bushing 20 has a hexagonal actuating hole 18 intended for an allen key the thread bushing 20 with the rotatably connected abutment means 16 thus is axially adjustable relative to the coupling device 7 in its threaded hole 19 referring once more to fig3 it is to be seen that the elongated actuator 6 in its lower end preferably is provided with an external thread to be connected to the hole 27 of the closure element 5 furthermore the actuator 6 is in its upper end suitably provided with an internal thread to be connected to and receive an actuating device 10 with an externally threaded pin 23 the actuator 6 is to be inserted through the hexagonal actuating hole 18 in the thread bushing 20 for the intended connection of the actuator at both ends the upper end of the actuator can also be provided with a diametrical groove 22 which is used to allow a complementary tool not shown to adjust the connection of the actuator 6 to the closure element 5 the actuating device 10 is preferably connectable to the coupling device 7 by an annular fastening means 24 the fastening means 24 is suitably provided with an internal thread which is adapted to a complementary external thread of the coupling device 7.