Carriage Bolts are mostly used in wood or metal to wood applications with a domed top and a square body under the head. This square body pulls into the wood as the nut is tightened. When using carriage bolts always use a flat washer between the wood and hex nut to eliminate pull-through of the nut in the wood. Drill pilot hole slightly larger than body diameter of bolt. Insert the carriage bolt so the headed end is on finished side of project for appearance or safety. Length is measured from under head to end of bolt. Use stainless steel where corrorsion is and rust is of concern or for exterior applications. 18.8 Stainless steel is an alloy steel that provides one of the highest degree of corrosion resistance when compared to a steel zinc finish. Ideal for outdoor projects such as decks, fences, wood structures and with treated lumber installations.