Discovered that one of my toolmaker’s vises had a defunct screw securing the jaw to the body: one side of the head simply vanished over the years. Hadn’t mattered up until now, but I really wanted the jaw to not ride up when clamping the workpiece and the screw was rather loose.
It’s difficult to tighten a screw with half a slot…
Eventually I figured out that the screw has a 6-40 thread. My Brownells Gunsmith screw assortment (which they no longer offer) has, among others, 6-48 and 8-40 screws in flat, fillister, socket, and round head, but nothing in 6-40.
After exhausting all other possibilities, I looked in the assortment box again and found four round-head crosspoint screws with captive lockwashers that I must have salvaged from some dead piece of gear.
Of course, they turned out to be 6-40. Whew!
Chopped off the lockwasher, added a dab of Loctite, and it fit perfectly. I flipped the sliding plate over, as it appeared somewhat worn, but I’m certain that didn’t make any difference.
The screw must have a flat head (and you can’t flip the plate over) if you mount the vise on a machined V-way, but that’s not anything I’m likely to do. In that event, maybe I’ll just file the top off the screw and be done with it.