This 2-56 stud will hold the mirror shaft into whatever helmet mount I eventually decide on. It’s a pan-head screw that miraculously fits snugly inside the cut-down shaft section, held in with a delicate epoxy dribble around the edge.
The head abuts the end of the smaller shaft section, so the two no longer slide. I think a length of heat-shrink tubing will stabilize them in rotation, although perhaps I should have just slobbered more epoxy into that joint.
After the epoxy cured, I sliced off all but 2 mm of the screw thread with an abrasive wheel and cleaned up the wreckage with a file. I actually remembered to spin on a nut before cutting, which ensured I finished the threads properly.
The business end of the mirror has far too many moving parts: two indented plates for the balls on the mirror and shaft, a screw, and a nut. That’s one too many ball joints, at least, and Wouldn’t It Be Nice If the mirror had a watertight seal around its perimeter?
For now, I just epoxied the nut in place after scuffing up the plate and nut with some sandpaper to give the epoxy something to grip:
You can’t see the new washer and rubber grommet under the screw head that provides a bit of compliance to hold the balls more securely, plus a dot of low-strength Loctite in the nut to discourage things from falling apart on the road.