PFC Carriage Bolts – Carriage Bolts or Carriage Screws have a square neck that keeps them from turning once tightened. Use in square holes for wood-to-wood and wood-to-metal fastening. Length is measured from under the head. Fasteners are typically made of low-carbon steel. Similar to a flat-head style, the oval head style has a slightly convex top surface; it is typically used for appearance. Coarse threads are better when working with brittle materials; they are sturdier and are easier to thread and unthread compared to fine. Coarse threading also allows for thicker coatings and platings.