Glossary
crankshaft
The crankshaft turns the pistons' up-and-down motion into rotation. Each piston's connecting rod rides a journal on the crank, and the spinning shaft is the engine's output.
The crankshaft is the heavy shaft at the bottom of the engine that turns the pistons' up-and-down motion into rotation. Each piston's connecting rod is attached to a journal on the crank, so as the pistons move the shaft spins, and that spin is what the rest of the drivetrain uses.
It is the engine's output and its hub. The flywheel or flexplate bolts to one end, smoothing out the separate power pulses into steady rotation; a pulley on the front drives the belt that runs the alternator and other accessories; and the offset throws of the crank are arranged so that the cylinders fire in a smooth order.
Questions usually pair it with the camshaft. The crankshaft collects the pistons' motion and turns it into rotation, and it drives the camshaft at half its own speed — which is why the two are linked by a timing belt or chain whose alignment matters so much.
Taught in
Related terms
- camshaft — The camshaft is the rotating shaft whose egg-shaped lobes open and close the engine's valves, letting the air-fuel mixture in and the exhaust gases out at the right moments.
- piston — A piston is the close-fitting plug that slides up and down in an engine's cylinder. It receives the push of combustion and passes it to the crankshaft through the connecting rod.
- alternator — The alternator is the belt-driven generator that makes electricity while the engine runs, using it to charge the battery and run the lights, ignition and electronics.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)