Abstract: The invention relates to a pressurized fluid mechanism comprising a plurality of pistons (14) and a plurality of different cylinder capacities. A particular cylinder capacity is selected by means of two identical slide valves (17, 18) which serve selectively to neutralize either a first overall cylinder capacity (45a) or a second overall cylinder capacity (45c), or both or neither of said first and second cylinder capacities. One application of the invention is to providing an efficient and compact motor which is cheap to produce.