I think Jon's onto something here- - - -the lower bearing carries most of the load, and usually doesn't have an oil groove. It's possible you've got two upper bearing shells, and no lower. How about the other mains- - - - -do all of them have grooves in both bearing halves? Jon, the 54 and 1st. series full pressure 235 and 261 main bearings use dowels in both bearings, like a 216 and spray-oiler 235. 1955 2nd series engines and later ones use a locating tab at the end of the bearing on the upper shell, which makes replacing mains a much simpler procedure.
Jerry
