An epicycling device with improved sealing. A housing is provided with adjacent walls spaced apart to allow a substantially planar partition to pass. A piston is linked eccentrically to a drive shaft and positioned within the housing, has a cross sectional shape for mating with the housing walls and includes a circular recessed surface at its corners. Seal housings with a hollow cylindrical cross sectional shape are provided at each corner of the piston housing. An inner seal element is positioned within each of the recessed surfaces with the inner seal element including an elongate cylindrical body with a slot therethrough. An outer seal element is positioned within the seal housings with the outer seal element including an elongate cylindrical body with a slot. A partition is positioned between each pair of the inner and outer seal elements and slidably engaged with the slots in the seal elements.