Because Blair is running a large-radius 4340 steel crank, it's necessary to use H-Series Clevite main and rod bearings. Notice Marvin has opened up the oil-galley passages to improve oil flow. Remember, crankshaft journals roll on a solid cushion of pressurized engine oil. Oil isn't just a lubricant; it is a coolant as well. He wants clearances that aren't too tight, which would restrict oil flow. The right measure of clearance and oil flow carries heat away from the bearings.