Abstract: An injection pump for diesel-cycle internal combustion engines includes a casing (1), a cylinder (6) within the casing and communicates, via at least one radial aperture (51), with a fuel intake chamber (3), and a piston (7) sealedly slidable within the cylinder and which, by external command, can also rotate about its axis to assume different angular orientations about a reference plane. In its jacket, a helical groove (72) opens into an axial groove (71), wherein the top, or roof, of the piston (7) is connected via a hole (77) to two channels (178, 179) provided at different levels in the piston jacket.