Which screws are you referring to? I can't think of any screws which hold the valve in, other than the valve retaining nut inside the gun. My later model KP-3 with the cylindrical rear plug has a small screw on each side, where the rear plug on older KPs was held in with a pin at the bottom. Are you referring to the screws which hold that rear plug in? If so, I believe they are 4-40 threaded.