[Adger Smith]

Could be taper on the journal or in the main housing.
With small journal I'm betting on crank flex.
Back in the early days when we ran those we had to put a little more clearance than what the big journal could get away with. Depending on lifter bore clearance & rod side clearance thin oil may not be friendly. I would rather have a thicker oil with good film strength than the miniscule amount of HP the thin gives.
As previously stated Dial bore the mains and put the Michrometer on the crank... Check the crank for cracks, too..
Good luck!