Borrall Wonnell says...

Blliteration aside,

Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle)
that seems to be off-center on my '02 MTB (with 68mm BB shell). I
think I have installed it correctly...the fixed cup (DS) is screwed in
as far as it can go, such that the flange is pressing against the side
of the BB shell. The free cup is inserted as far as it can go (with a
couple of threads showing). The BB is square taper.

The gap between crankarm and chainstay is larger on the NDS by about
2-5mm. I swapped the crankarms and placed them in various positions on
the spindle to rule them out as a source of the problem...in all cases,
the gap was larger on the non-drive side.

Assuming the frame is symmetrical (it appears to be, and is in good
condition), is this kind of discrepancy acceptable/expected? Have I
installed the BB wrong? Am I doing something else wrong?

Thoughts are appreciated,
Dave

An Isis BB I installed recently had a spacer and an o-ring that were
supposed to be used or left off depending on your BB shell width and
desired chainline. The o-ring is only there to cover up the gap on the
non-drive side cup because it can't be fully inserted on a 68mm shell.
The spacer was for the drive side 68mm shell. With a 73mm shell, both
were left off. The new outboard bearing cranksets all have spacers on
the drive side for MTB models with a 68mm shell and I believe MTB's have
the drive side crankarm further from the BB than the non-drive side by
about 5mm to accommodate triple chainrings and wide, thick chainstays to
clear wide tires. For Isis and Octalink, MTB BB's have spindle widths
of about 113mm and narrower 108mm spindles for road doubles. There are
other typical spindle widths for road triples and downhill MTB's. The
only way to know for sure the correct spindle width is to know the
manufacturer specs, because some cranksets don't follow the typical
patterns. Are you sure you have the correct spindle width? Were there
spacers included that maybe you should have used?