Abstract: A novel motion translation mechanism is provided for translating rotary motion to reciprocal motion or reciprocal motion to rotary motion. The mechanism is adapted for use in a positive displacement machine for pumping and compressing fluids. A shaft is journaled for rotation within a stationary housing. An inclined annular bearing surface having an annular bearing thereon is integral with the shaft intermediate the journals. Reciprocal motion is imparted to a piston by a yoke ring journaled within the housing and interconnected to the annular bearing. A continuous seal arrangement is carried by the piston. Novel valve means control fluid flow within the machine.