What Are Main Bearings?
The crankshaft — the central rotating shaft that converts piston movement into rotation — is supported in the engine block by main bearings (also called main journals). Like the rod bearings, these are thin precision shell bearings that provide a smooth, oil-cushioned surface for the crankshaft to rotate on.
Most four-cylinder engines have five main bearings (one between each cylinder and two at the ends); V6 and V8 engines have more. The rear main bearing also seals the back of the crankshaft — a failed rear main seal causes an oil leak at the rear of the engine.
What Main Bearing Knock Sounds Like
Main bearing knock has a distinct character compared to rod knock:
- Lower-pitched, heavier rumble — often described as a thud or boom rather than a sharp knock
- Less sensitive to individual cylinders — it affects the whole crankshaft, so it doesn't change as distinctly when one cylinder is shorted out
- Varies with RPM and load — louder under acceleration, but less RPM-sensitive than rod knock
- Sometimes a "thump" at very low RPM that smooths out as RPM rises
At idle with a fully warm engine, the knock may be a distinct, heavy "thud-thud-thud" that you can feel through the seat.
Distinguishing Main Bearing Knock From Rod Knock
The quickest differential test: change the engine oil and filter, and have the new oil checked after 500 km. Metallic particles in the oil indicate which component is generating them. An engine oil analysis lab can identify if the particles are bearing alloy.
An experienced mechanic can usually distinguish the two sounds by listening carefully with a stethoscope: rod knock is sharper and more distinctly rhythmic; main bearing knock is heavier and more diffuse.
Causes
All the same factors as rod knock: oil starvation, infrequent oil changes, very high mileage, and overheating. The rear main bearing is also susceptible to damage from a failed rear main seal — when oil leaks past the seal, it can contaminate the crankshaft journal area.
What Happens Without Repair
Unlike piston slap (which may be stable for years), main bearing failure is progressive. A failed main bearing will eventually allow the crankshaft to drop out of alignment, causing catastrophic engine failure.
Repair Costs
- Main bearing replacement (no journal damage): $1,500–$3,500 NZD as part of engine rebuild
- With crankshaft regrind: Add $500–$1,500 NZD
- Engine replacement: Often the most economical path — $2,500–$5,500 NZD fitted