Carriage Bolts

Carriage bolts have a smooth, rounded head above a square neck that embeds into wood or a square hole, locking the bolt against rotation so it can be tightened from the nut side alone. The finished dome head leaves no sharp edges, making them the standard for wood construction, fencing, playground equipment, and furniture.

How the Square Neck Works

Below the round head, a short square section pulls into the workpiece as the nut is tightened. In wood, the square neck crushes into the fibers; in metal, it seats into a matching square hole. Either way, the bolt is held against turning, so no tool is needed on the head side — a key advantage for through-bolting where the head is inaccessible.

Materials and Finishes

Steel with zinc plating: economical, for interior and dry applications. Hot-dip galvanized: the standard for outdoor wood construction, decks, and fencing. 18-8 stainless: corrosion resistance for exterior and damp environments. 316 stainless: maximum resistance for coastal and marine use, and for ACQ-treated lumber that corrodes ordinary steel.

Dimensional Standards

Inch carriage bolts conform to ASME B18.5; metric versions to DIN 603 / ISO 8677. Available in coarse thread, partial and full thread, across a wide length range.

Selecting the Right Carriage Bolt

Match the finish to the environment — galvanized or stainless for outdoor and treated lumber, zinc for interior. Size the length to pass through the materials plus the nut and washer; the square neck must fully seat for the anti-rotation feature to work. Always use a washer under the nut to protect the wood and distribute clamp load. For treated lumber, choose galvanized or 316 stainless to resist accelerated corrosion.

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