Bicycle Mechanics - 3 millimeters

I have a set of Shimano 600 cranks (FC-6400), and am looking for the ideal BB to pair with it. The recommended pairing long ago would have been the BB-6400, with a spindle length of 113.

An earlier model BB (BB 6207) is on ebay right now. Length is 116.

My cranks are not NOS. They show a bit of use, and often a longer spindle for older cranks is recommended because the holes have been worn a little. Is 3 mm significant, i.e., 1.5 mm per side, in terms of chainline? Is it perhaps recommended because the cranks are used? I'm building a double.

Thanks for any help!

You don't need the exact replacement that's spec'ed - 113 Un5x series shimano square taper cart are widely available.

You don't need the exact replacement that's spec'ed - 113 Un5x series shimano square taper cart are widely available.

Thanks for the quick reply. Good to know.

I thought the recommended bb for the Ultegra 8-speed double crank (6400) was 115 mm. I remember a BB-UN72 in 115 mm as the matching bb. In any event, a mm here or there isn't that critical.

Thanks, all, for the input. Yeah, I don't want to drive myself crazy over a millimeter or two -- but this is my first build involving derailleurs (I've built fixed gears) and didn't know if chainline is even more crucial for smooth shifting. I'm gathering parts slowly so I have time to get the correctly spec'ed length, but I have a silly fondness for the old serviceable-type BBs.

A.Winthrop, you may be correct about the 119mm for 6207; my research on it was not thorough.

I thought the recommended bb for the Ultegra 8-speed double crank (6400) was 115 mm. I remember a BB-UN72 in 115 mm as the matching bb. In any event, a mm here or there isn't that critical.

Yeah, that's what my 8-sp Ultegra bike came with. But it is has a fat seat tube and needs the crank a little more outboard to allow for shifting to the small ring. I think a skinny seat-tubed bike would be better with a 113mm but would still work fine with a 115mm or 116mm.