Displacer piston assembly

A displacer piston assembly comprising a displacer dome, a displacer base, displacer piston rings and a displacer guide ring, in which said displacer dome and displacer base are held together by corresponding internal and external threads, in which the displacer dome has a shoulder (S1) with length L1 and diameter D and the displacer base has a shoulder S2 with length L2 and diameter D2, and the diameter D is approximately equal to the diameter D2, and in which the displacer guide ring is positioned between the shoulders S1 and S2 in which the lengths L1 and L2 are less than the length L4 of the displacer guide ring.