replacing a bottom bracket
 

So I picked up an old trek 400 elance cheap off craigslist the same as this model (http://www.vintage-trek.com/TrekBrochure1987.htm#page13). To make a long story short I wont have the bike for another month so I can't measure anything, but I wanted to have some new components ready to put on it when I get it. I bought a new shimano 175 mm 105 FC-5502 crankset compatible with V1 octalink bottom bracket. I will be running a 9 speed setup. I know the old trek has a square tapered bb, so I will have to replace it. I've never replaced a old style bb with the new cartridge style so I'm not sure what size I will need. Has anyone replaced a similar style bb before or have any advice?

You'll need to install an Octalink BB to match the new cranks. It will be an easy swap.

Just remove the old BB cups, bearings, and crank axle. Clean the threads. Grease the threads. Install the new cartridge BB.

Thanks. But does anyone know what size bb I will need?

68 X 109.5 for a double crankset, 68 X 118 for a triple.

That's correct and your Trek will be English threaded (BS 1.37x24tpi). Remember the drive side cup is left-hand threaded!

sweet thanks guys