Abstract: An ancillary unit drive adapter is disclosed for use in an engine block which has at one end a lateral flange formed with an aperture for mounting of the ancillary unit. The drive adapter comprises a casing (112) for mounting on the end face of the engine to overlie the aperture. A shaft (116) journalled in the casing, has a formation (122) at one end for coupling to the input shaft of the ancillary unit and a cog (120) solid with its opposite end for meshing with an engine driven gear. Faces (120a, 120b) on the opposite sides of the cog (120) serve in use as bearing surfaces to withstand axial forces acting in both directions on the drive shaft (116).