Stock 18T pinion gear on Fury Mega - Set screw size?

[Overspeed]

Can anyone tell me what is the size of the set screw that holds the stock 18T pinion gear to the motor shaft? Part # 310124. The motor shaft has no flat edge on it, so I was trying to get it as tight as possible but it stripped. Please and thank you.

 

[Dreadstar]

M3x3mm grub screw.