An air motor comprising a piston and cylinder unit and control means mounted wholly within a housing secured to the unit. The housing comprises two parts (11, 12) at the ends of the cylinder (10) and a manifold assembly (28, 30) extending alongside the cylinder (10) and these end parts. The control means comprises a changeover valve (25) in the manifold assembly and two control valves (26, 27) in the respective end parts (11, 12) which are operated by mechanical actuators (K 1, K2) engaged by the piston (13) when it reaches the ends of its stroke and thereupon cause the changeover valve to change over the supply of compressed air from an inlet (60) in the manifold assembly, from one end of the cylinder to the other thereby to reverse the movement of the piston so that the piston reciprocates in the cylinder. The piston may comprise two parts (51, 52) which are adjustable to vary its effective length and hence its strocke.