You may need to remove the old one to find out what type it is, one of the main things is to measure the length of the central axle as they are listed in order of length. I think there are different thread types as well.
If you buy a one piece sealed unit like mine it screws in from one end and right through to the other end so it will only fit from one end.
oh, here's a help file on how to identify yours
http://www.chainreactioncycles.com/H...CategoryID=144