Abstract: A compact, rotary gas-operated motor having only two moving components, including valving. Pressurized gas causes an elongate rotor to rotate end-over-end within a generally triangular chamber and causes a crank to slide in a slot in the elongate rotor. A disk rotating with the shaft includes ports to admit pressurized gas and exhaust spent gas. The inlet port is shaped and positioned to provide early cut-off of inlet gas and provide up to 2:1 expansion ratio of the pressurized gas. Still earlier cut-off of inlet gas, at a location between the rotating disk and its housing, provides expansion ratios as high as 9:1. A movable guillotine may be used to provide continuously variable expansion ratios of from about 3:1 to 9:1.