>>1837546Cop this idea--I know I've seen a 3d print version somewhere.
https://www.renehersecycles.com/tire-fit-gauge/But I've also made a cereal box cardboard version for someone to use at the co-op.
Almost always the 'tight spot'/least clearance is going to be between the chainstays, just behind the bb. In your second pic, it's around the 15" mark.
>>1838582How did that damage happen? Steel headset, and you didn't have it tightened?
I think what you described is possible, but not advised because the threaded steerer tube could break at the bottom of the threadless stem. Because with a threadless stem you're levering on the steerer tube, and the steerer tube has a reduced diameter at the root of the threads. (With a typical threaded setup, the steerer tube is not exposed, and you're levering on the vertical part of the stem.)
There are several ways to fix, but none of them are typical bike shop things. You can have the threads filled in with bronze & recut (pic). This should be the simplest, and cheapest option.
http://www.yellowjersey.org/forkthrd.htmlBut realistically, you can probably get a China/Taiwan fork for about the same cost, it just won't match your paint or style exactly.
You could also have the steerer cut & graft a good threaded section on. Same procedure as extending a threaded fork steerer tube, but you would be adding zero length. This is more complicated, adds weight, and can interfere with the quill stem wedge if not done right.
https://www.youtube.com/watch?v=TtOeCtNBUwg 