Your crankshaft rides on the Ford Main Bearing Set in the interior of your engine. By the main bearings being in the correct tolerances is why the crankshaft can turn at thousands of revolutions per minute and still remain in place in your Ford engine.
There are generally up to only four main bearings in a set of this type of component made for a Ford engine. They are there to provide a replaceable part so when this clearances of the engine block and crankshaft need to be renewed, the engine does not have to be changed out.
The main bearing set provides not only the correct clearance between the crankshaft and the caps holding it in place, but they also provide a channel so the oil can lubricate the crankshaft as it is turning. Without this steady supply of oil to the moving crankshaft, it would seize up in a very short period of time.
The clearance for each set of Ford main berating sets is different for every engine. The precision of each of the main bearing sets is to the thousandths of an inch. This level of precision requires special tools to accomplish. The most commonly one used is a micrometer. There are many Ford owners that know how to use this instrument and take an accurate reading with one. If you are unsure, then take this repair job to an engine machine shop. If for any reason just one of the bearings in the main bearing set is off, it could have dire consequences to the operations of your engine and shorten its life span dramatically. This is a component where you only have one chance at getting it right.