So I had a little accident. I bought a fork off of ebay thinking it was ISO and turns out the crown race is actually 27.0mm aka JIS.

So, I have a couple questions. Is it possible to use a JIS fork in an ISO frame? I figure I should be able to just sand down the crown race until the ISO headset I have fits properly but is this actually the case or just wishful thinking? In terms of the steerer size will there be any other issues? I seem to remember that a JIS steerer tube is a different size but I'll double check that. Anyways, any input is greatly appreciated. Thanks

-5cagm

Tange makes (or used to make) a headset for that application - ISO cups and JIS race.

LEVIN CDS HEADSET 30.2/27.0

Looks to be out of stock, though.

Depending on the headset you have, you could try to find the JIS race for it and see if it will mate with the bigger cups.

Thank you for the suggestion. I also forgot to mention that the fork is threadless. It's a 1" CF Kinesis. Yes I know, blasphemy

Yikes - you're on your own.

diameter of the (1") steerer itself should be the same no matter which crownrace diameter. I have used a JIS race together with a ISO HS without any issue, but maybe I was lucky.

I dont think it was luck.

Thw whole JIS/ISO headset thing is way over rated and a bit silly if you ask me.... every old headset I've seen or sold was avaialble with either a 26.4 of 27.0 fork race. It was always, "I need a headset with 27mm fork race", or "I need need a headset iwth 26.4 mm fork race. It was never, "I need a ISO headset with a JIS race".

head tube size is the same, steerer diameter is different. A 27.0 inner-diameter race and a 26.4 inner-diameter race both locate the raceway at the same distance from their relative steerer tube. Either will work with the same head tube cup.

Alright sounds good and thanks for all the info. I believe I can figure it out now haha

I must've bought the exact same Kinesis CF fork on Ebay and discovered the same $%^#@#! when I tried to install the crown race. About 30 minutes of careful work with a fine file and I had the problem licked. Alternatively, if you are using a cartridge bearing headset, you could split the crown race (like some MTB headsets).

It's trivial to mill a JIS crown race seat to ISO dimensions if that's what you're asking.

If the steerer diameter (OD) is different between the two, what stem are you going to clamp on it? Do you need a JIS stem to fit a JIS steerer and an ISO stem for an ISO steerer...I don't think so.
Maybe the nomenclature is getting muddy, here: if we are talking about 1" threadless steerers (the OP) they have to have the same OD for a standardized stem to clamp. If talking about threaded 1" steerers, then according to Sutherlands both JIS and ISO are 25.4mm x 24tpi, and "Italian, Japanese and English 1" steering columns all have the same diameter and number of threads per inch".
(ignoring the exceptions that don't pertain to this like French, Raleigh-specific and early English makes like Chater-Lea)

So trivial I did mine in 5 minutes with a piece of sandpaper.

Haha well this is getting fun isn't it. It is indeed a 1" threadless steerer but strangely enough, when I asked the ebay seller about the steerer width they said that it was standard, just the race was JIS 27.0mm. I'm using a shim and a 1-1/8" stem (Easton EA70) that was on sale for $20. With regards to the headset, the one I have has the split crown race that you have described so maybe it is a mtb headset (at 104 grams with needle bearings, I could care less). I've dropped the frame and fork off at a shop downtown to press the headset etc. so I'll have to see if everything works out, I have a feeling it will. Again, thank you for all of the input, I'll let you know how things turn out when I get the frame and fork back on Monday.