Piston

A cylindrical part which reciprocates in the cylinder bore of an engine and transmits the force of the gas pressure through the connecting rod to the crankshaft.

Bore and Stroke

The bore and stroke of the piston sets the swept volume. The ratio of bore to stroke is a good indicator of the engine characteristic such as how high it will rev and basic fuel economy. More simply it also defines some of the basic dimensions of the engine such as height and length.

Bore versus stroke for a number of production engines.

Crown

The top of the piston; the piston head.

Head

The top of the piston or that part of the piston against which the gas pressure acts.

Oil-Control Rings

The piston ring, usually located at the lower part of the piston, that prevents an excessive amount of lubricating oil from being drawn up into the combustion space during the suction stroke. Also known as oil ring and oil scrapper ring.

Pin

A pin that rests in two bored holes in the piston and passes through the eye of the connecting rod, to join the two together flexibly.

Also known as the gudgeon or wrist pin.

Pin Bearing

The bearing either in the eye of the connecting rod or in the bored bosses of the piston, in which the piston pin rocks.

Pin Boss

That part of the piston on the inside, through which the hole is made to take the piston pin.

Pin Lock

The device used to hold or lock the piston pin in the piston.

Ring

A split ring placed in a groove of the piston to form a leakproof joint between the piston and the cylinder wall.

Ring Gap

The space between the ends of the piston ring when it is in the cylinder bore.

Ring Grooves

Grooves cut in the piston barrel to hold the piston rings.

Ring Land

The part of the piston on the outside surface located between the piston-ring grooves.

Skirt

The part of the piston below the piston-ring grooves.

Stroke

The movement of the piston from one end to the other of the piston travel in the cylinder bore. The piston stroke is equal to twice the throw of the crankshaft.

See also: Blow By, Blowby, Internal Combustion Engine, Piston Ring, Power Stroke, Stroke.