Within 3-5 mm how important is chain line on a multispeed drivetrain?

I'm building up a hotrod 12 speed from a handbuilt 531 American frame using some favorite but unmatched components. I'm using some odd Italian, Japanese, American and French parts which don't ordinarily go together. Now, I'm almost finished and have to decide on the "correct" BB for a lovely Topline double crankset.

The Topline is ISO taper (I presume) and fits differently on every BB I've tried - Campy, Shimano, Hatta, Sugino, Suntour, Edco. There's as much as 3+ mm difference in depth of fit because of square taper differences.

Also, there are various axle lengths to consider, from 112 to 116 mm.

I think that if I have sufficient clearence from chainstay AND have sufficient depth of fit on square taper that the variance in chainline can be compensated for in available derailleur adjustments.

Sheldon says that in spite of the JIS/ISO/? differences he's never had a problem compensating. I tend to agree (in theory).

Comments?

The center of the two rings in the front should be quite close to the center of the freewheel or cassette.

"quite close" is the key phrase. What does that mean? How many mm? I think you confirm my thoughts that it is variable, and there are no hard rules.

If it's just a 12 speed and you're using friction shifting, close should be good enough. You might need to trim your FD a bit more than usual if the chainline is off and you'll want to limit the cross chaining but everything should work.

eyeballishly close. you don't have to build a jig to measure it.

If your front derailleur will throw it, call it good.

chain line schmain line on old friction rides.