Abstract: The method of making a piston unit for an internal combustion engine incorporates the steps of pinching a melting core in a mold cavity of a piston mold wherein the melting core includes an inner ring that forms a ring shaped passage within a piston body, an outer ring and a plurality of coupling portions connected with between the inner ring and the outer ring. The method further includes pouring molten metallic alloy into the cavity and solidifying therein, removing the outer ring of the melting core, filling the space left by removal of the outer ring with reinforced material, and then melting the inner ring and the coupling portions of the melting core.