In summary, there is provided a hydraulic apparatus comprising an eccentric rotatable about an axis, a plurality of piston feet arranged around a running surface of the eccentric and wherein each piston foot is configured to reciprocate by rotation of the eccentric. In addition there is provided a retention ring having an inner surface which defines therein at least one depression, an outer surface defined by the outer circumference of the retention ring; and first and second lateral surfaces each extending between the inner surface and the outer surface. The piston feet each have a seat portion and a lateral flange, and at least two of the piston feet have a protrusion that is sized and shaped in relation to the size and shape of the depression, such that the protrusion can move from the first lateral surface of the retention ring towards the second lateral surface of the retention ring, via the depression, when the protrusion is in a first position with respect to the depression, such that the retention ring then surrounds both the lateral flange and the eccentric.