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Carriage Bolts

Used in many different large construction projects, carriage bolts are threaded bolts with a round, smooth mushroom head. The shallow mushroom head of carriage bolt distinguishes it from most other bolts. Carriage bolts are threaded along either the full length or part of its shank, largely depending on the length of the bolt. The square neck right beneath bearing surface of the head seats into the hole in which it is placed, usually in wood, to prevent the bolt from rotating when the nut is tightened or removed. When properly positioned, carriage bolts becomes self-locking due to the unthreaded section right beneath its head forming into a squared neck. As a result, only a single tool such as a wrench is required to install the fastener.
Flat Head, Square Neck Round Head, Ribbed Neck
Carriage Bolts - Flat Head, Square Neck
Carriage Bolts - Round Head, Ribbed Neck
Round Head, Short Square Neck Round Head, Square Neck
Carriage Bolts - Round Head, Short Square Neck
Carriage Bolts - Round Head, Square Neck
Track Bolt
Carriage Bolts - Track Bolt
Extensively used as a security fitting, carriage bolts have a smooth domed head which prevent them from being loosened from the top (head) side. When the nut is tightened or removed, a carriage bolt resists turning due to its square shoulder under a rounded head. Carriage bolts may be used for surfaces which require a smooth finish. Some examples are construction of decks, swings sets, and docks. In short, carriage bolts are self-anchoring bolts designed for wood-to-metal or wood-to-wood applications.

Since carriage bolts are used predominantly in wood-to-wood or wood-to-metal applications in wood, they are usually supplied in low tensile steel. Carriage bolts are also available in a variety of materials and finishes depending on their application. Because they are often used in wood applications, they typically do not need to be of high grade steel. They are often used outdoors and consequently exposed to the elements. As such, they are available in corrosion resistant materials or corrosion resistant finishes:

Aspen Fasteners offers carriage bolts in lengths up to 24” and diameters from #8-32 to 7/8" in steel and stainless steel 18-8. In addition to the different materials, carriage bolts are available in different forms reflecting their specific applications for which they are being used. These include:

Uses of carriage bolts

Some of the uses for carriage bolts include:

Arbor Construction

Carriage bolts can be used to join two pieces of wood together in a solid manner. The carriage bolt is secured in place with a nut and a washer after it’s inserted through pre-drilled hole. Furthermore the low profile mushroom head has a large bearing surface to provide support without the use of a washer and has a relatively appealing finish.


Carriage bolts are often used in the construction of the underlying frame and supports of wood decks. They provide an important role in maintaining the structural integrity of the deck. As this use is typically outdoors and exposed to the elements the material of the carriage bolt is of great importance. Stainless steel 18-8 or hot dipped galvanized bolts are most often the choice here to provide some level of corrosion resistance.


Carriage bolts are an important part of dock construction. The use of carriage bolts is required on many areas on a dock. Examples of this are the boards, frame and rails. In this case due to the constant exposure to moisture, either stainless steel or galvanized steel would be preferred to extend the life span of the bolt and underlying structure.

Building projects

An important part of many construction projects, carriage bolts are manufactured in many different sizes including small bolts for DIY home construction and big-size carriage bolts for large building projects. This is the reason carriage bolts are demanded by both big construction companies and homeowners. In order to join segments together, carriage bolts are often used within a structure. Far superior to nails and other bolt types, carriage bolts are extremely sturdy.

Swing sets

The construction of children’s swing sets or play structures is where carriage bolts are regularly employed. In fact, carriage bolts are an integral part of swing sets or play area construction. Because of their smooth rounded low profile heads, the offer a unique ability to join structures together without large and sharp protrusions on which children could easily injure themselves.

Carriage bolt dimensions and technical Specifications

Carriage Bolts - Round Head, Square Neck (ASME B18.5-1990)
Bolt Diameter Body Diameter Head Diameter Head Height Square Width Square Depth
Max Min Max Min Max Min Max Min Max Min
#6 0.1380 0.122 0.108 0.313 0.278 0.086 0.071 0.160 0.135 0.094 0.062
#8 0.1640 0.173 0.157 0.328 0.298 0.102 0.083 0.169 0.155 0.108 0.078
#10 0.1900 0.199 0.182 0.469 0.436 0.114 0.094 0.199 0.185 0.125 0.094
#12 0.2160 0.225 0.206 0.500 0.468 0.149 0.125 0.215 0.197 0.135 0.105
1/4" 0.2500 0.260 0.237 0.594 0.563 0.145 0.125 0.260 0.245 0.156 0.125
5/16" 0.3125 0.324 0.298 0.719 0.688 0.176 0.156 0.324 0.307 0.187 0.156
3/8" 0.3750 0.388 0.360 0.844 0.782 0.208 0.188 0.388 0.368 0.219 0.188
7/16" 0.4375 0.452 0.421 0.969 0.907 0.239 0.219 0.452 0.431 0.250 0.219
1/2" 0.5000 0.515 0.483 1.094 1.032 0.270 0.250 0.515 0.492 0.281 0.250
5/8" 0.6250 0.642 0.605 1.344 1.219 0.344 0.313 0.642 0.616 0.344 0.313
3/4" 0.7500 0.768 0.729 1.594 1.469 0.406 0.375 0.768 0.741 0.406 0.375
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