Pistons, Rings, and Connecting Rods

The piston is a cylindrical plug that slides up and down inside the cylinder bore. It is the component that actually receives the force of combustion and transfers it to the crankshaft through the connecting rod. Pistons are made of aluminum alloy because aluminum is light — and since the piston changes direction thousands of times per minute, keeping it light reduces the forces on the engine.

Each piston has a set of rings that fit into grooves machined around its circumference. Most engines use three rings per piston. The top ring and second ring are compression rings — their job is to seal the cylinder and prevent combustion pressure from leaking past the piston into the crankcase below. The bottom ring is the oil control ring — its job is to scrape excess oil off the cylinder wall and return it to the oil pan so it does not get into the combustion chamber and burn. When the rings wear out, compression drops and oil consumption increases. Blue smoke from the tailpipe often indicates worn rings allowing oil past.

The connecting rod is the link between the piston and the crankshaft. The small end of the rod connects to the piston through a wrist pin. The big end of the rod wraps around the crankshaft journal. The connecting rod converts the straight-line up-and-down motion of the piston into the rotational motion of the crankshaft. The rod and its bearing must be incredibly strong because they transmit all of the combustion force while changing direction thousands of times per minute.

The wrist pin — also called a piston pin — passes through the piston and the small end of the connecting rod. It allows the rod to pivot relative to the piston as the crankshaft rotates. The wrist pin is a hardened steel cylinder machined to very tight tolerances. A worn or loose wrist pin produces a distinctive double-knock sound — two quick taps in rapid succession — that is most noticeable at idle and under light load.