Usually, the crankshaft is ground perfectly round, so is the main bearing bore, and there's a slight egg shape built into the bearings. That can be measured by torquing up the bearings with the crankshaft removed and using a dial bore gauge to check the diameter at various positions around the circumference of the inside of the bearings. They're SLIGHTLY larger diameter at the parting line where the halves meet and there's a tolerance specified for oil clearance- - - -usually something like ".001-.003". In practice, .0015" is the target to shoot for, particularly on the rear main bearing where clearance approaching the maximum tends to cause the rope rear bearing seal to leak from the crankshaft bouncing up and down. We are quite literally "splitting hairs" here, as an average strand of hair is less than .002" thick.
Back in the dark ages of poured Babbit bearings, a new bearing was coated with Prussian Blue, and the bearing material was scraped until there was a smooth blue smear as the shaft was turned, indicating "no high spots"- - - -then shims were added to establish the right oil clearance. The accepted procedure before the invention of Plastigauge was to sandwich a thin slice of shim stock between the bearing cap and the shaft, and add thin layers of shim stock between the cap and the block until the shaft could be turned with a firm tug on the flywheel. That method still works- - - -Plastigauge is for rookies! Shimming stovebolt rod and main bearings is a holdover from the days of poured babbit- - - -as production techniques and tolerances improved (mid-1950's) quality control and machining accuracy eliminated the need for shimming. In certain situations where incompetent machinists get into the mix, a little tweaking with shims might be necessary.
Yes, there's enough hydrodynamic lift on a crankshaft at idle speed to center the shaft in the bearings, as long as sufficient oil volume is supplied. As volume and pressure increases with speed, theoretically there should be no metal to metal contact at all in an engine, even at the camshaft.
The engine failure I referenced was caused by the crankshaft bouncing up and down on bearings with a diameter .010" bigger than the crankshaft- - - -on both rod and main bearings. That's over 3 times the maximum oil clearance. I'm amazed it didn't spin a bearing somewhere, instead of just beating up the crankshaft!
Jerry
