Abstract: A piston and cylinder machine of the kind in which there is a wobble mechanism for reciprocating the piston in the cylinder or an arrangement in which a piston assembly and a cylinder assembly rotate about respective inclined axes so that such rotation is accompanied by relative reciprocation of the piston and cylinder, is provided with a piston having a part which has a surface exposed inside the cylinder to define at least a part of one end of a chamber therein and which, to provide sealing of the chamber, has a peripheral surface forming a sphere. Improved torque transmitting or reacting means for maintaining the correct relative orientation of the piston and cylinder are provided.