I have an oil leak between the drive upper and the top cover (item 12 in diagram). I assume the o-ring is the problem - or maybe it just needs to be torqued down? But, I don't know what kind of socket to use for the bolts (item 16) that tighten the cover down. It is not a standard bolt head pattern. None of my tools fit it. What kind of socket do I need and what size?