of the compressibility of the clamping ring 10 the clamping ring 10 will exert in the state shown in fig5 on the close off member 4 a force which tends to press the close off member 4 back out of the cylinder body 2 that is to say in a direction opposed to the direction of arrow a this movement of the close off member 4 is however counteracted by the coupling of the coupling ring 8 that is produced in the two peripheral grooves 7 and 14 this is shown in fig6 on reaching the position as indicated in fig6 the coupling ring 8 rests against a substantially circular portion of the peripheral groove 14 in the inner surface 16 of the cylinder body 2 and the coupling ring 8 also rests against a substantially circular portion of the peripheral groove 7 in this way on reaching this end position radially directed forces are no longer exerted on the coupling ring 8 as a result of this preferred embodiment of the peripheral grooves 7 and 14 this is avoided this is an advantage over the construction as proposed in u s pat no 6 263 779 a snap connection which is.