If you're talking about the one going into the case, I believe it's either M10-1.5x25m/m or M12-1.75x25m/m - although that length may be either 30m/m or 35m/m. (Roughly 1", 1-1/4", or 1-1/2")
If it looks like it's just over 3/8", it's the M10. If it looks like it's about 1/2", it's the M12. I believe the M10 will take a 15m/m socket, and the M12 will take a 19m/m.
You can also check to see if I have it listed on my Screw Size Chart at WiP - link in sig, and go to the Tech Archives.