A carriage bolt, also known as a coach bolt, has a domed or countersunk head and the shank is topped by a short square section under the head of the bolt. The square section grips into the part being fixed, preventing the bolt from turning when the nut is tightened. Carriage bolts are often used to fasten wood panels or board to masonry or to one another. The square shank of a carriage bolt allows it to lock into place when inserted into a round hole in wood or a square hole in a metal strap, the unthreaded square shank pulls into the wood creating a tight connection.
Carriage bolts are available in 18-8 Stainless Steel, Hot Dipped Galvanized 307 Grade A Steel, and Zinc Plated 307 Grade A Steel. Each finish offers its own benefits including hardened metal and corrosion resistance. Choose a type of metal to view different carriage bolt sizes.
18-8 Stainless Steel Carriage Bolt
Stainless steel carriage bolts used primarily for long lasting applications, due to its corrosion-resistant nature and durability. Scratching or burring the metal will not create surface rust as the corrosion resistance exists within the metal itself. Stainless steel is a soft metal due to the low carbon content, therefore most stainless steel bolts are cold-formed and not heat treated or thru-hardened. Cold forming and threading causes stainless steel bolts to become slightly magnetic, some fasteners will be more magnetic than others depending on size and how quick the cold forming process is. Stainless steel carriage fasteners are typically a clean silver color, which also makes them common in finishing and decorative applications. Stainless Steel should never be used with aluminum, corrosion may occur.
(18% Chrome, 8% Nickel, .08% Maximum Carbon) – 18-8 Stainless refers to 300 series stainless steel. 303 and 304 Stainless are the most commonly listed grades, the standard grade for stainless steel fasteners. They are corrosion-resistant and durable. They are often used in marine applications in freshwater environments, but will not work as effectively in a salt water environment as 316 stainless. Stainless alloy resists oxidizing and rusting, however it can tarnish over time.
Hot Dipped Galvanized Steel Carriage Bolt
Hot dip galvanizing is a process that adds a layer of corrosion resistance by dipping fabricated steel into a kettle containing molten zinc. The HDG process creates a tightly bonded alloy for superior corrosion resistance and harsh environment applications. Hot dipped galvanized carriage bolts must be used exclusively with other HDG fasteners when applied, meaning that a plain zinc nut or stainless nut cannot be applied to an HDG bolt.
Zinc Plated Steel Carriage Bolt
Zinc plated steel is very common in the fastener industry. Zinc adds a layer of corrosion resistance to plain alloy steel. Clear zinc has a light, shiny color and blue zinc has a bright blue hue. Zinc plated steel is not suitable for extreme environment use, as it is not as resistant to corrosion as hot dipped galvanized.
Hot Dipped Galvanized Steel Timber Bolt
A Timber bolt, also known as a Mushroom head bolt or Dome head bolt, is similar to a Carriage Bolt in appearance but with an oversized, heavy duty low profile head. Timber Bolts are typically used in marine and wood applications to protect timber structures from the elements. The underside of the oversize head of a timber bolt features rings that help prevent water from entering the hole and nubs or fins which prevent the bolt from turning. The timber bolt's oversize head eliminates the need for a washer on the bolt head side. Our Timber Bolts come in 3/4" and 5/8" size diameters.
A hot dipped galvanized finish consists of running a steel fastener through a molten zinc bath that creates a tightly bonded alloy finish. This finish creates a highly corrosion resistant finish in the fasteners. HDG products must be used exclusively when applied, meaning that a plain zinc nut or stainless nut cannot be applied to an HDG bolt.
To learn more about the different metals used check out our Fastener Grades, Strength and Materials page.
How to Install and Remove Carriage Bolts: