Flat Head Carriage bolts may be available in other diameters however the only standard diameter is in the 1/4" with the pictch of 1/4-20. These have a bearing surface that is countersunk to a 150 degree angle. This bolt has a protruding square under the head for locking the bolt into a mating square hole. These are used mostly in the garage door industry. See below for the lengths they are available in.
