1991 (I think) Trek 850 MTB bottom bracket replacement. Maybe similar to 830? I have a Trek 850 MTB that needs the bottom bracket replaced. Is there a standard size to order? If I take off the bike will there be a number that will help me order a new one or do I measure somehow? Thank you.
Bicycle blue book says the 850 came with a BB-Lp26 bottom bracket, which was 68mm wide, 110mm spindle length, square taper. There are some new (old stock) on ebay, but I don't see why a newer BB-UN26 couldn't also be used (or any other standard ISO bottom bracket made for a 68mm wide shell, with a 110mm spindle).

I'd measure the BB shell and spindle width, just to make sure. Shell width is self-explanatory. You'll need to pull cranks to accurately measure the spindle width, and a caliper would be best. If you pull the BB, this info might be legible somewhere on the BB. I'd try pulling it before ordering a new one. Remember that the drive side has reverse thread, so clockwise to take it off. If it's from 1991, it may be seized pretty well. You may need to clamp a BB tool into a vise and use the entire bike frame as leverage to get it loose. I've had to do this in the past.