Conerod

A connecting rod, or con rod, is a rigid link that connects a piston to the crankshaft in internal combustion engines and other piston machines. It converts the piston's linear motion into rotational motion of the crankshaft, transmitting force from the piston to the crankpin.

A rod has a small end that attaches to the piston via a wrist pin (gudgeon pin) and a big end that fits around the crankpin. The big end is typically split into two halves with a bearing shell between them and the crankshaft, forming a cap that is bolted to the rod. The small end may house a bushing or needle bearing to accommodate the piston pin.

Rod length, cross-section, and bearing clearances influence piston acceleration, side loading, and engine balance. The small end may incorporate a bushing or needle bearing; the big end uses a plain bearing or bushing for rotation around the crankpin. The geometry and tolerances are chosen to minimize friction and wear while maintaining strength.

Materials and manufacture: Most rods are made from forged steel alloys for strength and fatigue resistance. Lighter or high-performance designs may use aluminum, titanium, or advanced composites. Common shapes include I-beam or H-beam cross-sections to optimize stiffness and weight. Manufacturing methods include forging or casting, followed by machining and heat treatment.

Design and maintenance: The rod must withstand alternating tension and compression, bending, and torsion. Lubrication of the big-end bearing is critical to prevent wear. Common failure modes include bearing wear, cap bolts loosening, and fatigue cracking due to improper lubrication or misalignment. In some two-stroke engines, a master-and-articulating rod arrangement is used.