How to Mate a 1" Threaded Japanese Fork to a Pre-War American Frame?

[Ranger Dan]

I have a nice plate-crown fork (apparently of Japanese manufacture circa 1984) that I'd like to mount on a pre-war American frame that came without a fork. The frame also came without a headbadge but the previous owner said that it was a Rollfast, 1940.

It looks like a modern 1" threaded BMX headset should fit the head tube properly. What seems to be at issue is the I.D. of the crown race, which would normally comply with the American standard I.D of 26.4mm, while my Japanese fork is presumably sized to the J.I.S. standard of 27.0. (I need to knock off the crown race and put some vernier calipers to it to verify this.)

Short of going to the trouble and expense of milling the fork's crown race "shelf" (or whatever it's called) to fit the American-sized race, does anybody have another solution?

I really want to use these forks, as they have cantilever brake bosses already attached to allow me to mount front brakes.

I'm convinced this problem must have been met by others before me, and I'd be grateful for any suggestions -- thanks!

--Dan

 

[Andrew Gorman]

That's not much of a difference- I'd just try the JIS crown race with the BMX cup with loose balls. Or just take a file to the crown race seat.0.3mm isn't much to take off.

 

[Ranger Dan]

That's not much of a difference- I'd just try the JIS crown race with the BMX cup with loose balls. Or just take a file to the crown race seat.0.3mm isn't much to take off.

Thanks Andrew; yeah ... I guess you're right about the removal of 0.3 radius being minimal, and on further reflection it need not be perfectly concentric as long as the race ends up fitting snugly.

Unless I'm missing something, I think the issue is limited to the matching of crown race I.D. to seat O.D., though. For what reason might I need to remove balls from the retainer? (The headset doesn't come with a J.I.S. crown race, otherwise the issue would be moot!)

 

[rustjunkie]

I used to source a HS that fit the frame then if the race was a bit too big I'd give the fork a few twists through the knurler, put a little green loctite on and seat the race:

If it the race is a little too small you should be able to file the fork down a bit where it's flared to seat the race.
These options allow you to use all of the HS parts that came together.