A pyrotechnic tightening device 1 of a safety belt unit having a force limiting unit, comprising—a gas generator, and—a piston 19, which is guided in a pipe and which closes a pressure chamber inside the pipe, wherein—a pressure can be applied to the pressure chamber by the gas generator, and—the piston can be driven by the pressure present in the pressure chamber to perform a tightening movement which can be transmitted by means of a force transmitting unit to the safety belt, wherein—the piston is formed in two parts consisting of a main body and a contact part arranged on the side of the main body facing the force transmitting unit, wherein—the contact part has a greater strength than the main body, wherein—the main body surrounds the contact part form fittingly at the end face facing the force transmitting device.