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
Labeling parts of a piston takes thirteen names. Numbers 1 to 8 stand down the left of the drawing, from the crown to the rod cap, and 9 to 13 down the right. Write each name on its line.
Read the full guide
Piston word bank
In alphabetical order: bearing shells, big end, bushing, circlip (lockring), compression rings, connecting rod, crown (head), oil control ring, piston pin, rod bolts, rod cap, skirt, small end.
Numbers that get swapped
Of the three dark rings, 2 is the upper pair (compression rings) and 3 the slotted one below (oil control ring). In the window, 9 is the pin in the middle (piston pin), 10 the dark arc (circlip), 11 the sleeve outside that (bushing) and 5 the outermost ring (small end). At the bottom, 7 points at the upper half of the round end (big end), 8 at the lower half (rod cap), 12 at the lining of the hole (bearing shells) and 13 at a bolt (rod bolts).
When marking, wrist pin and gudgeon pin are right on line 9, snap ring on line 10 and head on line 1.
Two bolts are drawn, one each side of the big end. Number 13 stands for both.
Questions and answers
What is a piston made of?
Usually an aluminum alloy, cast or forged. Early pistons were cast iron, and a lighter alloy made the engine easier to balance.
What are piston rings made of?
Most are cast iron or steel. Each ring is split at one point, so it springs outward and presses against the cylinder wall.
What is a full floating piston pin?
A pin free to turn in both the piston and the small end of the rod, usually kept in by circlips. A semi-floating pin is usually fixed in the piston and turns only in the small end.
What is the difference between the big end and the small end of a connecting rod?
The small end is at the top and fits round the piston pin. The big end is at the bottom and fits round the crankpin of the crankshaft, usually on a plain bearing.