US Patent 7387062 Seal ring free piston structure

A piston includes a cylindrical body having a neck formed on an outer periphery of the cylindrical body so as to form a first annular disk and a second annular connecting to the first annular disk via the neck, multiple through holes defined through the second annular disk to communicate with a first space among the second annular disk, the neck and the first annular disk, an annular recess defined in an outer periphery of the second annular disk and multiple slits defined through a bottom face defining the annular recess to communicate with a second space below the first annular disk after passing through the neck.