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.
18-8 Stainless Steel Carriage Bolt
Stainless steel is 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 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.