Abstract: A cast piston which includes a metal insert and a combustion bowl, with the metal insert defining a sharp undercut structure at an upper portion of the combustion bowl. The sharp undercut structure of the insert contains combustible gases and helps improve emissions. The metal insert also strengthens the head of the piston. The cast piston is fabricated by positioning the metal insert in a casting mold and casting a piston in the mold. The final shape of the combustion bowl is produced by a machining process which forms the sharp undercut structure on the insert.

Abstract: A piston for connection by a piston pin and connecting rod to a crankshaft for reciprocating in a cylinder of an internal combustion engine. The piston has a crown and top land region, an intermediate region including at least one additional land, and a skirt region. Cross sections through the crown and top land region transverse to the cylinder axis are ovals whose major axes are generally parallel to the longitudinal axis of the piston pin. Cross sections through the intermediate region are either circular or oval, major axes of each of the ovals being generally transverse to the longitudinal axis of the piston pin.