If the engine being built does not have a pin or other method of locating the crank main thrust bearing (some Chevrolet). The thrust must be set by tightening all main bearing caps to spec except the thrust bearing. The thrust bearing cap is tightened enuff to set" the cap. Then the crankshaft is moved back and forth to properly locate the thrust. Then torqued to normal specs..
This method can also be used to increase/decrease the thrust clearance.
To try this after the mains are torqued/engine assembled, is a waste of time.