Originally Posted by
pete4120
I'm looking to replace my bottom bracket + crankset on a mid-1980's Trek 830. It has a 68mm shell with the standard 24 threads/inch, so I'm good there.
Have decided to go with a Shimano UN54 BB but am unsure about what spindle length to choose. My spindle from the original "cup and cone" BB measures ~126mm (see picture). It's a "3T" designation, from Japan, as
mentioned by Sheldon Brown.
If I go with a Shimano 7/8 22/32/42 crankset, I've seen it written that a 110mm or 113mm BB spindle is required.
Which spindle length do I go with? Close as possible to the original length or the one recommended/required by the crankset?
Thanks a lot!
Considering that the "original" BB spindle is 126mm, and the recommendation for your "replacement" is 13 to 16 millimeters shorter than that, I'd say that you should install the longer (113mm) recommendation. If there's too much clearance and the derailleur doesn't shift to the outer ring, install the shorter. If you install the shorter BB and hard parts start running into each other, you stand the chance of damaging the paint.
When combining parts of unknown specification (what's a "Shimano 7/8 crankset"?), sometimes the best answer is "Try it and find out".