A piston arrangement including a track and a piston moveable within a cylinder; wherein the track is adapted to rotate relative to the cylinder about an axis of rotation and has a cam surface and an edge surface extending away from the cam surface; wherein the piston is coupled to the track by a follower running on the cam surface; wherein the cam surface is shaped such that, as the track moves relative to the cylinder, the piston head moves in reciprocating motion within the cylinder in accordance with the path of the cam surface; wherein a stabilising element is connected to the piston, the stabilising element extending below the piston head and comprising a contact surface which engages the edge surface of the track.