Bearing caps hold the main bearings for a crankshaft and piston rods in place in an engine block. Proper machining and clearances between the bearing and the crankshaft are crucial for engine performance. Main bearing caps are machined as a unit in a process called line grinding.
A bearing cap can be described as a number of things, but the most common reference is the part of the engine block that contains the main bearings for 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 placed in the block on top of the main bearing half. The other half of the main bearings are in the bearing cap and sit in order on the crankshaft and the corresponding other half on the main saddle. The bolts are then tightened to specification, thus holding the crankshaft in place.
Piston rods also use a cap to hold the piston rod on the crankshaft. Like the main bearings, the piston rod is divided into two pieces: the rod and the cap. Stem bearings are split into two pieces, with half of the bearing going to the bottom of the stem and the other half to the top of the stem. The rod slides into place over the crankshaft rod through the journal. The rod bearing cap sits on the bottom of the rod and around the crankshaft, completing the joining of the piston rod and crankshaft with connecting rod bolts that are torqued to specifications.
Perhaps the most important task inside an engine, the cover must be properly machined to allow for the correct amount of oil to support the crankshaft and piston rod. The crankshaft doesn’t actually ride on the bearing; the crankshaft is actually supported by a layer of oil between the bearing and the crankshaft. This same principle is practiced between the rod bearing and the crankshaft. Proper machining and clearances between the bearing and the crankshaft are achieved by adjusting the bearing cap. A cap that is too loose or too tight can cause the engine to fail prematurely.
While the piston rods are mounted and machined individually, the main bearing caps are machined as a unit in a process known as line grinding. In line grinding, each bearing cap is ground down to size and then tightened onto the engine block. The block is then put into a machine, and a long polish is passed through all the tops, honing them to proper specifications. This ensures that each cap fits perfectly on the crankshaft.
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