A casting unit for a diecasting machine, comprising - a casting chamber body including a casting chamber which is capable of being filled with casting material and comprises a casting material inlet and a casting material outlet, and - a casting piston which is capable of being moved forward in a longitudinal direction of the casting piston in the casting chamber, in order to discharge casting material from the casting chamber under pressure via the casting material outlet, and moved backward, in order to feed casting material into the casting chamber via the casting material inlet, - wherein 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 being formed between an outer lateral surface of the casting piston moved forward into the casting chamber and an inner wall surface of the casting chamber body lying opposite said outer lateral surface transversely in relation to the longitudinal direction of the casting piston, by an outer cross section (d) of the casting piston being correspondingly smaller than an inner cross section of the casting chamber body, characterized in that - a casting piston temperature control device for actively controlling the temperature of the casting piston, at least in certain regions, is provided and designed for allowing the temperature of the casting piston to be actively controlled according to a predeterminable temperature profile along at least part of its length that can be moved forward into the casting chamber, and/or - a sealing element for sealing off the casting piston passage is provided at an inner side of the through-passage so that it presses sealingly against the passing casting piston by the pressure of the casting material in the casting chamber, where an outer diameter (d) of the casting piston substantially equals a diameter of the through-passage, and/or - the casting piston has an outer diameter (d) which at least over a part of the casting piston that can be moved into the casting chamber and through the through-passage is constant and substantially equals the diameter of the through-passage, where an annular gap remains as the area of free space of the casting chamber between the casting piston portion moved forward into the casting chamber and the radially opposed inner wall surface of the casting chamber body which has an inner diameter greater than the outer diameter (d) of the casting piston, said annular gap permanently belonging to the casting chamber volume not blocked by the casting piston. - a casting chamber body including a casting chamber which is capable of being filled with casting material and comprises a casting material inlet and a casting material outlet, and - a casting piston which is capable of being moved forward in a longitudinal direction of the casting piston in the casting chamber, in order to discharge casting material from the casting chamber under pressure via the casting material outlet, and moved backward, in order to feed casting material into the casting chamber via the casting material inlet, - wherein 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 being formed between an outer lateral surface of the casting piston moved forward into the casting chamber and an inner wall surface of the casting chamber body lying opposite said outer lateral surface transversely in relation to the longitudinal direction of the casting piston, by an outer cross section (d) of the casting piston being correspondingly smaller than an inner cross section of the casting chamber body, - a casting piston temperature control device for actively controlling the temperature of the casting piston, at least in certain regions, is provided and designed for allowing the temperature of the casting piston to be actively controlled according to a predeterminable temperature profile along at least part of its length that can be moved forward into the casting chamber, and/or - a sealing element for sealing off the casting piston passage is provided at an inner side of the through-passage so that it presses sealingly against the passing casting piston by the pressure of the casting material in the casting chamber, where an outer diameter (d) of the casting piston substantially equals a diameter of the through-passage, and/or - the casting piston has an outer diameter (d) which at least over a part of the casting piston that can be moved into the casting chamber and through the through-passage is constant and substantially equals the diameter of the through-passage, where an annular gap remains as the area of free space of the casting chamber between the casting piston portion moved forward into the casting chamber and the radially opposed inner wall surface of the casting chamber body which has an inner diameter greater than the outer diameter (d) of the casting piston, said annular gap permanently belonging to the casting chamber volume not blocked by the casting piston.