I replaced my bottom bracket and now my crank arm bottoms out on the square shaft before it can be torqued. What did I do wrong?

Question

I just replaced my bottom bracket using another one of the exact same make and size (Sunlite 68mm x 103mm square tapered shaft cartridge). It installed smoothly and everything seemed to be going well until I tried to attach the crank arms. For some reason, the sprocket on the right arm is bottoming out on the square shaft before it can be torqued down at all, so the whole crank arm/pedal is forced to either be loose or to scrape against the flange of the bottom bracket.

This image better describes what I mean:

Here's what the crank looks like on the old bottom bracket - notice the gap, which is what I want:

The brackets are clearly different, but they have exactly the same text printed on them. What can I do to fix this?

Answer 1

Of note is that the amount of gap you have before the chainring bottoms out on the old cup is itself unusually close. That's fine if it works, but it helps to explain what's going on here.

There is some very small difference between the two bottom brackets. That is self-evidently true, although I can't say what it is or why. The gap you had before is so small that it's possible there's no production change that happened and both parts are within tolerance. It's possible to imagine that for a low-end BB like this the spindle length tolerance by itself is enough to cause this. It's also entirely possible that it's from a supplier change and the new company's 103 is a little shorter, or the dimensions of the cup are a little different. This is a basic repair part that J&B (jbi.bike) just buys with their part numbers stamped on it, i.e. from Cheng Huar or SR Suntour. They could have changed supplier, or buy some from each. Usually the difference in gap wouldn't be noticeable or meaningful, but it is here because it's so minimal even on the good one.

Options:

• If the chainline will still be acceptable, go to a size longer spindle, like 107. If a 103 was chosen before to get optimal chainline, most bikes won't care if its out 2mm more.
• On this kind of bottom bracket, the drive side cup is usually stuck to the bearing with retaining compound. If you can get it off without hurting the bearings, you could try replacing it with a non-shouldered cup that's meant to mate with exactly the same bearing (same internal diameter). The non-shouldered type nestle completely inside the shell. If you have access to a big pile of used bike parts (community shop etc) this could be practical. The cup has to be a correct match though - if it's only close, it can be creaky on the bike. You may need to put retaining compound back on to avoid slop. This solution is a little bit hacky, but if you can find a cup that has the same important measurements as the one that's on there, it's a good one.
• Similar to the above, you could cut off the shoulder of the DS cup if that still leaves you with deep enough tool splines to work with and doesn't compromise the part structurally (in other words if cutting it off left it able to look and work like a shoulder-less cup).
• If the above is impractical and you need this chainline, you could shop around for a non-shouldered-cup 103mm BB. Phil is the only consistently available one I know of. They're expensive but last a long time.