an expansion head for expansion tools for hollow work - pieces , particularly for the ends of plastic pipes and metal - plastic composite pipes , comprising a union cap having a guide flange directed radially inward , and a set of expandable jaws of sector - shaped design , each received individually by an inner flange sector overlapping the guide flange so that they are radially movable in the union cap , wherein the set of sector - shaped expandable jaws is movable between a closed state and an expansion state , and in a closed state , an approximately cylindrical lateral area is formed by an envelope of the outer walls of the set of expandable jaws , wherein characterized in that each expandable jaw is provided with at least one recess extending in the direction of the longitudinal axis of the expandable jaws so that in a closed state of the expansion head , the surface of the outer walls of the set of expandable jaws corresponds to 25 % to 75 % of the surface of the envelope of the outer walls of the set of expandable jaws .