If you take out your old bb, it will probably be labeled with what its dimensions are. If not you can measure it with calipers (cheap@harbor freight, or borrow a pair). The nomenclature is shell width X spindle length. After this you can always put it back in while you wait for the new one to arrive. Also, it is unlikely but possible that you bb is coming loose from the frame. If that is the case, snugging it down should help. When you install the new one, be sure to start the threads by hand so that you do not cross thread it as that is VERY easy to do. Also, the drive side cup is left hand thread.