Carriage Bolts

A carriage bolt has a smooth, domed head with a square neck beneath it; the square section seats into wood or square holes to prevent rotation. Use them for joining wood-to-wood or wood-to-metal where a smooth head is desired on one side and when you want the bolt to lock in place without a nut turning

Select a diameter that matches the load and material thickness (common diameters range from 1/4" to 3/4"); choose a length long enough to pass through all materials plus the nut and washer thickness, allowing at least two full threads past the nut for secure engagement

For exterior or corrosive conditions choose stainless steel (e.g., 304 or 316) or hot-dip galvanized carriage bolts for improved corrosion resistance; zinc-plated is suitable for indoor or dry environments but offers less durability outdoors

Installation typically requires a drill for the hole, a hammer or mallet to seat the square neck, and a wrench or socket for the nut; using flat washers under the nut is recommended to distribute load, and a lock washer or thread-locker can help resist loosening

Carriage bolts are machine-threaded and used with a nut, featuring a domed head and square neck; lag bolts are large wood screws with coarse threads and a hex head that screw directly into wood without a nut; carriage head screws (coach bolts) look similar but may have different thread styles—always match fastener type to the intended fastening method

Carriage bolts are often sold in small packs to bulk boxes; prices vary by diameter, length, material, and pack size, with common household or small-project packs typically available under $50 and larger or specialty stainless/gallon packs at higher price points

Ensure proper hole fit so the square neck fully seats, use appropriate washers and a properly tightened nut to recommended torque, and add locking methods such as lock washers, nylon-insert lock nuts, or thread-locking compound for applications subject to vibration