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Since the aluminum block "moves around" and "grows" a lot as with an increase in temperature, the tolerances like those for the main bearings are tightened to reduce the amount of oil flopping off the crank. For an aluminum block, Urban sets the main bearing clearances at .0018 inch, versus .0025 inch on an iron block (this is on everything but the rear main). On the rear, he runs .0025 inch versus .0030 inch with iron. Urban told me, "Stielow, you'll shift this thing at 7,000 rpm, and I want a little oil on the thrust bearing to prevent a problem." We've brought him back problems before! Here's a tech tip: Urban always line hones and measures the mains with the oil pump bolted to the main cap. "It's amazing how much the torque of the oil pump stud will pull the main out of round," he says, "So I always bolt up a dummy oil pump before doing that machine work."