Using Plastigage to measure main bearing clearances is fairly easy. Begin by cutting off several sections of the strip and placing them on each crankshaft journal. Make sure that all of the lower main bearings are in place and free of debris. Next, torque the main caps into place and then remove them, being careful not to disturb the strip.
There are a few more terms to go over before discussing bearing sizing. Spread comes to mind first. This refers to the bearing's slightly larger arc in comparison to the block's bearing journal. In short, the spread is what keeps the bearing in place during assembly. You'll notice this from the snapping sensation when positioning a bearing in the groove. Crush also refers to an oversized condition, but in a different way. Bearings are manufactured so that when they are placed in the block and main caps their edges protrude above the block's or main cap's parting surfaces. The protrusion is minimal-just enough to barely grab your fingernail. The added material results in a crushing condition in which the two bearings are forced together upon tightening the main caps in place. This condition, along with a series of tangs, keep the bearings in place as the crankshaft spins, helping prevent what is known as spun bearings. However, excessive crush can hurt more than it helps. Too much crush causes the bearing to bulge inward, reducing oil supply and potentially causing metal-on-metal contact. Other bearing tricks include chamfered edges for fitting filleted crankshafts and dowel holes for when using aluminum connecting rods. Often times with aluminum rods it's necessary to permanently locate the bearings since such rods' thermal expansion rates do not allow for as much bearing crush. Finally, you need to consider a bearing's oil grooves. More or larger grooves take away bearing surface area, making them ultimately less capable of handling high loads. But often times there's little you can do here since eliminating oil passages isn't an option. Fortunately, some aftermarket bearings are offered with specially shaped grooves and holes that put forth the best compromise in surface area versus oiling.
The crushed strip can then be compared with the Plastigage packaging to determine the oil clearance. Wider sections reveal smaller clearances while thinner sections mean clearances are looser. It's a trial and error process in which several different sized bearings must often be tested.
It's actually only partially true that the bearings support the load of the engine's crankshaft. In reality, the film of oil on top of the bearings supports the crankshaft, thus transferring load to the bearings. As such, bearing clearances as well as oil weights play major roles in just how much oil pressure an engine actually gets. Depending upon where the oil pump is located, a single improperly sized bearing can disrupt an engine's oil pressure, causing catastrophic failure. Loose clearances lead to low oil pressure conditions while tight clearances lead to higher pressure ones. But assuming tighter is better isn't always a good idea as it's easy to restrict oil away from the bearing, completely causing it to contact the crankshaft (higher pressure can lead to less volume). Metal-on-metal contact in terms of moving parts rarely works effectively. But erring on the tight side is becoming more popular as today's engine's feature far more sophisticated oiling systems and the machining processes used for engine building are far more accurate. To put the importance of bearing clearances into perspective, consider this: by simply doubling the space between the bearing and the crankshaft, as much as four times the volume of oil can be distributed.
Plastigage is offered in different sizes and offers measuring instruments for both standard and metric calculations. | 
Typical bearing clearances fall somewhere in the neighborhood of 0.0005-0.0025 inch depending upon the manufacturer, year of production and sophistication of the particular engine. Newer engines have the capability of running much tighter and far more precise clearances than their older counterparts. | 
An inside and outside micrometer combination like this can be used to measure connecting rod internal diameters to determine what size bearings will fit. The process is a bit more accurate than using Plastigage but is also more expensive. |