Abstract: A hydraulic retarder for braking a shaft has a rotor comprising a rotor shell mounted on the shaft end and rotatively movable relative to a stator shell forming part of a stator enclosing the rotor. The shaft end projects from a housing upon which the stator is mounted by an apertured structure surrounding a driving pulley mounted on the shaft end. The apertured structure comprises lugs on the housing and a toothed ring on the stator, the teeth in the ring co-operating with the lugs so as to hold the stator centred on the axis of the shaft end irrespective of the angular position of the stator. Recesses between the teeth of the ring are disposed and dimensioned so that they provide the necessary openings for moving an endless belt into or out of the structure simply by moving the stator through an angle.