Crown (head)The top of the piston, also called the head. The burning fuel pushes down on it, and it is the hottest part of the piston.
Compression ringsThe top two rings. They seal the combustion chamber, so the gases push the piston down instead of leaking past it.
Oil control ringThe bottom ring, which scrapes oil off the cylinder wall. It lets the oil drain back through its slots, keeping it out of the combustion chamber.
SkirtThe lower part of the piston, below the rings. It keeps the piston from rocking in the cylinder.
Small endThe top end of the connecting rod. It fits round the piston pin.
Connecting rodThe rod that joins the piston to the crankshaft. It is shaped like an I-beam, which makes it strong but light.
Big endThe large bottom end of the rod, which goes round the crankshaft. Its hole is split so it can be clamped on.
Rod capThe bottom half of the big end. It bolts to the rod to clamp it round the crankshaft.
Piston pinThe hollow, hardened steel pin joining the piston to the connecting rod. Also called the wrist pin or gudgeon pin.
Circlip (lockring)A spring clip at each end of the pin. It stops the pin sliding out and scoring the cylinder wall.
BushingThe sleeve lining the small end, which the pin turns in.
Bearing shellsTwo thin half-round bearings lining the big end. They sit where it turns on the crankshaft. A small tab stops each one turning.
Rod boltsThe bolts and nuts that clamp the rod cap to the rod.
How this list was compiled: Read 2026-09-27. (1) US Army, TM 9-8000, Principles of Automotive Vehicles (1985), paragraphs 3-5 to 3-8 and figures 3-12 to 3-40, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 : the piston head, the hottest part; the skirt, which keeps the piston from rocking; compression rings and the oil control ring with drain slots, in ring grooves; the hollow casehardened steel piston pin; lockrings (snap rings) at the pin's ends; the connecting rod bushing; the connecting rod, an I-beam, whose lower end is split to clamp round the crankshaft; the cap, bolts and nuts; insert bearings held by a locating tab. (2) Wikipedia, Piston, https://en.wikipedia.org/wiki/Piston ; Piston ring, https://en.wikipedia.org/wiki/Piston_ring ; Gudgeon pin, https://en.wikipedia.org/wiki/Gudgeon_pin ; Connecting rod, https://en.wikipedia.org/wiki/Connecting_rod : the crown; the top two rings as compression rings and the bottom one as the oil control ring; the gudgeon, piston or wrist pin, secured with circlips; the small end with its bearing, the rod and the big end, bolted round the crankshaft journal. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
These are the parts of a car engine's piston and the connecting rod that joins it to the crankshaft. A window cut in the skirt shows how the pin holds the two together.
Read the full guide
The piston
The burning fuel pushes on the crown. Just below it, two compression rings seal the gases in, and the oil control ring scrapes oil off the cylinder wall. The skirt keeps the piston from rocking.
The rod
The piston pin passes through the piston and the rod's small end, turning in a bushing, with a circlip at each end. The rod runs down to the big end, clamped round the crankshaft by the rod cap and bolts, and lined with two bearing shells.
Rings from the top: compression, compression, then oil. That is the most common layout in a car engine.
Questions and answers
What are the rings on a piston for?
The top two, the compression rings, seal the gases in the combustion chamber. The bottom one, the oil control ring, keeps oil out of it. The rings also carry heat from piston to cylinder wall.
What is a wrist pin?
Another name for the piston pin, also called the gudgeon pin: the hollow steel pin that joins the piston to the connecting rod's small end.
What does a connecting rod do?
It joins the piston to the crankshaft, turning the piston's up-and-down movement into the crankshaft's turning.