Nigeyy

If I recall correctly, there was a thread in the past where someone vented about Shimano compability (or lack thereof), particularly when it came to rear cassettes.

Well, recently I had to replace the BB in a bike frame. No problem I thought, I'll use an old BB of the correct size I have lying around, it's pretty good quality -only problem is that the removable cup was broken when I removed it a couple of years ago. I'll see if I can use the current removable cup from the Shimano BB in there.

So guess what? On one Shimano BB the removable cup is on the left, on the other (also Shimano brand) BB the removable cup is on the right.... WTF? I can't interchange the cups as obviously the threads aren't correct. But why do that? There is absolutely no sense to that at all to me.

Anyone any theories on this?

d_D

If you have a UN72 then neither cups are part of the shell and can be removed. Given a bit of use either cup can end up attached to the BB when it is removed from a frame. A bit of persuasion should free the cup attached to the bb shell.

mechBgon

In many cases, the ones with the fixed cup on the left are actually adjustable-bearing (if you have the unusual tool required to reach in there and do that... very few people outside a bike shop would, though). Perhaps it suited their engineers better to put the adjusters on the left due to epicyclic torque considerations, or so the adjustment can be done without removing the driveside crankarm. I dunno.

Big picture: your LBS can probably supply you with the cup you need

Nigeyy

I don't think it's a UN72, but a lesser model. I'll give it a go to see is the cup does come off on the other side too though. It seemed to be very solid when I tried (as I did think it might be the case that both cups could be removable).