I need to replace the entire drivetrain on my circa 2000 MTB commuter. Quality groupsets all seem to use external bearings these days. Replacing my current cartridge bottom bracket for a modern external bearing BB is my main compatibility concern. What information do I need from my frame and a prospective groupset to determine if it will fit my bike?
Off the top of my head, I guess I need to know my frame's BB shell width and thread and the chainline. What else? Where do I find the corresponding info for the new parts, particularly Shimano? Anything I need to look out for?
Update: The bike needs new a new bottom bracket, chainrings, chain, 8speed cassette and front derailleur. Replacing them individually costs almost as much as a complete modern groupset (which includes new levers, rear mech and two more gears).
Why not buy a new bike? Three reasons. One economic: the steel frame is sound, the wheels and brakes are new, and bikes of comparable quality are much more expensive than a new groupset. One philosophical: I don't like throwing things out while they still work. One emotional/irrational: I like my bike, it's been good to me, and I want to do right by it.