ASME B18.5 Round Head Square Neck Bolts

ASME B18.5 round head square neck bolts, commonly referred to in the North American market as carriage bolts, are heavy-duty mechanical fasteners designed with a smooth, domed head and a short square section immediately beneath it.

The square neck penetrates into timber or seats into a matching square hole in steel plates, providing a self-locking mechanism that prevents the bolt from rotating when tightening the nut from the opposite side.

Compared to its European metric counterpart, the DIN 603 round head square neck bolt, the ASME B18.5 bolt differs primarily in thread standard, dimensional tolerances, and head proportions.

While DIN 603 relies on coarse metric threading (ISO pitch) with tighter European head geometry, ASME B18.5 follows Unified National Coarse (UNC) or Fine (UNF) thread series, featuring slightly larger head diameter options and rounded inch-based square neck proportions tailored to American structural specifications.

ASME B18.5 round head square neck bolts are heavily utilized in heavy timber construction, outdoor decking, highway guardrails, agricultural equipment, and structural wood-to-metal framing where a flush external head profile is needed for security and safety.