ASME B18.2.1 Square Head Bolt

Technical Drawing, Designations & Weights

The square head bolt has a square style head, the top of which is fully chamfered with the diameter of chamfer circle equal to the maximum width across flats, F. Compared to hex bolt, the square head bolt is not so prevalent in modern engineering. It is usually required during refurbishment of old project.

A typical drawing for square head bolt manufactured to ASME B18.2.1

The square head bolt can be designated as the following example: Square head bolt per ASME B18.2.1, 3/8″ – 16 × 2-1/2″. Steel per ASTM A307 Grade A, Zinc plated per ASTM F 1941 Fe/Zn 3A. It indicates that nominal size – 3/8″, threads per inch – 16, bolt length – 2-1/2″. The weight of a square bolt can be calculated using the following equation, Weight_square = ρ × (HF² + πE²L/4), ρ is the density of bolt material.

Full Size Body Diameter, E & Head Height, H

Width Across Flats, F & Width Across Corners, G

Radius of Fillet, R  & Nominal Thread Length, L_T

For L ≤ 6″, L_T equals twice the basic thread diameter plus 0.25″; For L > 6″, L_T equals twice the basic thread diameter plus 0.50″. For bolts that are threaded full length, L_G defines the unthreaded length under the head, and L_G ≤ 2.5 × P for sizes ≤ 1″ while L_G ≤ 3.5 × P for sizes > 1″ (P = thread pitch). If bolt size ≤ 1″ and L ≤ L_T + 2.5 × P, or, if bolt size > 1″ and L ≤ L_T + 3.5 × P, the bolt shall be threaded for full length.