2a thread class, right-hand thread direction, 0.438 inches minimum and 0.500 inches maximum thread length, 0.500 inches nominal fastener length, flat countersunk head style, 0.238 inches minimum and 0.279 inches maximum head diameter

An externally threaded fastener whose threaded portion is of one nominal diameter, no 0 (0.060 in./1.5 mm) or larger, designed to be held or driven with either a wrench or an inserted driver or both (excluding internal socket or internal multiple spline types), in sizes below no. 10 (0.190 in./5 mm). No. 10 and larger sizes must have a head designed for any type inserted driver (excluding internal socket or internal multiple spline types), but may also be designed for external wrenching. A locking feature may be incorporated in the design of the head or threads. Excludes:bolt, clevis; bolt, externally relieved body; screw, externally relieved body; and screw, assembled washer. See also, screw, instrument; bolt, machine; bolt, internal wrenching; and screw, cap socket head.