Seal ring free piston structure

A piston adapted for a cylinder, the piston comprising:

• 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 disk connecting to the first annular disk via the neck, multiple through holes defined through the second annular disk to communicate with a first space confined by a bottom face of the second annular disk, an outer periphery of the neck and a top face of 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,
• whereby when the piston is moved relative to the cylinder, a part of fluid inside the cylinder flows to the annular recess via the slits to substantially form hydraulic sacs around the piston so as to maintain balance of axial movement of the piston inside the cylinder.