In natural gas vehicle refueling stations, the reciprocating compressor is the key element in defining both the refueling process efficiency and the operating costs of the whole station. Great attention is therefore devoted at increasing both the performance and reliability of the machine. The valve-retainer is a component that plays a key role in determining the compressor performance, by introducing significant pressure losses, which affect the suction and discharge phases. However, the valve-retainer is essential for the proper orientation of the valve during the compressor operation and guarantees a simple and reliable design of the machine.