This has gotten to be a worse problem. Just bought a Tange-Levin headset advertised as J.I.S. and while the crown race was indeed 27.0mm, the cups are 30.2mm. It is to be noted that Sheldon says he puts 30.2mm races into 30.0mm head tubes, but my head tube (nice old Miyata 710) measures 29.85mm on both ends, and the races measure exactly 30.20. that's a 0.35mm differential and I'm not comfortable with that.

I will probably end up going to the LBS and have them ream the head...it still has paint remnants on the original milling. I was thinking of having them do a more honest j
ding the rear triangle to 130mm so that I don't have to keep jamming the wheel in! ...combine the pain