3a thread class, right-hand thread direction, 0.506 inches minimum and 0.556 inches maximum thread length, 0.766 inches minimum and 0.796 inches maximum fastener length, flat countersunk head style, 0.449 inches minimum and 0.507 inches maximum head diameter

A screw, machine or screw, cap, socket head whose unthreaded portion is of specified grip length, plated or unplated, and is machined to a tolerance of one thousandth (0.001 in.) (0.025 mm) or less of a specific diameter for items (1.000 in.) (25 mm) or less in diameter. Items over (1.000 in.) (25 mm) in diameter shall have a tolerance of fifteen ten thousandths (0.0015 in.) (0.038 mm) or less. The nominal major diameter of the threads shall be at least one thousandth (0.001 in.) (0.025 mm) below the minimum shank diameter, but not below the minimum major diameter for applicable class of fit, as shown in the screw thread standards, fed std h28 and/or other nationally recognized standards. The head is designed to be held or driven with an inserted driver. It is not designed for rotation by the thumb and fingers. A locking feature may be incorporated in the design of the head or threads.