the channel 106 normally the diameter of channel 106 constitutes one of the design data and consequently determines the dimensions of the other parts on the side of the first abutment surface 17 opposite to sleeve 9 and therefore above it the main body 3 comprises an upper portion 19 able to engage with a corresponding upper portion 20 of slider 4 formed on the opposite side relative to the first abutment surface 18 to that of the central portion 6 this upper portion 19 of the main body 3 is advantageously cylindrical having an inner diameter d 3 gt d 1 the bottom portion 19 of the main body 3 has in the vicinity of its upper end namely opposite to plane u03b1 a third and a fifth abutment surface denoted by 25 and 27 in fig2 respectively these surfaces are advantageously formed on a circumferential projection 23 projecting towards the inside of main body 3 and with an inner diameter d 5 the abutment surfaces 25 and 27 therefore define two conical surfaces having opposite vertices with respect to the projection 23 and are advantageously inclined relative to axis x x by about 15 u00b0 and about 45 u00b0 respectively the upper portion 20 of slider 4 terminates in an end 22 opposite to the bottom portion 8 where the outer diameter d 4 is such as to engage with slight interference with the upper portion 19 having a diameter d 3 so as to prevent any relative movements of slider 4 and main body 3 the upper portion 20 may advantageously have a cylindrical section with an outer diameter d 6 not indicated in the figures where.