Abstract: A fluid actuated cylinder assembly including a piston and piston rod and having a cylinder head capable of shifting in any direction throughout 360.degree. along a plane perpendicular to the long axis of the cylinder. The piston and piston rod are rockable throughout 360.degree., the piston maintaining a fluid tight seal with the cylinder at all times. The cylinder, piston rod and shiftable head are preferably made of acetal resin or equivalent thermoplastic material. The piston comprises a sleeve made of acetal resin or equivalent thermoplastic material mounted on the piston rod and having an integral enlarged flange portion with a peripheral surface carrying a seal maintaining a sealing engagement with the cylinder. Alternatively, the piston comprises a spherical rubber member mounted on the piston rod and making the seal with the cylinder. The cylinder assembly can be pneumatically or hydraulically actuated and may be of the single-acting or double-acting type.