US Patent 7464684 Composite piston for an internal combustion engine

The invention relates to an assembled piston (1) for an internal combustion engine, said piston consisting of an upper part (4) and a lower part (5) that are interconnected by means of an internal hexagon screw (6) consisting of an upper half(37) and a lower half (38). The upper half (37) of the internal hexagon screw (6) comprises an external thread (39) which is parallel to the external thread (40) of the lower screw half (38) and has a larger lead angle than the same (40). The lead angle resulting from the difference between the lead angles of the two external threads (39, 40) is very small and contributes to a high strength of the screw connection.