Main Bearings

Main bearings are specialized plain bearings that support the engine's crankshaft within the crankcase. Bolted into the main bearing saddles of the engine block, they allow the crankshaft to rotate freely under the intense downward forces of the pistons. They consist of two semi-circular halves that form a smooth, circular sleeve around the crankshaft's main journals.

Unlike ball bearings, main bearings do not use rolling elements. Instead, they rely on a constant supply of pressurized engine oil from the oil pump. The oil forms a microscopic film between the rotating crankshaft journal and the soft bearing metal, ensuring they never physically touch during operation. This is known as hydrodynamic lubrication, and it reduces friction and wear to near-zero levels.

If the engine loses oil pressure or if the oil becomes contaminated with dirt or metal shavings, the lubricating film collapses, causing metal-on-metal contact. This quickly generates extreme heat, melting the soft bearing material. Symptoms of worn main bearings include a low oil pressure warning (especially at idle when the oil is hot), a deep metallic knocking noise from the bottom of the engine (main bearing knock), and oil filter debris.

Replacing main bearings is a major engine repair that requires removing the engine from the vehicle and disassembling the crankcase. During an engine rebuild, technicians measure bearing clearances using specialized plastic gauges (Plastigage) to ensure they meet tight factory tolerances.