Quick question,

Tange Levin threaded headsets come with either 27.0mm or 26.4mm races. What dictates which race diameter I should get? Stem?

Those numbers refer to the crown race diameter, JIS and ISO respectively.

So, measure your crown race.

If you're replacing a 27.0mm headset, you might be better off getting your fork crown cut down to take a 26.4mm race. I think the only options for 27.0mm headsets are the steel Levin and the pricey Hatta Swan. Both are fine, but if you don't want to deal with maintaining a loose ball headset you'll have a lot more options to choose from in the world of 26.4mm crown races.

That is an excellent suggestion, m. I socred a NIB 105 headset for cheaps [$10!], and had the lbs mill the crown race down to 26.4 from 27.0, to fit the headset. I think it cost me something like $10. That was over a year ago, and there have been no problems whatsoever.

I think shimano makes some threaded headsets with a JIS crown race. At least, that's what the spec/instruction sheet seemed to imply which came with shimano 105 headset.