Abstract: An oscillatory rotary actuator is provided for controlling an adjustable valve. The actuator comprises a freely rotating, cylindrical housing with concentric inner and outer walls about its center. The housing has at least one depending vane, which functions as a rotary piston, that is radially disposed between the inner and outer walls. The rotary actuator further comprises an annular chamber casing which has two concentric walls that are designed to mate between the inner and outer walls of the housing member to receive the depending vane(s) between the concentric walls of the chamber casing. At least one radially extending chamber wall is interposed and attached to the concentric walls of the chamber casing. When the housing member is fitted over or receives the annular chamber casing, the housing, chamber casing and chamber wall collectively form at least one rotary chamber for the arcuate movement of the depending vane, or rotary piston, therein.