Hillman 5/16 In. x 8 In. Grade 2 Zinc Carriage Bolt (50 Ct.)Carriage bolts are designed to fasten a wooden face to either a wood or metal frame. Its flat head is designed to eliminate the need for a washer and provide a semi flush finish to applications. The square underside of the head is self locking to eliminate spinning and the need for a driving tool. The zinc plating is rust resistant and provides a beautiful, consistent finish, but, may not work for all exterior projects.