All bearing clearance measurements...
All bearing clearance measurements should be taken along the vertical axis, or crown, 90 degrees opposed from the parting line, since a complete set of bearing shells forms an eccentric shape.
Sizing an engine's main and connecting rod bearings is relatively easy, but there's also a hard way. Of course, the hard way's more precise, but more than a few successful race engines have been assembled using the easy way. We're talking about Plastigage. But first the hard way. To size bearings appropriately, most machinists will use a dial bore gauge. The process involves using the dial bore gauge to measure the bare crankshaft bore's internal diameter. The crankshaft journal's outer diameter must also be measured and subtracted from the previous figure also taking into account the necessary clearances for bearing thicknesses. Bearings must be measured using an outside micrometer fitted with special ball-shaped anvils to fit the curvature of the bearing. An alternative method would be to install an arbitrary set of bearings into the block (or rods), torque the main caps (or rods) in place and measure the inner diameter with the dial bore gauge. For improper clearances, undersized or oversized bearings can be added and re-measured. Most manufacturers offer bearings that vary in sizes as little as 0.0002 inch from one another while most aftermarket bearings are generally offered in standard, 0.010-inch and 0.020-inch oversizes. Such large sizes require the crankshaft to be reground but allows for just about the closest tolerances you could imagine. It's important to note that's it's not at all necessary for each crankshaft journal to use the same size bearings. It's not even necessary for a single journal to use two bearings of the same size. What is important is that the final clearances are as close to one another as possible. If two different size bearings are going to be used on the same journal, just make sure they are within 0.0005 inch of one another and that the thicker bearing is placed on top for rods and on the bottom for the mains. Additionally, all measurements must be taken along the vertical axis (90 degrees from the parting line), where the brunt of the load is placed. This is especially important considering the bearings' eccentric shape once installed. The thicker area along the axis is referred to as the bearing crown; the thinner is the parting face. Despite the method chosen, before measuring bearing clearances, it's important to ensure that the block's main bore and the crankshaft journals are perfectly round and that the connecting rods are straight. To do this, you'll need a dial bore gauge.
Plastigage is not just an easier method of bearing sizing, it's also a lot cheaper. A good dial bore gauge and outside micrometer setup will set you back at least a few hundred dollars. Plastigage costs roughly two bucks and will last you through an entire eight-cylinder rebuild. Begin by cutting off a strip of the waxy string and place it on a clean crankshaft journal. Repeat the process for each journal. Make sure the bearings are already positioned in the block and on the main caps, free of contaminants and especially assembly lube. Next, install the main bearing caps and torque them to the proper specifications. Be careful not to turn the crankshaft, as it will distort any measurements. Remove the main caps. Take care when setting the caps down and when pulling them off that the Plastigage is not unnecessarily disturbed. Notice the crush process the material has undergone and compare the results with the Plastigage packaging, which features a metric scale on one side and standard measurements on the other. Note that the wider the strip, the smaller the clearance. Whether you'll be swapping bearings and re-measuring or bolting everything together for final assembly, it's important to clean off all of the excess Plastigage as it can easily disrupt oil flow across the bearings.
The perfect bearing would be a single piece encompassing the entire crankshaft journal, with the entire clearance between them occupied by a film of lubricant. The surfaces would not deflect under load conditions, and there would be no interruptions in the bearing surface such as oil holes, grooves, and irregularities. Corrosion would not be an issue and a sacrifice between embedability characteristics and strength would not have to be made. Unfortunately such a case doesn't exist. For the time being, the best engine builders can do is to ensure that the clearances are as optimal as possible and just as importantly, consistent.