Friend of mine brought his bike over for me to look at. Apparently the bottom bracket fell out during a ride. Rare as I felt it maybe, I offered to tighten it for him. Initially it looked like it'd be an easy job. just take out the arm, tighten BB, put arms back. done.

Upon taking off the crank arms and inspecting, i found out that the threads on the drive side of the bottom bracket shell was stripped. so much that i can just slide the BB (which visually still have all the threads intact) all the way in so the lip is flushed against the shell.

here are some info:

- cannondale aluminum frame, 2011 HT model. forgot exactly the model name. around $900 full retail
- English thread
- taper BB
- not sure what brand BB it was, but not shimano.

he says after he noticed the crank is loose and the bb fell off, he tried to push it back in into finish his ride. he is not very mechanically inclined so i don't think he was fixing it nor did i think he could've done much damage either. it was just a local dirt trail, nothing steep nor technical.

what i found strange and mysterious is how could this have happen. the only confirmation i know is the bb became loose and fell out. there are two possibilities:

- some how the bb wasn't tighten correctly and became undone. and somehow him trying to continue riding (just a few miles) managed to completely strip the thread - is it even possible for a loose BB to strip the shell's thread completely, in such a short amount of time?

- the thread was stripped due to poor installation or defect. perhaps it was overly tightened to start with, and over time the bb just worked it way out.

i have seen and known many people with bikes, this is the first time i have ever heard a BB can fall out in the middle of the ride. and i am having trouble understanding what could've caused it.

anybody has any experience with something similar or have a possible explanation?