There is a small crack in my non drive side BB shell. Its just a Shimano plastic one. Does anyone know how this could have happened? Also, can I buy a replacement or do I need to replace the whole BB? If I can get a replacement should I get a metal one? I think this may be the source of the creaking in my cranks.

Thanks

perhaps you cranked it down too hard and it opened up over time.
if you can find a replacement cup go for it but i don't know where you would find one - i'm pretty sure they are all the same for the UN's..

Your LBS should have a replacement cup. Generally those plastic cups are trouble free, although they occasionally crack. If the cracked cup was the source of the creak, the only way to find out is to install another one and see if that solves it.

I save the metal ones from the old BB's and use then instead of the plastic ones.

Which is completle unecessary if a decent mechanic installed it. The reason the plastic cups were introduced were to PREVENT noises.

I wouldn't bother. Crack could've been from over-torquing or external damage.

source needed