Engine Bearings: The Friction Surfaces Inside Your Engine

Engine bearings are the soft-metal-backed shells that allow the crankshaft and connecting rods to rotate at high speed inside the engine block. They’re sacrificial — designed to wear instead of the harder steel they support, and to be replaced when the engine is rebuilt. A bearing failure is one of the most dramatic engine failures — the engine knocks, then seizes, often without prior warning.

Types of bearing

Main bearings support the crankshaft inside the engine block — usually 4-6 per engine depending on cylinder count. Connecting rod bearings (rod bearings or big-end bearings) support the connecting rod at its crankpin connection — one per cylinder. Thrust bearings handle the crankshaft’s axial loads (typically one set, at the centre of the crank).

How they wear

The bearings ride on a hydrodynamic oil film. As long as oil pressure and quality are maintained, the soft bearing surface never actually touches the hard steel of the crankshaft or rod journal — they slide on a film of pressurised oil. When oil pressure drops (low oil level, failing oil pump, blocked filter, low-viscosity oil) or oil quality is compromised (dirty oil, contaminants), the film breaks down and the bearing material starts to wear.

Symptoms

A faint knock at idle from a worn main bearing. A heavier knock under load (acceleration) from a worn rod bearing. Reduced oil pressure (worn bearings let more oil escape, dropping system pressure). On the worst-case progression, a sudden catastrophic failure with the rod punching through the block.

Replacement

Bearing replacement is an engine-out rebuild job in most cases. The crankshaft and conrods come out; the journals are measured for wear and undersize-ground if needed; new bearings sized to match are fitted. It’s a workshop job, not a DIY one.

Brand

Glyco, King, ACL, Mahle, and Federal-Mogul cover OE-equivalent bearings for most European engines. Specification accuracy is critical — bearings come in standard and several undersize ranges (0.25, 0.50, 0.75mm) matched to journal grinding.

Find engine bearings for your specific engine on the Engine Bearings collection. Bearing work is rebuild territory — get the spec right and the engine lasts another 200,000 miles.