Thought I would dig up this thread, for I too have some impending headset woes. I have a 1991 Specialized Stumpjumper Comp, with Tange Prestige tubes, one-inch headset, and so on. Originally it would've had a Specialized headset (probably Tange-built) that was replaced at some stage with a Shimano Ultegra unit with cartridge bearings.
I want to replace the headset because the stack height on the Ultegra is too tall. Since there aren't enough threads on the steerer to include the tabbed washer, the locknut has to bear directly on the top cup, and it's difficult to get the headset correctly adjusted and difficult to keep it adjusted. I put up with it for a long time but not any longer.
I thought bikes from that era, Spesh included, used JIS headsets, and I was about to buy a good old Tange Levin CDS from Spa Cycles, but I decided to remove the Ultegra headset to doublecheck things. Here's where it gets strange.
Both cups on the Ultegra measure 30.0mm OD, and the headtube ID is 29.76mm on the top and 29.80 on the bottom. Those sound like normal JIS measurements to me. But the fork crown seat OD is not 27.1mm, not 27.2mm, it's not 27-point-anything. It's somewhere between 26.3mm and 26.48mm, and that says ISO to me. The crown race itself is a bit corroded in places, and has an ID of 26.0mm in some places and 26.2mm elsewhere. Perhaps it's a rusty 26.4mm.
It's almost as though the headset shipped with a mismatched crown race and whoever installed it just hammered it on good and proper. Would it actually manage to carve away a third of a millimeter of steel in the process? It wasn't desperately difficult to remove, however, with my toolmaker's clamp and a rubber mallet. A JIS crown race would rattle around something terrible with the fork the way it is currently.
Thoughts? And potential solutions anyone?