A bearing cap can be described as a number of things, but the most common reference is the part of the engine block which contains the main bearings of a crankshaft. In an engine block, the part of the engine that the crankshaft runs through is split. Once the main caps are removed, the crankshaft can be set in the block on top of one-half of the main bearings. The other half of the main bearings are in the bearing cap and are placed in order onto the crankshaft and the corresponding other half of the main saddle. The bolts are then torqued to specification, thereby holding the crankshaft in place.
Piston rods also use a cap to hold the piston rod onto the crankshaft. Like the main bearings, the piston rod is split into two pieces: the rod and the cap. The rod bearings are divided into two pieces, with one half of the bearing going into the bottom of the rod and the other half going into the rod cap. The rod is slid into location onto the crankshaft rod through the journal. The rod bearing cap is placed onto the bottom of the rod and around the crankshaft, completing the union of the piston rod and the crankshaft with the connecting rod bolts being torqued to specification.
Perhaps tasked with the most important duty inside of an engine, the cap must be machined properly to allow the correct amount of oil to support the crankshaft and piston rod. The crankshaft does not actually ride on the bearing; in reality, the crankshaft rides on a layer of oil between the bearing and the crankshaft. This same principal is practiced between the rod bearing and the crankshaft. The proper machining and clearances between the bearing and the crankshaft is accomplished by the fit of the bearing cap. A cap that is too loose or too tight can cause an engine to fail prematurely.
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While the piston rods are fitted and machined individually, the main bearing caps are machined as a unit in a process known as line honing. In line honing, each bearing cap is ground undersized and then torqued onto the engine block. The block is then placed into a machine, and a long hone is run through all of the caps, honing them to the proper specifications. This ensures that each cap is the perfect fit to the crankshaft.