purpose this width is measured by means of sensors not illustrated if a reduction in the gap width is desired the relevant guide part 14 and or 15 is displaced in the direction of the gas outlet opening 9 by the motion of the above described presses in the process the stop rib 19 is stressed in a sprung manner so that in the event of a required movement in the opposite direction it shifts the guide part 14 or 15 carrying it back in the direction of the hot gas chamber 8 in order to carry out this task the presses respectively associated with the same guide part 14 or 15 together reach an axial force which corresponds approximately to 10 times an axial force induced by operation and exerted by the gas stream 6 on the relevant guide part 14 or 15 in this case both axial forces act in the direction of the gas outlet opening 9 and are added to each other the deformation energy absorbed by the stop rib 19 during its deformation is stored when a guide part 14 or 15 is displaced in the direction of the gas outlet opening 9 and in the event of an opposing movement serves to generate a restoring force this restoring force is greater in every position of the associated guide part 14 or 15 than the axial force exerted on the latter by the gas stream 6 and induced by operation the restoring force is preferably about 2 to.