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Cup head bolts, also known as cup head square neck, coach, or carriage bolts, are bolts with dome-shaped heads and square necks that are normally used in wood applications. They are optimal when used to fasten wood to metal however, they can also be used to fasten two timbers together as well. Carriage bolts are inserted into pre-drilled holes and nuts are threaded on to the shank to fasten the materials in place. The square neck of coach bolts locks itself into the wood to prevent it from spinning when tightened. Cup head bolts are available in various sizes and lengths.