ring is permanently mechanically deformed radially inwardly into the groove as shown in fig4 thereby to create a mechanical interference between the ring and the rod this will prevent the piston from moving axially or becoming detached from the rod in addition the mechanical interference is effected in a manner that minimizes stress concentrations in the functional portions of the mating components in particular the stress concentration caused by deformation of the lock ring is isolated from the piston proper overall this arrangement provides better fatigue properties than prior art designs as well as better strength and ease of assembly the lock ring 16 should be small enough in radial dimension to allow for adequate deformation of the lock ring into the groove 20 and yet large enough to provide adequate resistance to forces imposed on the piston and rod assembly when in use preferably the lock ring is sufficiently deformed to fill the groove the lock ring also may be mechanically deformed into the locking groove sufficiently to provide a radial as well as an axial interference that prevents rotation of the piston relative to the rod while also preventing axial movement of the piston relative to the rod as above mentioned a permanent and fluid tight seal may be formed when the lock ring 16 is mechanically deformed into the groove 20 this seal may be sufficient for various applications for higher pressure applications for example additional sealing may be desired to this end.