I have a stock 2008 Specialized Singlecross, currently setup as a SS. Stock Cranks are Sugino RD cranks with 103 BB for a 45mm chainline. Based on some info from Sheldon, rear chainline is a hair under 44mm.

As I understand, most SS/FG cranks and hubs utilize a standard 42mm chainline. If I were to swap out either the stock wheelset or the stock cranks, how closely do I need to match the 45mm? Is +/- 3mm going to be noticeable?

Thanks in advance.

Most hubs use a 42mm chainline. Cranks have a certain amount of "displacement", but their half of the chainline can be adjusted with a different BB spindle. The straighter you can get the better.

Thanks Kayce. I understand I want the chainline as straight as possible, but when I look at 3mm on a ruler, it seems fairly inconsequential. I'm just wondering if it would be noticeable at all.

It will wear everything out faster, but not noticable probably. If you are getting new cranks you will probably need a new BB any way, so it would make sense to get one a few mm off to get the right line.

Right...but that would only work with traditional BBs and not an external bearing BB(?).

Correct.

So, if I've got this right, external bearing BBs are out. If a given crank recommends a BB spindle length X to achieve 42mm chain line, I would really need to get a BB with a spindle length of X+6, since I need to move the drive side 3mm?

Chainline varies with different cogs(formula hub+eai cog=43.5ish,formula+origin8=41.5 for example).Don't stress over 2-3 mm.Many "singlespeed" cranks like the RD have 45mm CL because they're road doubles being sold with only one ring.A crank that is designed for track specific chainline will give you 42mm regardless of internal or external bearing(my s-300 is dead on).