ISO frame and Chrome JIS fork what headset!
 

Ok doing a favor for a friend.

Replacing a fork on an old Trek 700 ( crash broken brake bosses).

Fork purchased (not by me) is a fully chromed and the crown race is 27.0. Frame is ISO.

He tried to force a ISO crown race on is and it split. No now it is a new headset as well.

So now I am trying to help him out. The bike is a beater so not looking to spend a ton on it.

The shops wont machine the crown race due to the chrome dulling their cutter

Will a JIS headset solve this problem? I know the JIS headset cups are smaller, but only .2 mm

Can you purchase just the JIS crown race for this headset? Many have the crown race available as a separate part. Also, there are mixed-diameter headsets available with JIS and ISO parts as needed. JIS cups will be a loose and sloppy fit in an ISO headset and will probably damage it over time.

I'm surprised the shops won't mill the JIS crown race seat down to ISO diameter as it seems unlikely the crown race seat itself is chromed.

As far as the headset goes, it is an OEM trek headset so who knows. And 10 + years old, so getting a crown race may not be easy.

Who makes a mixed diameter headset?

The fork is a cheap , and the whole thing is chromed. from fork tips to thread.

Amazon.com : Sunlite Threaded 700c Hybrid Fork, 200 x 120 x 22.2 x 27.0mm, Silver : Bike Rigid Forks : Sports & Outdoors

Here's one: Bike Tools Etc. - 1000's of bicycle tools and parts for the home mechanic!

Unfortunately it's out of stock at Bike Parts but it shows they exist. You might do a search for Tange headset and see if one of these can be found.

Amazon has several 27.0 crown races for around $5. You should be able to find one that will work.

You could sand or grind the chrome off the race seat before milling it to ISO diameter.

+1 chrome plate is hard on the cutting tools .. carborundum grit 'sand' paper is the way to go.. to take it off.

Tange, for one. As HillRider pointed out above, it's actually not unheard of to find headsets and framesets with mixed ISO and JIS parts, even. The Tange Passage headset is available in a few combinations. Here's one with 30.2 mm (ISO) cups and 27.0 mm (JIS) crown race:
Tange Passage New 1" Threaded 27.0 Chrome