Glossary
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.
The camshaft is the rotating shaft that opens and closes the engine's valves. Egg-shaped lobes on the shaft press each valve open as they come round and let it snap shut again under spring pressure, so the shape and position of the lobes set the engine's breathing.
It is driven from the crankshaft, at half crank speed in a four-stroke engine, because each valve has to open once for every two revolutions. The connection is a timing belt or a chain, and keeping the two in step is the whole game: if the camshaft and crankshaft fall out of alignment the engine loses power, runs badly, or the valves meet the pistons.
A camshaft low in the engine uses pushrods and rocker arms to reach the valves, while an overhead cam sits in the cylinder head and acts on them more directly. Mechanically it is a set of cams on a shaft — the machine that turns steady rotation into repeated in-line motion.
Taught in
Related terms
- 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.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)