Hillman Zinc Carriage Bolt 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. 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.