Checking the main bearing clearance can be done one of two ways. First, you can assemble the main bearings in the block without the crank, torque the caps down, and then use an inside micrometer to determine the ID of the bearings. Then, by using an outside mike and measuring each journal, you simply do the math. A much simpler way is to use Plastigage. This thin, plastic-like material is designed to be placed between the crank and bearing. Once torqued down, the Plastigage is measured and its width determines what the clearance between the crank journal and bearing is.