if the crank caps are fitted to the block and then the bolts are torqued up, each of the 5 bores that the bearing shells fit into must be perfectly round and you need a bore gauge to check this. YBs (and some other engines) typically go egg shaped which means that they close up across the parting line so the width becomes less than the height and the bore is egg shaped. the same problem is frequently found in the con rod bores.