Hello. It is my understanding that JIS headset cups are 30.0 mm in diameter and ISO cups are 30.2 mm in diameter, no? My question is this:
I have an old Ross Super Grand Tour (insert unremembered Roman numeral here) that I am purchasing a Surly Steamroller fork for, and in the process am going to convert to a threadless 1" stem/ headset. I know the new fork has a 26.4 crown race diameter, so a standard 1" headset crown race should fit no problem.
The original fork used a 21.1 diameter stem, so that set me to wondering if it was a JIS headset. I measured the old cups, and they came out at 30.1 at the widest spot. I measured the headtube, and it is 30.2 in some spots, and 30.1 in others. Will ISO 30.2 cups fit in this frame? It seems to me that one tenth of a millimeter would not be that much of a difference, a difference that could probably be sanded off if it's tight. I have very little experience with headset installation, so I thought I might ask before ordering my new headset. Thanks to all who reply.
The 21.15mm stem isn't related to ISO/JIS as far as I know. Schwinn used this stem size the most, I've got several that use the 21.15mm stems with ISO headsets (the only difference is the locknuts are sized to fit snugly around the 21.15mm stems). If your headtube is 30.2 in places, 30.1 in others (let's call it 30.15? ), it's on the upper end of the ISO spec and, relatively speaking, not close to the JIS spec.
It seems that you may be concerned that a headset cup with a slightly larger diameter than the inside diameter of the headtube won't fit properly. That's actually the way it's supposed to be, it's what gives the proper "interference" fit and is the reason the cups have to be pressed into the headtube-