Originally Posted by
himespau
So the actual head tube diameter is different between JIS and ISO forks?
Yes they're different 30.0 JIS vs. 30.2 ISO. How important that difference is depends on the specifics of the situation. On the fork there's no margin for error because the crown races are stout and brittle, so there's no way that a .6mm difference can be accommodated.
However it's muddier with head tubes. the standard .05mm interference (head tube is actually below the nominal by .05mm, ie 29.95mm, not 30) is based on the lugged steel frames with steel headsets in use when it was adopted. But changes in wall thickness and materials of both the headsets and headtubes changes the amount of interference both possible and desirable. So it's possible to press an ISO cup into the thinner walled head tube of a lugless Tig welded steel or aluminum frame.
However, if for example you were to force a cup into an undersized carbon head tube, there's a great chance of cracking it. Fortunately JIS carbon frames are very rare (if they exist at all).
BTW- the reverse, putting a JIS headset into an ISO frame will never work right. The cup will never be tight, and the headset will always act like it's adjusted too loose.