I am hoping someone can help me figure out the threading on a new fork I just bought. Gorgeous Italian steel job, but no info on it. I was assuming it was either Italian or ISO, but there is a flat channel filed into the threading on the back of the steerer. No groove, but a flat section obscuring the threads. I heard this might indicate that it's French. Anyways, I'm about to buy a headset for the project, but now don't know what I need. Does this filed section mean it is definitely French? Is there an easy way to figure it out? Thanks for any help!
If it's a French fork, the steerer tube will have a 25.0mm outside diameter instead of 25.4mm, which is ISO or Italian. You'll need calipers to determine that. Also, even if it's Italian threaded, Sheldon says an ISO headset will work with the threading. According to his chart, the inside diameter of the crown race you would use on an Italian fork is 26.5mm or 27.0mm, so if that's the case you might want to take it to a bike shop and have the crown race seat on the fork milled down so that it will accept a 26.4mm ID crown race. But if it's 27.0 crown race ID, you should be able to easily find a crown race that will work, they're common.
Here's a link to Sheldon's chart (you'll have to scroll down to see it): http://www.sheldonbrown.com/gloss_ha-i.html#headset