M 8 x 40 Coach Bolts + Nuts. Bzp.(Also known as Carriage Bolts or Cup Square Hex Bolts). Pack of 50. Low round head with square underneath to lock into wood. Timber to Timber or Timber to Metal applications. The bolt is inserted in hole in material & as the nut is tightened it draws the square section of the shank into the hole/wood & locks it leaving a low domed head finish.