After making the dam for the open face, I drilled a 12mm dia. hole in a piece of 1" dia. aluminium rod. Had to taper the rod a bit to fit into the steeply drafted casting. I cut groves around the rod, like I did for the casting, to form a key, then glued it in place with more "Chemical Metal". I chamfered the bottom edge of the hole prior to glueing, this self centred on the chamfered edge of the bush bearing. I covered the face of the bearing with masking tape while glueing. Once set I sanded the bottom face flush on a belt sander using a crude jig, I  made the dam and rod slightly long so they could be sanded back to the hight of the screw fixing bosses. I now have a gasket face to work with.