A pump comprises multiple, axially stacked positive displacement fluid device sections, such as circumferential piston pumps having chambers and contra-rotating chambers. The device can be similarly employed as a fluid motor. The stacked sections are arranged within an outer retaining barrel in one or more stages. The pump is particularly suitable for installation downhole in the casing of a wellbore. Each section comprises a pair of rotors fit to shafts which are rotatably supported on hard faced bearings between the shafts and the bosses. Each pump section draws fluid from an inlet port and discharges fluid to a common and contiguous discharge manifold. The inlets of the pump sections for a suction stage communicate with a fluid source. Cross-over sections route fluid between stages. Successive pressure stages draw fluid from the cross-over fit to the preceding stage's discharge manifold.