The nylon locking ring in the nut didn't quite engage the threads, so I bored out another 0.1" and now the nut now tightens securely:
While I was at it, I added some bevels. The innermost bevel makes threading the nut more comfortable. The large outer bevel just makes it a slightly better visual match for the other hand cranks and looks a little better. I didn't measure any of the bevels, just eyeballed them.
Here's what the profile on the back looks like:
The upper part is just a copy of the shaft in the back of the existing crank, and the index wheel fits over it with the spring in the groove. The bottom part is slightly longer than it really needs to be and has my attempt at a J4-like profile cut into it. (Probably it will shred the belt and I'll start over after grinding a new attempt at a form tool.) The dimensions aren't critical.
While I was at it, I added some bevels. The innermost bevel makes threading the nut more comfortable. The large outer bevel just makes it a slightly better visual match for the other hand cranks and looks a little better. I didn't measure any of the bevels, just eyeballed them.
Here's what the profile on the back looks like:
The upper part is just a copy of the shaft in the back of the existing crank, and the index wheel fits over it with the spring in the groove. The bottom part is slightly longer than it really needs to be and has my attempt at a J4-like profile cut into it. (Probably it will shred the belt and I'll start over after grinding a new attempt at a form tool.) The dimensions aren't critical.