Pistons, Rings & Connecting Rods

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.

Pistons are cast or forged from aluminum alloy. The forging process increases strength and resistance to damage, and forged pistons are generally used for extreme conditions like racing. The piston rings fit into grooves near the top of the piston, called the head. The most common construction is 2 compression rings with a 3rd ring below them for oil control. Most oil rings consist of upper and lower rails with a center expander to maintain tension.

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.

The piston skirt is the area between the bottom ring groove and the bottom of the piston. This is the bearing surface of the piston that contacts the cylinder wall. Wear in this area, called “scuffing”, can be caused by lack of lubrication or lack of clearance due to bore distortion. To help prevent scuffing, many pistons today come with moly coated skirts to reduce friction. It is common for the connecting rod big end bore to stretch due to the millions of direction reverses it undergoes during normal operation. Rods can also become bent or twisted, evidenced by abnormal wear patterns on pistons skirts and rod bearings.

Ring material depends on application and the ring’s location on the piston. To handle the heat, top rings are made of ductile iron or steel with a face coating of plasma molybdenum or chromium. Chrome-faced rings are very durable and ideal for engines operated in dusty conditions. Second rings are not exposed to as much heat, and can be made of cast iron. For optimal ring seal, the cylinder wall finish must match the type of ring. Compression rings are split to facilitate installation, leaving a gap when the piston and rings are installed in the cylinder. This gap must be within specs or excessive blow-by can result.