A PISTON FOR COLD-CHAMBER INJECTION MACHINES comprising a support (2) and an external body (3) with a sealing ring (5), joined by a bayonet lock ring (6) independent of the external body (3) and divided into two parts (6a) fitted onto a groove (21) in the support (2) and connected by long screws (7) inserted into threaded holes (61). The piston preferably includes a spring bush (4) as a part that adjusts to the container, independent of the external body (3) and of the bayonet lock ring (6), under which a conical spring ring (8) is included which exerts pressure on the two parts that form the bush (4), tending to expand the same. The piston also includes alternating transverse grooves (81) along the edges thereof for receiving conical metal tips (9) inserted through holes (63) in the bayonet lock ring (6).