Every flange pair is joined by a ring of bolts passing through the bolt holes on the bolt circle. A bolt has a cylindrical body with a screw thread at one end and a head at the other. Its counterpart is the nut, the female fastener that threads on from the far side. Steel is the most common material, with the grade and finish chosen to suit the service.
Most flange bolting uses hex bolts, which have a six-sided head. A six-point spanner or socket grips every face, so there is less risk of rounding the bolt when you apply torque.
A 12-point head lets a spanner engage from more angles, which helps in tight spaces. More contact points also let you apply more torque before the head rounds off.
Tip Use a spanner with the same point count as the head. A 12-point spanner on a 6-point head touches only the corners, which concentrates the load there and makes rounding more likely.