Coach bolts, also known as carriage bolts, are extremely heavy duty, used to fasten metal to wood or other heavy woods. They are ideal for structures which require a sturdy joint. Famous for its shallow mushroom head and that shank cross-section of the bolt is circular for most of its length. However the piece under the head is square to prevent twisting. The square piece locks into place when used in a square hole. Coach bolts are broadly used in security fixings where the bolt must be only removable from one side. They are fastened using a wrench or spanner.
Where are coach bolts used? Working with timber Building and construction DIY