3a thread class, 0.311 inches minimum and 0.378 inches maximum thread length, 1.353 inches minimum and 1.373 inches maximum fastener length, 0.157 inches minimum and 0.172 inches maximum head height, 0.492 inches minimum and 0.502 inches maximum width between flats, 0.3108 inches minimum and 0.3117 inches maximum grip diameter

An externally threaded fastener whose unthreaded portion is of a specified grip length, plated or unplated, and is machined to a tolerance of one thousandth (0.001) of an inch (0.025 mm) or less of a specific diameter for items one inch (25 mm) or less in diameter. Items over one inch (25 mm) in diameter shall have a tolerance of fifteen ten-thousandths (0.0015) of an inch (0.038 mm) or less. The nominal major diameter of the threads shall be at least one thousandth (0.001) of an inch (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 not designed to be held or driven with an inserted driver, nor is it designed for rotation by the thumb and fingers. A locking feature may be incorporated in the design of the head or threads. For items having close tolerance, and fabricated from material having 160,000 pounds per square inch (1100 megapascals) minimum tensile strength, see bolt, shear. For items fabricated from material having less than 160,000 pounds per square inch (1100 megapascals) minimum tensile strength and not having a close tolerance, see bolt, machine.