7 the part 5 is preferably integral with the cylindrical part 3 an internal cylindrical portion 10 projects from an inner side 8 with respect to the body 2 of said part 5 with which it is integral and extends as far as the second end 7 of the body 2 this internal portion 10 hence forms a part of the body 2 i e is in one piece with it the internal portion 10 is coaxial with the cylindrical part or outer wall 3 of the body 2 between these there being a recess or interspace 12 the portion 10 and said wall 3 are connected together by ribs 11 k which stiffen the body 2 and prevent its deformation with time due to thermal cycles vibrations or atmospheric agents as will be described this recess 12 of the body 2 is in communication with a compartment 13 present between the closure part 5 and a first end 14 of a cylindrical member 15 inserted coaxially into the internal cylindrical portion 10 of the body 2 and retained within this latter by interference with said portion said member 15 is of yieldable material such as natural or synthetic rubber or equivalent material and comprises a second end 17 projecting from the.