a blow head for extruding a tubular plastic preform from an outlet gap between a fixed die core and a die body allows a change of the width of the outlet gap as a function of the circumference using light and small actuating devices , e . g ., double - acting actuator cylinders , if the die body has its upper end , facing away from the outlet gap , pivotably mounted in the blow head and a pressure ring is positioned in the outlet gap , on which the actuating devices act .