Originally Posted by chaadster

I just put the calipers on my stock Nano headset, and it looks like 35.5mm (25.5 upper cup to locknot top, and 10mm for lower cup).

You're going to need to stay pretty close, obviously, because you'll need to have enough purchase on the steerer threads with the locknut. Of course you know that, probably, so I guess I'd say 1.5mm either way because there is a spacer washer you can remove and substitute with a little thread locker to drop height of a taller unit a bit if needed. I'm no pro, though, so verify that!

HTH...and yeah, the stock HS is a POS; not even sealed! Let us know if you find a good JIS unit. I've been wanting to replace mine, but really wanted a blue one, and could only find one for $200; I'd be happy with black at this point.