Abstract: A piston assembly adapted for use in nonlubricated high pressure compressors, comprising a piston having a plurality of peripheral grooves therein with a ring assembly in each groove constituted of one seal ring and one guide ring in a common groove. The guide ring is behind the seal ring in a direction away from the compression end and has a surface remote from the compression end which is at least partially conical in shape and abuts against a corresponding conical shaped surface of the associated groove. The seal ring and guide ring abut along planar surfaces, the guide ring being made from non-metallic material such as a carbon-graphite material providing higher resistance to shape deformation then the material of the seal ring which can be polytetrafluoroethylene. The guide ring thus prevents plastic deformation of the seal ring particularly at very high pressures of compressed gas.