So you need an ISO headset that has a stack height that fits your frame/fork. Stack height is Fork Steerer Height - Headtube Height.

From an old post by [MENTION=284466]coupster[/MENTION]

"First, measure the actual head tube of the bike. If the headset is currently out of the bike, great. If not don't take it into account. You want the measurement of the frame of the bike. Second, measure the length of the steerer tube of the fork. From the bottom of the crown race - if its still on the bike - or the top of the fork crown - if not - to the top of the threaded steerer tube. Subtract the head tube measurement from the steerer tube measurement. That is the maximum headset stack height you can use with that bike and fork combination. If you have any required spacers; brake hangers, accessory brackets, etc, measure their thicknesses and subtract them from your previous total. Any headset with a published stack height of <= to your measurement will work. If its less you can make up the difference with some spacers or cut the steerer tube. (I'd go for spacers) If its more you will need new fork."