Connecting rod

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A connecting rod, or con rod, is the part that links a piston to the crankshaft in engines. It turns the piston’s up-and-down motion into the crankshaft’s rotation and must carry both pushing and pulling forces.

Parts in brief
- Small end: attaches to the piston with a piston pin (gudgeon pin) and lets the rod pivot as the piston moves.
- Big end: fits around the crankpin on the crankshaft and usually uses a bearing to reduce friction.
- Oil hole: holes in the big end spray lubricating oil to keep the bearing and piston rings working smoothly.

How it works
As the piston moves, the connecting rod pivots at the small end and rotates at the big end. The rod’s angle changes as the crankshaft turns, so the piston and crank balance forces throughout the engine cycle. In many engines the rod is lubricated through the big-end bearing to keep everything sliding smoothly.

Materials and designs
- Most car engines use steel rods.
- Lighter or stronger setups may use aluminum alloys, titanium, or forged/billet rods (machined from solid metal).
- Cheaper engines might use cast iron rods.
- Some high-performance rods are made as a single billet piece, not cast or forged.
- The rod can wear or fail under high RPM, improper lubrication, or if installed incorrectly.

Special arrangements
-Radial engines often use master-and-slave rods, where one master rod connects to the crank and others connect to it.
- Some V engines use fork-and-blade (split big-end) rods to reduce rocking forces between oversized piston pairs.
- Steam locomotives often have the rod connect the wheel’s crankpin to a crosshead that moves the piston rod.

Common problems
- The rod can fail under strong, repeated forces, sometimes breaking through the engine casing.
- Bad lubrication or bearing wear can damage the rod and piston seals.
- If the rod is misaligned or too long/short, it can wear the cylinders unevenly or reduce engine power.

A quick note on history
The idea of a rod connecting a piston to a wheel or crank goes back to ancient water-powered tech and evolved through early steam engines and later machines to modern internal combustion engines.