Engine Bearing (Plain Bearing)

Main bearings hold the crankshaft in the engine block, and rod bearings sit in the big end of each connecting rod. They are plain bearings: steel shells with soft bearing material on the inside, not ball or roller bearings. The clearance between bearing and shaft is only a few hundredths of a millimetre. That small gap fills with oil and carries the load.

Bearing shells are made in standard and undersize versions. When a crankshaft is reground, matching undersize shells restore the correct clearance.

The oil pump forces oil through drillings in the block and crankshaft into each bearing. As the shaft turns, it drags oil into a wedge-shaped film that lifts the shaft away from the bearing surface.

Low oil pressure, dirty oil, the wrong oil grade or long periods at very high load can wear bearings. Knocking from the bottom of the engine and an oil pressure warning are signs to stop the engine and have it checked. Bearing wear is often found by oil analysis or by measuring clearance with plastic gauge during an engine rebuild.