The pistons (4,5) fit closely over the radial arms (9,10) of piston carriers so that there is no play in the direction in which the pistons move in the ring-shaped cylinder. The pistons can slide over the arms by a small amount in the radial direction. Alternatively the pistons are mounted on the radial arms (9,10) via stacks of leaf springs which are mounted in cross-bores in the pistons and in the ends of the arms. The stacks of springs allow a small amount of radial movement of the pistons relative to the arms. The cross-sectional area of the pistons is smaller than the cross-sectional area of the cylinder. Any play is taken up by a set of broad piston rings.
