Glossary
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.
A piston is the close-fitting plug that slides up and down inside an engine's cylinder. Its job is to take the push from combustion and hand it to the crankshaft through the connecting rod.
The top of the piston faces the burning mixture and takes its heat and pressure. Rings fitted around its sides seal the gap to the cylinder wall, so compression holds and oil stays out of the combustion chamber. A wrist pin joins the piston to the rod, and the other end of the rod rides on a journal of the crankshaft.
Its travel defines the four strokes of a four-stroke engine: it descends on the intake stroke drawing in the mixture, rises on the compression stroke squeezing it, is driven down by the power stroke when it ignites, and rises once more on the exhaust stroke pushing out the burnt gases. A pump or compressor uses a piston for a similar back-and-forth job.
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.
- compression stroke — The compression stroke is the second of the four strokes in a four-stroke engine: the piston rises with the valves closed and squeezes the air-fuel mixture into a smaller space.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)