Slipper adjustment ring screw size?

[simplechamp]

What size hex wrench is supposed to be used for the screw that locks the slipper ring? On the screw that is in mine 2mm can work, but has too much play, and 2.5mm is too large. Likewise, 5/64 has too much play, but 3/32 is too large. The screw is definitely in perfect shape, not rounded or stripped, and the tools I'm using are almost brand new, also in perfect shape.

So far the 2mm is the closest, but every time I have to tighten or loosen it I cringe because it feels like it's on the verge of stripping out. If I wanted to replace the screw what is the proper size?