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?
On a bike of `89 vintage you will have a threaded bottom bracket shell and the bottom bracket bearings will be of the internal cartridge type - a unit containing the bearing and axle that threads into the frame and is secured with a lock ring, cranks attach to the axle on square tapers.
Cartridge BB bearings are available in different axle lengths to position the cranks at the correct distance from the center line of the frame. This is important to align the chainrings with the rear sprockets.
Cartridge BB bearings are still widely available and not particularly expensive. Shimano currently offers the BB-UN26 and BB-UN100. You just need to get one with the correct shell width and axle length. MTB shell widths are either 68 or 73mm, yours is likely 68mm. The width is the shell and the threads are standardized (knows as 'BSA')
You can measure the shell width and end-end axle length, or read what's printed on the old cartridge body (if legible after 30 years).
Installing a new cartridge BB is not very hard but requires a few special tools. A crank puller to get the cranks off the axle taper (do not drift them off with a hammer and screwdriver!) and a bottom bracket tool to engage with spines on the cartridge and its lockring. Technically a torque wrench is required to tighten the crank bolts but often you can ask a local bike repair shop to torque the bolts for you.
Tutorials and guides are not hard to find online. Park Tool has a good one.
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.
In addtion to other answers, from http://www.vintage-trek.com/Trek-Fisher-Klein-Lemond/1991specmanualTrek.pdf
Although the its a 1991 manual, there is a good chance its the same spec. Remove the BB and measure to confirm.
BB Shell 68mm, Axle 37.5/52/36 (total is 125.5)