Abstract: Experiments based on theories pertaining to elasticity, finite deformation and fluid sealing have been conducted to provide data for designing pistons and nylon rings. Radial displacement of surface of nylon ring, stress distribution in the whole piston and contact stress distribution on the surface of piston are determined by strain gage measurement, photoelastic method and pressure compensation method. Modification tests of the construction and the property of materials used for the nylon rings have also been carried out. From the results of experiment, it has been proved that the home-made triangular or rectangular nylon rings offer good protection to the heel of the piston. Adequate regulation of the tolerance between the nylon ring and the cylinder may reduce the surface contacting stress of the piston, and the property of materials used for making the nylon rings may be ameliorated by heat treatment.