Seal accessibility device

An adapter is disclosed for use in mounting an ancillary unit in an engine block which has at one end a lateral flange (10) formed with an aperture for mounting of the ancillary unit. The adapter comprises a casing (112) for mounting on the end face of the engine to overlie the aperture, which casing is provided with an O-ring (130) for effecting a seal between the casing (112) and the aperture in the lateral flange of the engine. The casing has a larger diameter portion separated by a shoulder (131) from a portion of reduced diameter, a retaining sleeve (132) axially slidable over the reduced diameter portion of the casing (112) and a O-ring (130) held captive between the axial end of the retaining sleeve (132) and the shoulder (131).