JIS uses a 27mm fork crown seat, and a 30mm headtube ID
ISO is 26.4 & 30.2mm
However within the same model of the same brand headset these are the only dimensions that are different - JIS or ISO.
So, for example if you bought a JIS fork for a bike that used to be ISO, all you'd need to replace is the crown race. (staying with the same brand/model)
I hope that clears it up.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.