Abstract: A housing has a cylindrical interior which has a peripheral seal chamber parallel to the cylinder axis. Inlet and exhaust ports are formed in the housing, preferably in or near the seal chamber. These ports are sealed from each other by a freely rolling cylinder which rolls on a rolling piston cylinder mounted on a shaft with an eccentric and with a bearing between the eccentric and the piston. The rolling seal also engages the seal chamber, and to reduce friction a rotatable support on a fixed axis is disposed in the seal chamber and in turn is sealed by a rubbing seal to the seal chamber wall. The rolling seal cylinder retreats into the seal chamber when the piston cylinder reaches the region of the seal chamber. This structure eliminates prior art rubbing seals which engaged the rolling piston.