Tap for a cylinder of gas under pressure

A tap for a cylinder of gas under pressure, said tap comprising a control body for fixing on a cylinder head and defining a chamber, means defining a gas flow passage for connection to the cylinder head, a valve for opening and closing said passage and provided with a valve rod, a sealing bellow, said valve rod being movable through said bellows, a pneumatic control piston slidable in said chamber and having piston rod connected to said valve rod, a return spring associated with the piston for biasing the piston to a given position for closing said valve, manual means combined with the piston for axially shifting the piston, said manual means comprising an axial extension of the piston rod, said axial extension having an end portion and adjustable abutment means comprising a hand wheel member, wherein said hand wheel member includes a cylindrical member having a cylindrical cavity and an inner end, said end portion of said axial extension being spaced from said inner end of said cylindrical cavity to define a gap therebetween, the hand wheel member being freely slidably engaged around said extension of the piston rod and having an inner end wall providing an abutment for said extension of the piston rod as said piston is axially shifted, said hand wheel member being screw-threadedly engaged in said control body for variably changing said gap while defining different abutment positions between said end portion of said axial extension and said inner end of said cylindrical cavity for varying the degree of opening of said valve.