The pistons are the mechanical components that bear the pressure from combustion that is ultimately turned into vehicle motion. Piston rings form the seal between the pistons and cylinder walls, and the connecting rods connect the pistons to the crankshaft, converting the reciprocating motion of the pistons to the rotating motion of the crankshaft, which turns the gears in the transmission, the axle shafts, and ultimately the wheels.
A connecting rod consists of a small end, attached to the piston by a wrist pin, and a big end, which is split so it can be attached to the crankshaft throw, separated by the beam. Today most connecting rods are forged from steel or cast from powder metal. The wrist pin can be a press fit in the rod small end bore, or it can be “fully floating” and free to turn. The bottom part of the split big end bore is called the rod cap. The bore is machined to accept plain bearings that ride on the crank journal on a thin film of pressurized oil. The cap is retained by bolts that thread into the rod or by bolts and nuts.
