Abstract: This invention relates to a hydraulic mechanism comprising a cam; a cylinder-block mounted to rotate with respect to the cam; a plurality of cylinders arranged in this cylinder-block; a plurality of pistons mounted to slide inside a cylinder; a plurality of rollers for abutment of the pistons on the cam, each roller being mounted on a piston, rotating by means of a bearing for rotation, being axially defined by two transverse faces, and being capable of penetrating, at least partially, inside the cylinder in which this piston slides, the end of the piston comprising two recesses corresponding at least to the intersection of the cylindrical surfaces of the roller and of the piston, being open in the upper part of the piston; and two wedging pieces which are each disposed in a recess between the roller and the cylinder, in order to ensure axial wedging of the roller parallel to its axis of rotation.