Countersunk bolt is a flat-headed fastener. Countersunk bolts are available in a range of sizes and materials, and are produced to meet various specifications. The countersink angle and diameter are defined by the fastener being used. Common angles are 82 degrees for inch-sized fasteners and 90 degrees for metric fasteners.
To create a countersunk hole, a pilot hole is formed before applying the screw. This is done using a countersink, which is a bit or drill that makes a funnel-shaped enlargement at the outer end of a drilled hole.