The present invention relates to a subassembly for forming a hydraulic motor after assembly onto an assembly including a shaft, said subassembly including a cover forming a housing element, a multilobe cam, a cylinder block opposite the cam, pistons guided so as to slide radially within the respective cylinders of the cylinder block and bearing on the lobes of the cam, and a valve intended to apply pressurized fluid to said pistons in series, wherein said subassembly includes means for temporarily attaching the cylinder block to the cover and means for accessing an element of the valve through the cover in order to enable, during installation, the angular positioning of said element with respect to the shaft, followed by the attachment thereof to the shaft. The present invention also relates to a method for assembling such a hydrobase.