Abstract: A piston-ring system has two, equal-radius, coaxial, axially-contiguous sealing rings, each ring having a split circumferential offset from the split in the other ring, each split being substantially Z-shaped in a plane normal to the axis of the rings to have a substantially-circumferential, overlappable middle portion. Each ring is urged, at least at the middle portion of its split, radially.

Abstract: A method of exchanging the displacement element of a refrigerator having a divided housing whose parts can be separated from one another in such a manner that the displacement element becomes accessible, and a refrigerator structure which facilitates the method. In order to be able to exchange the displacement element in the cold state without the danger of condensation in the displacement element chamber, a stream of protective gas is maintained during opening of the housing, during the exchange of the displacement element and during closing of the housing so that undesirable gases are unable to penetrate into the displacement element work chamber. To facilitate the method, one of the parts of the housing has the work chamber formed therein and high and low pressure gas ports are provided in this part.