Engine Bearings

There are many different kinds of bearings used on automobiles, including ball and roller bearings, needle bearings, and tapered roller bearings, but the bearings that are commonly used inside the engine are called plain bearings. These bearings are either cylindrical or are made of two inserts that form an enclosure. Components like the crankshaft and camshaft rotate on plain bearings supported by a film of oil. Oil is supplied under pressure by the engine oil pump.

Bearing construction varies according to intended use: stock, high-performance, and racing, and there are too many variations for an in-depth discussion here. In general, there are bi-metal and tri-metal bearings. Both have the lining material applied to a rigid and supportive steel backing. Bi-metal bearings have an aluminum alloy lining. The alloy may be strengthened with other metals, and the latest alloys contain silicone, which significantly increases surface hardness. Aluminum alloys with silicone are highly resistant to seizure. Tri-metal bearings have an intermediate layer of copper bearing alloy and an overlay of lead-tin-copper alloy. They have the strength, fatigue resistance and embedability (the ability to embed hard particles like dirt and debris that would otherwise become lodged between the bearing and journal, causing wear to the journal) that is the right combination for many applications.

Bores in the cylinder block are precision machined to accept the engine bearings. The camshaft and balance shaft bearings are cylindrical and pressed into the bores. The main and connecting rod bearing bores are split, with the bottom half of the bore known as the cap. Main and connecting rod bearings are called inserts that fit into bearing “saddles” formed by the top half of the bores in the block and connecting rods, and the caps. Where required, bearings have oil holes that must align with oil passages in the block, and some may have oil grooves. The thrust main bearing is flanged where it contacts the crankshaft thrust surface. Bearing halves for the crankshaft and connecting rods have a certain amount of material that extends above the bore parting line when placed in the saddle, which is known as bearing “crush”. This ensures that the bearings will be fully seated in the bores when the caps are torqued to specification.

For optimal performance and long life, engine bearings must have the right amount of oil clearance. This clearance is the difference between the shaft diameter and the diameter of the installed bearing, and is typically between 0.001” and 0.0035”. Main and connecting rod bearing clearance can be checked with the crankshaft installed using Plastigage, but a more precise measurement can be achieved by measuring the journal diameter with a micrometer and the installed bearing diameter with a bore gauge, and subtracting the micrometer dimension from the bore gauge dimension. If the clearance is excessive and the crankshaft is worn, it can be ground undersize and the appropriate undersize bearings used. However, the main or connecting rod housing bores could also be larger than specifications, requiring the block to be aligned honed or the big ends of the connecting rods reconditioned.

Although there are some performance bi-metal bearings, most performance bearings are of tri-metal construction. Many performance bearings also come with a proprietary dry film coating to reduce friction, provide startup protection, and increase strength and heat resistance. In addition to superior materials, they also have design features better suited to high power output engines. The eccentricity of performance bearings are tailored to the engine block and connecting rod bore characteristics of performance engine components to help maintain proper oil clearance, and they have higher crush for improved bearing retention. Performance bearings may be narrower than stock or chamfered to allow for the larger fillet radiuses of performance crankshafts, and they may have enlarged oil holes and/or grooves to improve lubrication.