I am getting ready to join the case halves of my O360 A2F and my old Lycoming book says not to use o rings on the center studs if they are doweled, this is what I will do unless told to do otherwise. The thing that makes me question is that there is a machined recess on one half that would fit an o ring, it would need to be slightly stretched to go around the dowel but it seams like it would make a good seal.