Abstract: A motor or pump assembly (1) comprises a cylinder block (4) with a plurality of radially positioned cylinders (3), a piston (2) arranged slidably reciprocable in each of the cylinders, a cam ring (7) cooperating with the pistons and being rotatably journalled on the cylinder block, and a distributor valve (9) arranged centrally in the cylinder block to control a flow of fluid to and from the cylinders in operation. The cam ring (7) is journalled on the cylinder block (4) with at least one ball bearing with balls (25) arranged on each side of the cylinder block. These balls run in grooves which are positioned close to the sides and periphery of the disc-shaped cylinder block. This provides a structure which has a very small axial length and which is simultaneously capable of absorbing great loads both radially and laterally.