The crankshaft turns the up-and-down motion of the pistons into rotation. Each piston pushes on a connecting rod, which is attached to an offset journal on the crankshaft, so the crank spins like the pedals of a bicycle. That rotation drives the flywheel or flexplate and the transmission, turns the camshaft through the timing belt, chain or gears, and drives the accessory belt through the crankshaft pulley.
Parts of a crankshaft
| Part | Job |
|---|---|
| Main journals | Ride in the main bearings in the block; the axis the crank spins on |
| Rod journals (crankpins) | Offset journals the connecting rods attach to |
| Counterweights | Balance the rotating and reciprocating mass |
| Snout and pulley | Drive the timing set and accessory belt |
| Flange | Mounts the flywheel or flexplate |
Crankshaft and stroke
The distance the rod journal sits from the crank’s center sets the stroke: how far the piston travels. A longer stroke adds displacement and tends to increase torque; see horsepower vs torque.
What does the crankshaft pulley do?
The crankshaft pulley, often combined with a harmonic balancer, drives the serpentine belt and damps torsional vibration in the crank; see what does a harmonic balancer do? and serpentine belt.
Crankshaft vs camshaft
The crankshaft is driven by the pistons and drives everything else; the camshaft is driven by the crankshaft at half speed and opens the valves; see what does a camshaft do?
Crankshaft position sensor
A sensor reads a toothed wheel on the crank so the engine computer knows its exact position and speed for spark and fuel timing. When it fails, the engine may stall or not start; see crankshaft position sensor.
Crankshaft problems
- Spun or worn bearings from low oil pressure; see spun rod bearing.
- Knocking that rises with rpm; see engine knocking.
- Rear main seal leaks at the back of the crank; see rear main seal leak.
Crankshaft design and firing order
The angles between the rod journals determine the firing order and how smoothly the engine runs. Most American V8s use a cross-plane crank, which gives the familiar burble; flat-plane V8 cranks rev more freely but vibrate more. Boxer engines place opposing pistons on journals that cancel much of each other’s vibration; see firing order and what is a boxer engine?
Forged, cast and billet cranks
Most factory engines use cast or forged cranks. Forged and billet cranks are stronger and used in high-output and racing builds. If a crank is damaged, a short block or long block is often simpler than a repair; see short block vs long block.
Trouble codes: P0335 Crankshaft Position Sensor · all OBD2 codes.
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