drive side shell, which is apparently super common. At my local bicycle co-op I picked up several cups that look like they would be the same, but they're not exactly the same - the cup depth that sets onto the cartridge body is shorter, and their overall depth is slightly greater.

When I thread them on, the bottom bracket seems very well secured, but there are still excess outboard threads from the left side cup. Aesthetics aside, is that mechanically acceptable? Or alternatively, is there a type of cup that I can use that won't have this problem, or is there a way of cutting the one (cutting a millimeter off the inboard side?) that I have so that there is no excess threading?

Obviously easy answer this question is "just buy a new bottom bracket dude, they are cheap", but I just want to see if there's a way of completing the rebuild with the things that I have (or without wasting this bottom bracket that is otherwise functional).

Original left hand side cup , would go completely flush if it were not broken and I could fully torque it:

original left hand side cup , would go completely flush if it were not broken and I could fully torque it

Another plastic Shimano cup that sticks out when fully seated:

 another plastic Shimano cup that sticks out when fully seated

Metal cup same deal:

metal cup, same deal

The three left cups I have in profile view:

 the broken cup, and the other two in profile view

  • I doubt trimming the end will help - these cups tend to have a taper on the inside, and that's what does the holding, so trimming the end off may not make a lot of difference. Have a look on the inside and see how it compares. I'd suggest a washer cut out of plastic to fill the gap. – Criggie Nov 25 '19 at 9:01
  • Shouldn't there be a locknut on the non-drive side. The right cup goes in all the way and the left side is used to adjust the free play before being locked. – Carel Nov 25 '19 at 14:17
  • There wasn't a lockring on it before, nor was there a place to put one; it was just flush. And in the book diagrams I have (Zinn) it's only the loose-bearing bbs that are shown with lockrings. – joseph_morris Nov 25 '19 at 16:22
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    A lockring is only found on the older style adjustable bottom brackets with caged ball bearings, the BB on the bike in the post looks like it has cartridge style bearings, these bearings are non adjustable and in general there is no lock ring used on this type. – Maarten -Monica for president Nov 25 '19 at 17:42
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As long as the BB is being held in place properly the stick-out of the cup shouldn't be an issue. Make sure the BB can't move from side to side when putting axial force on it. Also make sure the crank arm on the side of the cup that sticks out isn't rubbing against the cup. As long as those conditions are met you should be fine.

The forces on the bottom bracket in axial direction will be quite low under normal use conditions so I'm pretty sure the plastic cup will work fine but if you want to play it safe use the metal cup since the threads won't strip nearly as easily as on the plastic cup, also the tool-interface (the grooves that the tightening tool falls into for tightening the cup) can more easily be damaged on the plastic cups (I've had cases where they have become very stuck in the frame).

Do make sure to apply some sort of grease or anti-seize product on the threads especially on an aluminum frame to prevent the metal cup from seizing. They can be very difficult to remove in some cases

I have seen bikes which have spacers/rings in between the non-adjustable cup of the BB and the frame, similar to this: enter image description here

You could consider using such spacers to properly center the BB in the frame. Do keep in mind this may mess with your chain alignment a bit. The difference should be so little though that it shouldn't cause issues. if you find that it does cause issues you could potentially solve it by using a BB with shorter or longer axle length.

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  • I would be inclined to use a plastic cup rather than a metal one because there is more risk of the metal one damaging the threads in the frame which can be an extinction level event. It is better to get the cup tight and I think his suggestion of trimming the inside of a plastic cup is the best idea. – user68014 Nov 25 '19 at 18:50
  • @ user68014 I understand where you're coming from but I think it will be very difficult to strip the threads of the frame if they are in somewhat ok condition even with the metal cup since the tool you use to tighten the cup will slip/come out far before it is tight enough to damage the threads, this is assuming there is enough thread contact but it looks like at least 80% of the cup threads are making contact with the frame threads on the image in the OP. If you are somehow clamping the tool onto the BB shell you can tighten it a lot more so might strip threads then – Maarten -Monica for president Nov 25 '19 at 19:18
  • Thanks for the detailed response Maarten. One question on chainline since you brought it up: it seems like, if the right hand threads are in the same position as before (lip flush to the BB shell), then the cartridge body and spindle would have to be in the same lateral position, and so chainline should be the same as before, despite the different left-hand-cup . . . does that seem like correct logic to you? – joseph_morris Nov 25 '19 at 19:34
  • @ joseph_morris Yes indeed if the non adjustable BB cup is in the same position (and you are using the same axle length BB) then the chain line won't be affected. Only when you would use spacers/rings under the fixed side (which i suggested as one of your options) the chain line will be altered. However if you have a front derailleur you can most likely compensate for this by adjusting said derailleur slighty. – Maarten -Monica for president Nov 25 '19 at 19:38
  • @Maarten Wasn't thinking so much of thread damage during tightening but of any erosion that could happen if you ride around with it at all loose. – user68014 Nov 25 '19 at 20:25

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