Abstract: The machine has a holding pot containing molten magnesium in which a pumping chamber is submerged, a filling port is provided through a bottom wall of the pumping chamber, defined at least in part by a cylindrical wall surface, and an axially moveable control rod with a cylindrical shut-off part is moved between a position at which it is within the confines of the cylindrical wall surface and a position at which it is free of that wall surface. The clearance of the cylindrical wall surface and the cylindrical shut-off part of the control rod is such as to permit free movement of the rod part through the port, but, by virtue of the wetting of the surfaces by the molten magnesium, to seal the port against the pressures applied to it.