Bottom bracket replacement I have an old '89 Trek Singletrack 950 with the original Shimano Mountain LX group bottom bracket. I'd like to replace the bottom bracket with something new. Can anyone help me determine what will fit and what I should best replace it with for touring and single track? Would a new Shimano Deore xt BB-MT800 fit/work? Anything I need to know before I attempt this project?
You need to know the type and axle length of your previous bottom bracket. If the BB is original, it is very likely BSA JIS square taper and needs to replaced with JIS square taper of similar length. You can either measure the length or just uninstall the old one and read what is printed on it. BSA bottom brackets have left handed threads on drive side. Just searching for "bottom bracket replacement" should find enough instructions.

BB-MT800 is completely different type and requires different crankset to work.