Originally Posted by Greenfieldja

I know that some of the treks from the era your is from came with Campagnolo components which use ISO dimensions...this marries up with your current headset being ISO speced.

The ISO spec as it relates to a headset is the fork crown race seat dimension which is 26.4mm. Shimano speced this headset in both ISO and JIS (27mm). There are sources online for Shimano crown races in both JIS and ISO specs...the only two issues I am unsure of are:
1. Is your 7410 headset JIS or ISO spec...and does that matter other than the fit of the crown race on the steerer?
2. Whether the angular area that contacts the bearing balls on an obtainable replacement shimano crown race is correct for your 7410 headset.

-j

Originally Posted by miamijim

OK...JIS primarily refers to the head tube cup size. 30.0 is JIS while 30.2 is ISO, 27.0 fork races aren't exclusive to JIS headsets, they're also found with ISO headsets.

The question here is what size fork race do late 80's Treks take. 27.0 or 26.4?

Originally Posted by JohnDThompson

If this is the original fork, it should have left the factory trimmed for 42mm stack and the crown race seat milled to ISO 26.4mm diameter, and the head tube should also be ISO 30.2mm. A couple spacers should be all that is needed to make it work, assuming you can source the proper crown race.