A sealing and guiding device for the injection piston (3) of a hot chamber pump for corrosive alloys made up of a body (1) in which a chamber (6) and an injection cavity (2) therebelow are formed, wherein the piston (3) slides with a vertical reciprocating motion (V), includes a bush (8), having an outer diameter smaller than said chamber (6), interposed between centering members housed in the chamber (6) and made coaxial with the chamber (6) and with the piston (3) by a pair of opposite surfaces of revolution (12, 13) formed on said centering members, the upper surface (13) and the relevant centering member performing a pressure-tight sealing. The outer lateral surface (15) of the bush (8) is in communication with the injection cavity (2) whereas the upper side of the bush (8) is in communication with the crucible and immersed therein at a depth (L), measured from the lowest level (23) reachable by the molten alloy, which is greater than the maximum travel (C) of the piston (3). The inner surface (21) of the bush (8) is a surface of revolution with diameter just larger than the piston (3).