Abstract: This invention relates to a radial piston device with a rotatable casing useful as a pump or a motor. The device is highly efficient with an overall efficiency of 0.95 and a mechanical efficiency of 0.97, a wide rotational speed range of over 1000 rpm, and can be made with continuously variable or fixed displacement. The device is compact in size, light in weight and simple in structure making it easy to manufacture. Moreover, the device can be constructed from a variety of materials without restrictive material requirements, can operate with both filtered and non-filtered oil, and is not sensitive to environmental effects. When the device is mounted on a wheel to form a hydrostatic transmission system, the layout and performance of the vehicle are significantly improved. The displacement control means provided by the present invention is also applicable to other motors or pumps with eccentric shafts. This invention also provides a simple and reliable flat oil seal.