A reciprocatory machine (8) having a cylinder (12) defining with at least one piston (18) a working chamber (13a). The or each piston is reciprocable in the cylinder by means of a respective crankshaft (40). The machine has intake and exhaust ports (26 and 27) disposed at opposite ends of the working part of the cylinder and having respective valves (10 and 11) additional to the pistons for opening and closing the ports. The valves are driven by the crankshaft(s) (40) and have provision (68) for adjusting the timing relationship between the displacement of the crankshaft(s) and the valves and thereby the timing relationship between the respective valves. The location of the ports at opposite ends of the working part of the cylinder permits considerably greater adjustment of the timing than is possible with conventional closely spaced porting arrangements. The machine preferably operates as a two-stroke internal combustion engine but may also operate as a pump or compressor.