Carriage bolts are mostly used in wood, and are also known as plow bolts. They have a domed top and a square under the head. The carriage bolt square pulls into the wood as the nut is tightened for a very secure fit.
Available in a variety of diameters, carriage bolts are a common choice for any job. Carriage bolts are made from a variety of materials and different grades of steel to accommodate the abundance of applications in which they are used. Below are a few of the most common types of carriage bolts.
Zinc-plated bolts: Moderate protection against rust.
Steel grade 5 bolts: Medium carbon steel; used in high-strength automotive applications.
Stainless steel 18-8 bolts: This material of choice for exterior and marine applications is produced from an alloy of steel with a high corrosion resistance.
Silicone bronze bolts: Used in wood boat building this alloy of copper has better strength and corrosion resistance than brass.
Hot dipped galvanized bolts: Much more corrosion resistant than zinc-plated. These thick coated carriage bolts work with galvanized nuts for exterior use in the coastal areas.