Abstract: A method of manufacturing a piston for internal combustion engines which utilize a core molding or casting system for forming a hollow chamber for the passage of cooling oil therethrough. The manufacturing method includes the steps of preparing a core so as to have at least a first and second foot portion extending therefrom; inserting a bar member made of magnetic materials within each of the first and second foot portions; positioning at least one permanent magnet on a core box; and fixing the core to the core box via the bar members due to the magnetic flux of the permanent magnet.