The grade 2 steel carriage bolt with plain finish has a square neck and round head and meets American Society of Mechanical Engineers ASME B18.5 and ASTM A307 specifications. Grade 2 steel, composed of either low or medium carbon steel, is often used in applications where toughness is the primary consideration. The square neck prevents the bolt from turning once installed, even when a nut is tightened. The round head has a domed shape.
Carriage bolts, also called round head square neck bolts, are square-necked threaded fasteners commonly used in wood fastening applications. The bolt's square neck prevents it from turning once installed using a nut.