Bicycle Mechanics - JIS headset to ISO

I want to replace my JIS headset with an ISO one. I know I can cut the crown race down to ISO 26.4 but is that all I'll have to do? JIS cups have an outside diameter of 30.0 and ISO are 30.2, Is this going to be a problem or can I just squeeze it in there?

If the frame is steel, I'd say go for it. If aluminum or titanium, I'd get the headtube reamed out to 30.0mm for the 30.2mm cups. If carbon fiber, I'd stick with a JIS headset.

I once put ISO headset cups into my old steel MTB and didn't realize it used a JIS headset until I tried to put the 26.4 ISO crown race on the fork.

I want to replace my JIS headset with an ISO one. I know I can cut the crown race down to ISO 26.4 but is that all I'll have to do? JIS cups have an outside diameter of 30.0 and ISO are 30.2, Is this going to be a problem or can I just squeeze it in there?
You will need to have the head tube reamed. The crown race can be sized down with a crown race milling tool or a lathe.

The normal interference fit is .1mm or .004 inch. You can't install cups that would create three times that much interference. The head tube would need to be reamed.

I emailed Sheldon Brown this same question two years ago, here was his reply:



The difference in head tube diameter is so small as to be insignificant, especially as the head tube can "stretch" a bit. The main issue with ISO/J.I.S. is the fork crown race.


The head tube in question was steel.

New guy to posting here. I replaced my jis headset on my Schwinn Circuit. All that was needed was to turn down the crown race.