ASME B18.2.1 Hex Bolt

Compared to square head bolt, the six-sided head of hex bolt requires smaller tightening angle of each stroke hence may facilitate the assembly of bolted joint by a wrench. When being tightened, the contact surface between the wrench and head of the hex bolt is comparatively larger than that of a eight-sided (octagonal) head. This may provide a firm grip by the wrench.

Generally, a hex bolt can be designated as Hex bolt per ASME B18.2.1, 1/2″ – 16 × 4-1/2″, ASTM A193 Grade B7, galvanized. It indicates that nominal size – 1/2″, threads per inch – 16, bolt length – 4-1/2″. The weight of a hex bolt can be calculated using the following equation: Weighthex = ρ × [3 × 31/2 ÷ 8 × G2 + πE2L/4], ρ is the density of bolt material.

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