What Oil Clearance on Mains?

Just order a set of bearings with the undersize you need to get the clearance you want. But make sure your measuring is accurate.
King will make whatever you want.
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The first question that requires an answer is, are the journals std or ground undersize.
If std and the clearance is a bit on the large side, 0.001 and in some cases 0.002 undersize are available. If being real picky for a high performance build, 0.001 undersize can be used in the block or the caps. Just use them in one location or the other. Same applies to the con rod bearings but you could slightly alter deck height by moving the 0.001 under shell from the rod to the cap or the other way.
Now if the clearance is tight as shown and bearing selection is not an option, the machine shop can use the polish operation to take 0.0005" off to bring it up to a more reasonable 0.002". For street driving that is livable. Just as long as the crank spins freely after the main bolts are properly torqued.