i have an older olmo frame which i'd like to build up into something, but i'm already stuck at the headset. the fork crown race seat measures 27,2 ish with digital calipers. frame cup measures about 29,7 ish. All this tells me pure italian and it was never modified.

anyhow, all my extra headsets are ISO, so i am trying to source a JIS headset since those measurements match closest. even a cheap knock-off would do, but i only find ISO stuff these days. anyone have a recommendation? i'm not looking for something top of the line.

i thought about milling the fork to 26,4, but i expect that to be more expensive than a semi-cheapo headset.

thanks,
dan

There's plenty of vintage Italian headsets available on ebay. If you do not need cosmetically perfect they can be very affordable. Look for Campy Record from the 70's or Magistroni, which is what the Olmo would have came with (although branded as Olmo.)

If you are going to modify something, do NOT cut the fork crown. You can easily enlarge a 26.4 bearing race by chucking it in a lathe and using a die grinder and stone to open it up. Just go slow and measure a lot.

This is (or at least used to be...) a common procedure. Unless you're intending to use a $12 headset on your bike, you might as well have a shop mill the crown race seat for you. Milling the headtube is usually a little more expensive, but also doable. Modifying a frame and fork to fit an ISO headset is not a big deal.

If you do decide to use a JIS headset, keep in mind that most of the headsets that are sold as "JIS" really just have a JIS crown race, and the headset cups are still 30.2mm diameter, rather than 30.0.

Modifying a crown race to a larger size only makes sense if you happen to have a lathe, and some patience, and lack a crown race seat milling tool or the $15 it takes to have a shop do it for you.