Piston

Pistons are usually cast or forged aluminium alloy, chosen because it is light and conducts heat well. The top, called the crown, faces the combustion chamber; grooves around the side hold the piston rings; and a pin through the middle connects the piston to the connecting rod.

Diesel and high-performance petrol pistons often have shaped crowns or bowls that guide the fuel spray and the flame. Many engines also spray oil onto the underside of the pistons to keep them cool under heavy load.

On the compression stroke the piston rises and squeezes the air or mixture. When it burns, the pressure pushes the piston down, and the connecting rod turns that straight-line force into rotation of the crankshaft.

Damaged pistons are rare in well-maintained engines, but overheating, knock, a lean mixture or oil starvation can melt or crack them. Blue smoke, low compression on one cylinder and a knocking noise are typical signs that a piston or its rings need inspection.