CONNECTING RODS

In a piston driven engine the connecting rod is the part that connects the piston to the crankshaft. Together with the crankshaft they form a simple mechanism that converts linear motion into rotating motion. Connecting rods may also convert rotating motion into linear motion.

As a connecting rod is rigid, it may transmit either a push or a pull and so the rod may rotate the crankshaft through both halves of a revolution, which means that the piston is pushed down by the detonation of the fuel air mixture ignited by the spark plug and then the piston is pulled back up to TDC (top dead centre). In some two-stroke engines the connecting rod is only required to push. The connecting rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and being compressed with every rotation making conrods an engine part of high importance on any engine builders list.

Today, connecting rods are best known through their use in internal combustion piston engines, such as car engines and these rods are most usually made of steel for standard production engines, but can be made from different mixtures of aluminum alloys for lightness and the ability to absorb high impact at the expense of durability or titanium for a combination of lightness with strength, at higher cost. Titanium is the metal of preference for high performance engines. Engines used for racing usually are equipped with three different types of conrods; the first is a ‘billet’ rod, machined out of a solid billet of metal. The second is a ‘forged’ conrod, which is stamped out of a solid billet of metal and lastly a cast conrod, which is a conrod ‘cast’ from certain molten steel, making castings the weakest of the three processes.

The ‘small end’ attaches to the piston to the connecting rod with a gudgeon pin which forms a ‘floating wrist pin’ in design. The ‘big end’ connects to the bearing journal or crank throw. In most engines running on replaceable bearing shells accessible via the connecting rod bolts which hold the ‘bearing cap’ onto the big end journal.

The big end of the rod is fabricated as a unit and cut or fracture split in two to establish a precision fit around the big end bearing shell. Therefore, the big end ‘caps’ are not interchangeable between connecting rods, and when rebuilding an engine and therefore care must be taken to ensure that the caps of connecting rods are not mixed up. Both the connecting rod and its bearing cap are usually embossed with the corresponding position number in the engine block.

The simplest conrod setup which is practically universal standard production car engines is to use connecting rods where cylinders from both banks share a journal. This usually requires the rod bearings to be narrow, increasing the bearing load and the risk of failure in a high-performance engine and it means the opposing cylinders slightly offset and not exactly in line with each other.

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