I have finally worn out my Overland 4-8-4. It wants to bind because the slide rods on the valve gear have broken. There is a replacement part that appears to be connected to the main drive wheel with a hex head screw.

The questions:

1) Is this a screw or some kind of pin on the main drive wheel?

2) If this is a screw, what is the size of hex head? It seems like 2mm, but I am not exactly sure. It is soft metal and I only have so many chances before the hex head gets rounded and impossible to get off.