Never had to think too much about this before, but here goes.
I'm renovating an old frame and have some Octalink cranks that I want to use on it. I've taken out the old square taper BB which is a 68 x 116mm unit. Looking for an Octalink replacement but they seem to come in 113 and 117.5 overall lengths. Is that going to cause major chain line problems if I go with the 117.5 length as the closest replacement?

No techy number bods around at the moment then?

Bumpety bump I need a trump.

I wouldn't have said so 5 or 6 days ago only my bottom bracket has just started to fall apart on my main commuting bike and the couple of mm play that's there has started to move the chain off the rear sprocket with depressing regularity (it's a Rohloff setup). It seems like being out a small amount has a bigger impact than I'd have expected.

It's got to hold out until the end of the week when I get paid!

Your new BB will be working with a different chainset to your old one, so you can't draw comparisons with them both.BBs for Octalink were, by and large 113mm for a 68mm shell, 118mm for 73mm shell.

I can't see +1.5 mm making any difference, people mess about with 2mm spacers all the time. It 'might' have done if you were going the other way. Having recently tried to fit a triple on a double BB I can attest to that not working. Or you could put a spacer on the non drive side?

Guess I'll just have to suck it and see!