A diecasting machine casting unit includes a casting chamber body having a casting chamber with a casting material inlet and outlet. A casting piston is movable forward in a longitudinal direction in the casting chamber to discharge casting material from the casting chamber under pressure via the outlet, and is movable back, whereby casting material is feedable into the casting chamber via the inlet. The casting piston extends through a through-passage of the casting chamber body from outside into the casting chamber. An area of free space of the casting chamber is formed between an outer lateral surface of the casting piston and an inner wall surface of the casting chamber body transversely relative to the casting piston longitudinal direction by an outer cross section of the casting piston being appropriately smaller than an inner cross section of the casting chamber body.