I've been looking for a GT trip tri mountain bike and recently picked up a 20" frame in good condition. Where can I get info on modern bb square taper to replace standard bearings. Any advise for others who built one?
The Shimano BB-UN300 is usually a good replacement and one that I have used on several rebuilt GT's. They come in different sizes and spindle lengths. I think most GT Mountain bikes post 1990 have a 73mm shell, but do measure yours to check. More importantly, if you have the old BB make sure you measure the spindle length and buy the same length or as close as possible. This will give a good chain line and avoid shifting problems. You can get them on Amazon and elsewhere.
Also, clean those bottom bracket threads as much as possible and use lots of grease