Bicycle Mechanics - Bottom Bracket size

Alrighty then, I replaced the crank on my 2005 Giant Rainier because the bottom bracket went all to hell and the LBS had a good deal on a Shimano BB and crankset. So now I have the crankset from my Giant (Truvativ ISIS). I'm still in the phase of picking parts for my winter commuter, a 1993 Raleigh M50, and in order to save some cash I plan on getting another ISIS bottom bracket and using my crankset from the Giant. Nashbar has a house brand ISIS BB for cheap, but you have to specify size. How can I determine the size of BB I need. The crankset has 3 chainrings if that helps.
Thanks fellas, you have no idea how much you've helped me so far.

Where's your old bottom bracket?

From the raleigh? It hasn't had one in it in... 5 years maybe.

Take the frame and crankset to the LBS and let them tell you. Triple crank usually needs a longer spindle (BBs are often listed as "for triple"), and the bracket is probably 68mm English, if that helps.

I can imagine that you'd be little bashful about asking the LBS to help you decide what to order form Nashbar. The BB diameter is determined by your frame, while the spindle length is determined by your cranks. As already mentioned, a Raleigh from the 90's should have 68mm threading. And according to Sheldon Brown's BB size database ( http://sheldonbrown.com/bbsize.html#truvativ ) all of the Truvativ cranks use a 113mm spindle length. So, 68x113 should do the job.

thanks folks, much appreciated