Just noticed that the plastic non-drive side cup on my BB has cracked. I'm about 155 lbs, and there's only about 1000 miles on it. It's a Shimano UN-54.
One thing I am wondering - I installed it myself, using a torque wrench, so I'm pretty sure I didn't over torque it, but I did not chase and face the BB shell (not having the tools, and being too impatient to take it to a shop to have it done).. the frame was new, and while the threads were pretty smooth, there was definitely paint on the edge of the BB shell.
Could the uneven stress from the non-faced BB shell edge have caused the crack? Do plastic BB cups just suck?
Also, how bad would it be to keep riding on this for now?