I have an older Nishiki hybrid bike that I got new in 1999. I used it through the winter this year with studded tires and, over the weekend, started stripping it down to clean off all the grit and salt. The original crankset (a tourney stamped steel 48-38-28) is in pretty bad shape and I'm planning to buy a replacement. I'm planning on getting an Altus m311 48-38-28. The specs I've seen say that I should use a bottom bracket with 122.5 mm spindle. The BB I have now is a BB UN26 with 117 mm spindle. According to bikepedia, the original spindle was 120 mm, so I think my chainrings have been too far inboard by about 3 mm. (but maybe the old spindle was symmetrical so same length as the 117 mm on the driveside, but 3 mm longer on non-drive? I don't know. I don't have the original spindle anymore.)

As it is now, I'm not really having any problems with chainline. Even in big-big and small-small combos, the chain rub is not so bad that I completely avoid using them. Well, I never use small-small because it's never really useful, but if I'm coming up on a short steep hill in the big ring, I'll shift to the big-big briefly if I need to to get up it instead of shifting front and rear.

If I get a new crankset that's spec'd for a 122.5 mm spindle am I correct in thinking that that should move the chainrings inboard by an additional 2-3 mm? I'm just trying to decide if I should order a new BB along with the crankset. The old one still turns smoothly and there's no reason to replace it other than possible chainline issues. I'm much more likely to use the big-big combo than the small-small, so if my chainrings are a little too far inboard, I don't think it'll be a problem at all... Am I thinking about this right or am I missing something?