I put a compact Sugino crankset on my bike using a 118mm Shimano UN54 BB. Really this is too wide, but it's what I had laying around, and Harris Cyclery suggests a 107mm or 110mm. So I'd like to order one, but the non-drive side crank arm is spaced out about 4mm further than the drive side. I'm guessing that using a 2mm spacer on the drive side would even the cranks out. I have a 68mm shell and I'm wondering if I should order the 68mm style BB or if I should get the 73mm because I'll be using spacers. Thanks.

Order a 68mm BB with the right length spindle and you won't or shouldn't need spacers.

Normally the right crank is farther out than the left cor chainring/chainstay clearance. If your left is farther out I suspect that the spindle or BB cartridge is installed backward. Try reversing it before buying anything new.

Thanks for the reply. It's the non-drive (left) that sticks out further. There's only one way the BB can go in since one side is left-hand threaded and that side is part of the cartridge.

BBs come offset to the right, left and symmetrical. When shopping for a replacement check the spindle dimension specs carefully and you should be able to find a match. BTW- the spindle offset doesn't matter by itself. What counts is where the cranks end up. Measure chainring and arm tip clearance of both cranks to see how far in either side can go. When shopping do not try to get clearance super tight, because there's variation in how far the cranks go on the tapers and you can come up short.

It's a bit cryptic trying to figure out what BB's have what spindle offset. I went to the bike shop and I noticed that the left hand side of the spindle was exactly the same length on the 110, 113, and 118 Shimanos. Only the drive side differed in length.

Cryptic may not be the right word, but it isn't obvious either. Look at this (incomplete) database of spindle lengths. Start be mounting your cranks the way they were, and measuring the various clearances. Then measure your spindle length and left/right offsets. Now figure what you want to trim at each end and use the BB that closest meets that objective.