hokiefyd 

I have a low-dollar Asian MTB that came with a 1" JIS headset, with a 27.0mm ID crown race. I acquired (years ago) a Tange cro-mo fork for it with the intention of installing a 1" ISO headset (using a 26.4mm ID crown race). Figuring the most difficult part of the project to be fitting the crown race to the fork, I started that first, and ground down the crown race seat to accept the 26.4" crown race. This was years ago, and I'm just now coming back to this project. I bought the bike new in the mid-1990s, and it's a sentimental project to me. I plan to strip it down and paint-and-rebuild as a winter project.

Question for today: I have re-considered my headset plan, and would like to replace the existing headset with a standard 1" JIS headset. However, I'm afraid that I've irrevocably modified the Tange fork that I'd like to use. I know I can try to mix-and-match races and cups and try to make work what I have, but I understand the results can be inconsistent. A 27.0 slips right over my modified crown race seat, and has just a bit of wiggle. I suppose I could try to set it in a bed of JB Weld and assemble, and hope it sits straight. That seems like a bit of a bodge, though.

Any other ideas?

(I can use the stock fork with its JIS crown race seat -- but I'd prefer to use this Tange fork if I can manage it.)

JohnDThompson 

Most Tange headsets are available in either JIS or ISO dimensions; often crown races are available separately.

Unca_Sam

Have you read the information from the Gospel of Sheldon Brown?

In the mix and match section, a blessing is bestowed upon using different upper and lower bearing units, but not different standard crowns and cups (because they're a different standard and just won't work well). You won't be able to use the Tange fork (that's the one you modified, right? It's unclear) because you made it too small. You can punch the lower headtube race/cup to press the ISO cup there, and keep the JIS top hardware. That's the easy way to use the Tange fork, and half of the JIS headset.

Otherwise, shims made from thin enough/thick enough stock have been used. I'm not sure what the thickness of an aluminum beverage can's sides are, but you only need around .3 mm width concentrically for a better fit. I'm also not sure how much better that would be than epoxy, barring that it'll be easy enough to undo with typical shop tools to replace in the future.

AnkleWork

Seems you have to pick which bodge you dislike the least.

Foxonabike

I also have a low cost MTB (ToysRus) it had 1" headset and I wanted to fit 1 1/8" for new forks, I got some cups machined that pressed over the head tube and took the new bearings, had to make sure the new steering tube was long enough to span the new wider spaced bearings. Sounds like it may be too late for something like that. Done a lot to this, frame modded to take coil over oil rear shock, magnesium air front forks, converted to single speed with handmade sprockets, tensioner and bash guard, Cranks changed to 155s.

unterhausen

My lbs sells a 1" threaded Tange headset for $12 and you can get either crown race for it. They have a pile of JIS races in the drawer they store the headsets in.

2old

IMO, nothing wrong with a shim if that seems like the most viable alternative. That area isn't exposed to sideplay so a simple fix should be adequate. Last resort, JBWeld should work too; add too little, then assemble. That way the race will be in the right place; disassemble, add grease and reassemble.

hokiefyd 

Thanks, y'all. Yes, Sheldon's site has been really helpful (it's how I knew the dimensions), but I was not aware the Tange headsets are available with either crown race. The way I read that, I can get my 30.0mm JIS cups for the frame, and a 26.4mm ISO crown race to go with it for my bodged Tange fork (the Tange fork is the one I modified to fit an ISO crown race). I'll poke around the internets for that, or check with my LBS next time I'm there. Cheers.