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If the BB shell's drive-side and NDS threads are aligned on the same axis, can the cartridge be backed out of the drive side while the NDS cup is still in place in the shell?

Edit: When I tried to do this as an experiment the cartridge got pulled away from the NDS bearing, so that the drive-side spindle got shorter:

enter image description here

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Most cartridge BBs have a shoulder on the drive side, which is also the side that's integral to the bearings and spindle, plus a non-shouldered left cup. Some are the opposite, and some have no shoulders. Some have two similar cups, one of which may or may not be shouldered, and then the bearing and spindle unit is a third piece.

Drive vs non-drive side is unimportant to the question you're asking. Shouldered vs non-shouldered is though. What's important is that when you're done, it's all properly torqued against each other. If there's a shouldered side, that needs to be torqued against the frame, and the non-shouldered side needs to be torqued against the main BB unit.

If what you're asking is if on a common BB with a shoulder on the drive side and the bearings and spindle integral to that piece, and an unshouldered left cup, whether you can leave the left cup untouched when you take the the main unit out, no, that's not really ideal. Unscrewing the drive side in that situation can easily rotate the NDS side out a little, such that when you reinstall the DS, proper contact with the NDS cup isn't made. Or, if it doesn't move at all, the ways things settle could cause you to be torqueing the BB against the left cup and not the shoulder against the shell. The better practice would be take out the DS, unscrew the NDS a turn, torque the DS back into the frame, then torque the NDS down properly against the DS.

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  • I removed the cartridge in that way as an experiment, thinking it could confirm any thread axis misalignment. This is a problem BB shell (and the US distributor has agreed to replace), only 67mm wide in one place and the faces not parallel, and showing signs of slight welding distortion (lift) on the NDS of the shell. The NDS hole is like a double-exposed photo of a circle. When I removed the cartridge with the NDS cup in place, there was greater resistance than usual as it was being backed out. Once removed, I saw that the NDS bearing had been pulled away from the cartridge about 5mm.
    – Tim
    Mar 8 at 11:54
  • Shoulder drive-side, no shoulder NDS.
    – Tim
    Mar 8 at 12:02
  • @Tim Got it, sounds frustrating. Every time I've ever dealt with a bike like that, it was fine after tapping and facing the shell, but nothing else would come close to working. There was one where the BBs I had to work with at the time were all the plastic left cup type, and it kept cracking them. Fine after tapping/facing, felt perfect. That it's 67mm is unimportant if using something like a square taper cartridge, but would need to be corrected with a spacer to use most external BBs with it. Mar 8 at 17:49
  • I described it backwards: it was the cartridge that had been pulled away from the NDS bearing -- the drive-side spindle got shorter.
    – Tim
    Mar 8 at 22:22
  • @Tim Either way, it's the kind of thing that mostly only happens when the two sides are out of axis due to welding distortion. Even if something like that doesn't happen with the next one, bearing lifespan and feel will still suffer until it's fixed. Mar 8 at 22:26
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Yes. I've never seen a NDS side piece that locks into the cartridge.


I think you're seeing the cartridge as one part - looking something like this:

enter image description here

But remember its in TWO pieces - the main body that threads in from the drive side, and a much smaller supporting ring with a slight taper on the inside to support the Non-drive side.

Parktool website

The pictured unit should unscrew out the drive side once you remove the crank and spider and chainrings. In an extreme case you might be able to reach around with sliding pliers and slowly back the whole thing out, but that would be no fun at all.

Upshot

  • There are no two threads that have to align.
  • Even if they did, ones a left hand thread and the other's a right hand thread in 99% of bottom brackets (french and italian BB threadings are weird)
  • The NDS spacer generally has a lip that would stop it threading in too far, there's no good way to wind that all the way through the BB
  • And most threaded BB shells are slightly smaller in-between the threads, because that's the metal into which the threads were cut. That is, the crest of the threads is no-higher than the metal from which it was cut.
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    I assumed the cartridge could be backed out if the thread axes were aligned, that the spindle on the NDS would not move eccentrically if so. I've added an explanation of the issue as a comment to Nathan's answer above.
    – Tim
    Mar 8 at 11:58
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    The alignment I was speaking of was along the same axis. L cup ]----- axis-----[R cup
    – Tim
    Mar 8 at 22:02

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