Crankshaft

The crankshaft is the primary rotating component of the internal combustion engine, converting the reciprocating (up-and-down) linear motion of the pistons into the rotational motion that drives the vehicle’s transmission, ancillary components, and ultimately the wheels. Forged from high-strength alloy steel or cast from nodular iron, the crankshaft runs the full length of the engine’s crankcase and is supported by main bearing journals machined into the engine block. It consists of a series of offset throws (crank pins) to which the connecting rods attach, main bearing journals that ride in the block’s main bearing caps, counterweights that balance the rotating assembly to minimise vibration, and a front snout that drives the timing belt or chain, harmonic balancer, and accessory drive pulleys. The crankshaft is one of the most highly stressed components in the engine — each combustion event applies an impulse load of several thousand newtons to the relevant crank throw. Oil pressure from the engine oil pump is delivered to each main bearing and connecting rod bearing through drilled internal passages, creating a hydrodynamic oil film that separates the crankshaft journals from the bearing shells during operation. Crankshaft failure is rare in well-maintained engines but catastrophic when it occurs, typically from oil starvation, bearing failure, or detonation-induced overloading.

As each piston reaches top dead centre (TDC) on its compression stroke and the air-fuel mixture ignites, the expanding combustion gases push the piston downward with considerable force. This force is transmitted through the connecting rod to the crank throw, generating a torque about the crankshaft centreline that rotates the crankshaft. The four-cylinder engine has four such impulses per two crankshaft revolutions, each offset by 180 degrees of crank rotation, producing a power pulse every half revolution. The crankshaft’s rotational inertia — supplemented by the flywheel — smooths these discrete power impulses into a more continuous rotation. The offset counterweights on the crankshaft are precisely calculated and positioned to balance both the primary and secondary forces generated by the reciprocating piston-and-rod assemblies, minimising the vibrations that would otherwise be transmitted to the engine mounts, bodywork, and passenger compartment. The front of the crankshaft drives the timing belt or chain (synchronising camshaft rotation), the harmonic balancer/crankshaft damper (absorbing torsional vibration), and the serpentine belt that drives the alternator, power steering pump, air conditioning compressor, and water pump.

Main or big-end bearing failure from oil starvation: The most serious crankshaft-related failure. When engine oil pressure falls below the minimum needed to maintain the hydrodynamic bearing film — from very low oil level, a failed oil pump, a blocked oil pickup screen, or severely degraded oil — the bearing shells make direct metal-to-metal contact with the crankshaft journals. This produces a deep, rhythmic knocking or rumbling from the bottom of the engine that varies with RPM. Continued operation destroys the bearing shells and spins the bearing insert in its housing, eventually seizing the crankshaft or breaking a connecting rod. This is an immediate stop-engine situation — continued operation after an oil pressure warning light illuminates converts a bearing replacement into a complete engine rebuild.

Crankshaft harmonic balancer failure: The harmonic balancer (crankshaft damper) on the front of the crankshaft absorbs torsional vibration. Its rubber damping element deteriorates with age, eventually causing the outer ring to separate from the hub. A separated balancer destroys the serpentine belt, knocks the crankshaft position sensor out of alignment, and allows torsional vibration to damage the front crankshaft oil seal and timing cover.

Crankshaft position sensor fault: The crankshaft position sensor reads a toothed ring on the crankshaft to provide engine speed and position data to the ECU. A failed sensor prevents the engine from starting or causes misfires, stalling, and a no-start condition. It is one of the more common sensor failures on high-mileage engines.

Level 1 — Oil Pressure and Level Monitoring: Check engine oil level with the dipstick at every fuel fill or at minimum weekly on high-mileage engines. The oil pressure warning light must never be ignored — if it illuminates while driving, pull over and turn off the engine immediately. Running even 500–1,000 metres with oil pressure loss can destroy main and big-end bearings beyond economic repair. Listen for any deep knocking, rumbling, or tapping from the bottom of the engine, which indicates bearing wear. Also listen for any change in the accessory belt noise or visible wobble of the front harmonic balancer pulley.

Level 2 — Engine Oil Changes as Bearing Protection: The crankshaft main and connecting rod bearings depend entirely on the hydrodynamic oil film maintained by clean, correctly-viscosity oil at adequate pressure. Changing engine oil at the manufacturer’s specified interval — never extending beyond it — and using the exact specified viscosity grade is the primary protection for the crankshaft bearing system. Using an incorrect viscosity (too thin for high-temperature operation, or too thick for cold-start protection) reduces the oil film thickness at the critical bearing clearances. If oil consumption is increasing, investigate the cause before the level drops to a damaging threshold.

Level 3 — Professional Inspection for Bearing Noise: Any persistent knocking or rumbling from the engine bottom end requires professional diagnosis before mileage accumulates. A workshop can measure oil pressure at the engine to confirm the hydraulic system is functioning, and a mechanic can distinguish bearing knock from other engine noises. Crankshaft replacement requires complete engine disassembly, crankshaft removal, precision measurement of all journals, selection of new bearing shell sizes, and reassembly to manufacturer bearing clearance specifications — this is a specialist engine rebuilder’s job.