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Carriage Bolts

Carriage bolts are used to fasten wood to wood, wood to metal and metal to metal. They are a member of the round head bolt family and commonly have a "circular head with a low rounded top surface and flat bearing surface, and an integrally formed square neck under the head." Found on chain link fences and often on patio furniture and swing sets made of wood, carriage bolts are also known as: 

carriage screws, round head square neck bolts, coach bolts; shaker screen bolts

 (when fully threaded). Since the head is designed not to turn, they are usually tightened by torquing a nut. However, they can be used with an internally threaded (tapped) hole and tightened by turning the carriage bolt's square neck, if accessible.

Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse). Lengths of 6" and shorter are usually fully threaded; lengths longer than 6" are partially threaded.

Typically, carriage bolt sizes range from #10 to 3/4" in diameter while lengths span from about 1/2" to 20"—only larger sizes are available in long lengths. Measure length from under the head to the threaded end of the bolt.

Head diameter is roughly twice the size of the bolt or, for smaller sizes, a little more than double. The width across flats of the square neck is about the size of the bolt. Table 1 lists head diameter and height, and square neck width and depth.

Grades 2 and 5, steel, and stainless steel, are the most prevalent types of carriage bolts.

Common finishes for Grade 2 are zinc plating and hot dip galvanizing, and zinc plating for Grade 5. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Stainless steel, though, is a better choice when corrosion is of concern. Hot dip galvanized and stainless steel are usually recommended if the bolts (less than 1/2" in diameter) will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations.

The square neck prevents the bolt from turning as the nut is tightened. Consequently, access to the head is not needed so it can be installed in a counterbored (flat bottom) recess, if desired. When used with soft wood, one source suggests drilling the hole the same size as the carriage bolt's shank to ensure a tight fit for the square neck; use a "soft" hammer or mallet to drive the bolt into position and prevent damage to its finish. In hard wood, a slight counterbore for the neck is sometimes needed to prevent the wood from splitting and to ensure that the head will pull down flush. Metal parts, such as chain link fence tension and brace bands, are often square punched to accommodate the square neck, which prevents bolt rotation.

When installing in wood, use a large diameter washer under the nut to distribute clamping force over a larger area and minimize compression of the wood.

It is advisable to match materials and finishes of bolts, washers and nuts. When using hot dipped galvanized bolts, always use hot dipped galvanized nuts, which are tapped oversize (threaded larger than normal) to accommodate the thick zinc coating on the bolts (using a galvanized nut on a non-galvanized bolt will result in an unacceptably loose fit and using a plain finish or zinc plated nut on a hot dip galvanized bolt will result in a fit that is much too tight).

Although fin and ribbed neck carriage bolt types are available, the most typical is square neck and, sometimes, short neck, which is also square. Table 2 summarizes common names and standard designations for the various types.

Refer to American Society of Mechanical Engineers Standard ASME B18.5, Round Head Bolts, for specifications relating to carriage bolts.

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