I don't have a LBS (there are BS, but not L), so I generally order everything I need online. I need to replace the bottom bracket on my commuter bike, which I know is a VP-BC73 but that BB's available in sizes from 107 mm to 127.5 mm and I'm not sure what size I'll need. The bike has a 28-38-48 triple front chainring.
I plan on using a Sunrace BB as the replacement, which is available in 103, 107, 111, 113, 118, 122, 124, and 127 mm lengths (italics are out of stock at my preferred supplier).
The obvious answer here is to take the BB out and measure it; trouble is, there's no crank puller or BB tool available—I'll be ordering these with the new BB. I've also tried contacting the bike's manufacturer (a bicycle chain store—I know), but they've been non-responsive.
The best I've managed so far was an attempt to measure the distance from the outside of one crank to the other (at the BB), then remove the crank bolts and subtract the distances from the outside of the crank to the end of the bottom bracket axle. This gave a (rudimentary, at best!) measurement of 129 mm.
I can possibly order a couple of BBs and return one, but I'd really like to be able to narrow it down that much, at least. Is it better to be too long, or too short (or does it really need to be exact)?
UPDATE: I measured the chainline yesterday, it's 53 mm at the front (I know that's wide, but that's what it is) and 48.5 mm at the back. The rear cassette is a 7-speed Shimano 14-28.