OK, I've got an 11 year old Trek 4500 which was recently stolen, but recovered but in the 5 days the little scrotes had it they've pretty much knackered the drive train. Having recently bought a new road bike the missus WILL kill me if I buy a new MTB, so repairs are the only option (and yes I've tried the cost comparison excuses, but they just won't wash). So in order to keep costs down I'm going to attempt it myself. The bottom bracket sounds ok, but given the age could do with being replaced. What do I need to be looking for and if I want to fit these what do I need to do. At the moment the cranks are unspecified Shimano.