No, not 8/32"! 8-32. This is the set screw in the bridge (length may be slightly different than shown:https://www.vexrobotics.com/276-2385.ht ... gJ7w_D_BwE
It's not metric. Go to any hardware store and you'll find 8-32 machine screws.
I appreciate the nice drawing, spud, but I think the most solid approach would be to grind the end of the arm slightly conical. Then, the nylon screw could be screwed in far enough so when the arm is inserted, it really is wedged in by the screw. But try the nylon screw on its own first without modifying the arm (other than rounding off the sharp edges to it can slip past the nylon screw). You can move the arm but it won't fall out.