Abstract: The piston is provided with a pair of radial holes opened to an external circumferential recess and lodging and retaining one respective end of a pin, to which is coupled a driving mechanism to reciprocate the piston inside a cylinder. The radial holes are completely sealed in order that the axial extension of a bottom radial gap provided by a bottom bearing surface of the piston work together with the axial extension of a top radial gap provided by a top bearing surface to provide the required leakage restriction of the refrigerant gas and also the required bearing of the piston inside the cylinder. The axial extension of each of the top bearing surface and of the bottom bearing surface is insufficient to provide by itself said required leakage restriction of the refrigerant gas.