73mm bb shells, very common on mtb's. Also, there's outboard bearings, which have been resurrected in recent years, which put the bearings outside the shell. I don't know how it would make much difference for the OP's purposes, though. But I am kind of curious as to the water inside the hull of the submarine. I take it this water that's going to come in is the ballast that will allow the submarine to dive, but it seems surprising that there wouldn't be a separate ballast tank or compartment for that. Put it this way: I'm not sure I'd want to take a ride in this particular submarine.