rotational movements of the shaft 48 produce very small increments of movement of the chasers thus permitting very precise sizing adjustments fig7 a illustrates the position of the components after sizing adjustments have been made and just prior to threading in this position all of the components are in their limit of rearward movement fig7 b shows the positions of the components upon completion of threading but prior to opening of the die head the horizontal forward motion of the rods during the threading operation is indicated at 84 and the smaller horizontal forward motion at the headstock is indicated at 86 the differential motion being due to the rotation of the yoke 46 in a counterclockwise direction about the axis of shaft 48 producing a slight opening movement of the chasers to thereby produce the desired tapered thread the receding motion of the chaser is indicated at 92 fig7 c shows the positions of the control components and the position of the die head at the conclusion of the opening movement which is effected by rotation of the shaft 48 in a counterclockwise direction as viewed in fig6 and 7c as a result of the rotation of shaft 48 the yoke 46 is offset forwardly as indicated at 100 this in turn causes a forward movement of the shafts 34 rotating the bellcranks 28 in a counterclockwise direction to move the chasers radially outwardly to open position this movement being indicated at 102 to ready the die head for.