Abstract: The external rotor of a single-rotation machine has engagement spaces, whose boundary faces in the inward direction form a widening, which is adjacent in an outer region radial thereto to parallel surfaces or surfaces forming a widening in the outward direction. The side walls axially bounding these engagement spaces have a radially outer part, which is cast in one piece with the peripheral parts of the external rotor and an inner part radial thereto, which subsequently reduces to a circular opening the opening surrounded by the radially outer parts, so that the radial seal between side walls and a stationary sealing member surrounded by said opening is located radially within the peripheral parts of the external rotor and a sealing gap in the radial plane between the peripheral parts and the sealing member is avoided.