Bolt: Machine part, cylindrical threaded fastener with nut. A type of fastener consisting of a head and a screw (a cylinder with external threads) that is fitted with a nut for fastening two parts with through-holes. This type of connection is called bolt connection. If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a detachable connection. According to the thread tooth type is divided into coarse teeth and fine teeth two categories, coarse teeth in the bolt logo does not show. Bolts are divided into grades 4.8, 8.8, 10.9 and 12.9 according to the performance grade. Bolts above 8.8 (including 8.8) are made of low-carbon alloy steel or medium carbon steel and treated with heat treatment (quenching and tempering), generally known as high strength bolts, and bolts below 8.8 (excluding 8.8) are generally known as ordinary bolts