What Is a Crankshaft and What Happens When Main Bearings Fail?

What Is a Crankshaft?

The crankshaft is a precision-machined rotating shaft at the bottom of the engine, housed inside the engine block. Its job is to convert the reciprocating (up-and-down) motion of the pistons into rotational motion — the turning force that ultimately reaches your wheels via the gearbox, driveshaft, and differential.

Think of it like a bicycle crank: your legs pushing up and down (pistons) turn the crank into circular rotation. The crankshaft performs the same conversion inside your engine, thousands of times per minute.

How Does a Crankshaft Work?

Each piston connects to the crankshaft via a connecting rod. The connecting rod attaches to an offset journal (a bearing surface) on the crankshaft called a crankpin. As each piston fires and pushes down, it rotates the crankshaft — with the offset crankpins ensuring smooth continuous rotation even with multiple cylinders firing in sequence.

The crankshaft itself sits in main bearings (also called main journals) — precision-fitted bearing shells that allow the crankshaft to spin freely while supporting it against the enormous forces produced by combustion.

What Are Main Bearings?

Main bearings are thin semi-circular shells made from a soft metal alloy (often steel-backed aluminium or copper alloy). They sit between the crankshaft journals and the engine block. Engine oil is continuously pumped between the bearing surface and the crankshaft journal, creating a thin film of pressurised oil that prevents direct metal-to-metal contact.

As long as oil pressure and oil quality are maintained, main bearings last the life of the engine. When they don't — they fail.

What Causes Main Bearing Failure?

Signs of Failing Main Bearings

The most recognisable symptom is a deep, rhythmic knocking or rumbling noise from the lower engine:

• The knock is often described as a "rod knock" or "big end knock" — though strictly speaking, rod bearings (between the connecting rod and crankpin) knock slightly differently to main bearings
• The noise typically increases with engine speed (rpm)
• It may be louder under load (when accelerating)
• You may notice low oil pressure indicated on your dashboard

Other signs: oil pressure warning light, excessive engine vibration, metallic debris in the oil.

What Happens If You Ignore It?

Main bearing failure is progressive and accelerating:

1. The oil film breaks down; metal-to-metal contact begins
2. The bearing shells wear rapidly, increasing clearance
3. The crankshaft journal itself gets scored and damaged
4. The crankshaft can seize — locking the engine completely
5. In severe cases, the connecting rod breaks and punches through the engine block (catastrophic engine failure)

Once main bearing knock is present, every minute of running accelerates the damage. Driving with a knocking engine risks a complete engine write-off.

NZ Cost Estimates

For common NZ vehicles, sourcing a reconditioned Japanese-import engine via a wrecker is often the most cost-effective repair option when the crankshaft or bearings are heavily damaged.

Prevention

• Change your oil on schedule — this is the single most important preventive measure
• Check your oil level monthly (or before long trips)
• Address oil pressure warning lights immediately — don't drive further
• If you've had a head gasket failure, change the oil promptly to remove any coolant contamination

WoF Implications

A knocking engine won't directly fail a WoF, but an engine on the verge of catastrophic failure is a safety and reliability concern. WoF inspectors assess overall vehicle condition.

When to Book a Mechanic

Book a mechanic immediately — do not continue driving — if:

• You hear a deep knocking or rumbling from the lower engine
• Your oil pressure warning light is on
• You've been running the engine low on oil

Further Reading

• Find a mechanic near you
• Engine oil — why regular changes protect your crankshaft
• Pistons — the components connected to the crankshaft
• Oil pump — how it keeps the crankshaft lubricated