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):