I removed the cup and come bb from this 1991 rockhopper and there was a decent amount of pitting so I replaced it with a Shimano cartridge bb un300 – same bb shell length 73 mm and 123 mm spindle (the original may have been 122.5). I should have checked this before (my first build, making lots of mistakes) but now that things are going back together I noticed the crank arms are significantly off center. 17 mm from the chain stay on the left, about 35 mm from the chain stay on the drive side. Also measured distance from the down tube and it is definitely off. Is that normal and ok?
by avg_quality_person
2 Comments
Don’t know how to edit, but upon further inspection it’s the new cranks that account for the difference. Bucklos off Amazon. The drive side definitely flares out more. Is there a term for the lateral difference from where the crank meets the BB to where the pedals mount? I did try looking this up before placing an order but couldn’t find anything about it.
Sounds like you installed the left side of the BB first ([manual](https://si.shimano.com/en/pdfs/dm/MDFC001/DM-MDFC001-06-ENG.pdf), page 17).
Remove the left side. Thread in the right side all the way until it bottoms out, and fully tighten it — the lip has to contact the bottom bracket shell. Then reinstall the left side.