In this shot, the heated rod is inserted into the "gripper dies" to a predetermined depth that is marked on the rod with a sharp piece of soap stone. The dies are in 2 halves, split down the middle lengthwise so that the upper half can be moved up and down, in this case hydraulically. When the two halves come together, they basically "grip" the heated rod which has expanded due to the heat. keeping it from moving lengthwise.

Forging of crown staybolts