Most often, the Problem Solvers inbox has pretty random stuff. But last week, we had two questions/requests in a row regarding the same issue: 1" headset compatibilities. As Karl puts it: 

"I am building up a vintage 1994 Giant CFR1 carbon/aluminum frame with an aluminum fork and I have a problem. I need a headset. The headstock is an aluminum casting built for 30mm cups and the fork is a 1 inch threaded fork built for a 26.4mm ID crown race. The problem is I can't find a compatible headset. I can find JIS headsets with 30mm cups but they are built for use with a 27mm crown race. ISO headsets are built for 30.2mm cups and a 26.4mm crown race ID. There isn't enough material to mill out the headset for 30.2mm cups. If I had a crown race with a 26.4mm ID but the profile of a 27mm crown race, the problem would be solved. I could just use a JIS headset."

And from Louis:

"Had to fit a JIS headset (discontinued GeForce) onto a bike; Problem was, the crown race is 27mm, and the fork a standard 26.4mm diameter... Shimmed it with a carefully cut aluminum can strip + copper foil, but it would've been great to have a seamless, drop-on shim. Are there many people who need to fit JIS, or older Italian headsets to standard forks? I don't know, but I hope the simplicity of the needed part would justify its production." 

All those numbers in prose form give me a headache. So here's a chart (info hijacked from Sheldon Brown, of course):

Standard Crown Race ID Cup OD
ISO (current) 26.4mm 30.2mm
Italian 26.5 / 27.0 30.2
JIS 27.0 30.0

To sum up, what Karl and Louis are lookin' for is a 0.6mm shim to use 27.0mm crown races on forks designed for the newer 26.4mm standard. As far as justifying the production of a 0.6mm shim, that's harder to do. Thing is, there's enough variation in the manufacture of the fork crown race seat, as well as the races themselves, that would nullify the precision of the shim. Installation would be difficult, and I'd imagine this could do more harm than good. Hypothetically, we could design a race with an 26.4mm ID for use with the 30.0mm cups, but again, there's so much variation from manufacturer to manufacturer, we probably couldn't accommodate them all. So for now, you're stuck with the following solutions:

To adapt old forks (27.0mm OD crown race seat) to new (26.4mm ID) crown races, any shop that faces/chases frames & forks can shave the forks' race seat down that 0.6mm needed. 

To adapt new forks (26.4mm OD crown race seat) to old (27.0mm ID) crown races, the pop-can method is still the best bet. However, due to legalities (and our lawyer's fingers digging into my shoulder right now) we can't really endorse that.

Does anybody out there know another solution?