Connecting Rods – A Detailed Guide

Whether you’re building your dream engine or increasing output figures in a bespoke tune, engine internals are often parts that are first replaced. Pistons, connecting rods and crankshafts can all be changed to handle the extreme forces that come with higher horsepower and torque. Aftermarket options include better designs and stronger materials to create a package that is both balanced and noticeably better than stock parts. Central in delivering that newly-tapped power from pistons to crank to wheels is the humble connecting rod. 

What are Connecting Rods?  

The connecting rod, or conrod for short, is the engine part that connects the pistons to the crankshaft. Conrods are tasked with converting the linear movement of the pistons in different stroke cycles to rotational forces exerted on the crankshaft. During this process high heat, pressure and lateral forces act on the conrods, meaning a tune with the end goal of increased performance numbers needs to include conrods that are well-designed and of the best possible materials.  

Parts of a Conrod 

Conrods are complex feats of engineering consisting of several parts. The small or top end of the conrod is connected to the piston pin. The bottom or big end is attached to the side of the crankpin in the crankpin journals. Both ends are connected by the conrod shaft. Bearings at both the top and bottom ends provide for seamless integration with the piston and crank. Bush bearings connect to the piston pin and slide bearings to the crankpin journal. Pinholes in the bottom end bearings allow for lubrication between crankpin and conrod. Bearing caps along the bottom end affix the conrod to the journal at either side. To get the best possible tolerances, caps are tightened with high-strength bolts and nuts.  

Materials 

Different materials are used depending on the type of engine and its intended use. Many factors come into play considering the different stresses exerted on the conrod. High compression engines like diesels will exert more pressure and bending in the rods, while performance petrol engines ticking at higher rpm exert high tensile stress leading to stretching, or in worst-case scenarios, snapping. Materials are different in either case. 

Stock naturally-aspirated petrol and diesel engines often come with cast steel conrods with lower grades of steel. These are cheap and mass-produced but still reliable. Forged and heat-treated carbon steel can be seen in performance vehicles and the additional treatment leads to higher prices. Steel is high density and maintains integrity in high compression engines.  

Aluminium, on the other hand, is much lighter allowing engines to rev faster. Aluminium conrods are used in turbocharged and supercharged stock engines and engine rebuilds due to the material better handling the stretching required in every cycle. They do however have their limits, and cases of snapped aluminium conrods are often reported in racing vehicles. The lower fatigue strength compared to steel is a major factor here.  

Another material substituting both aluminium and steel is titanium. Titanium conrods are lightweight like aluminium and have comparable tensile strength to steel. These qualities, and the difficulties in producing higher numbers, make titanium conrods extremely expensive. They’re seen in drag racers and hotrods.  

Design Types 

Connecting rods come in two basic designs – I beam and H beam. They get their names from the cross-section cutouts of the conrod. I-beam conrods are more common in stock engines with moderate rev redlines. They are easier and cheaper to produce than H-beams owing to their added weight and thickness. However, variations exist here too. Different steel alloys lend to lighter I-beam conrods capable of large compression loads, high tensile strength and comparable performance to H-beam conrods. 

H-beam connecting rods are designed differently. They have two large, flat sides with thinner midsections, making them lighter than I-beam conrods. Manufacturing such designs is more difficult, making them more expensive. They are however the best choice for performance upgrades with included turbo installs.  

Common Conrod Issues 

Depending on the materials and designs, and the inflicted forces on the different parts of the conrods within each rotation, conrods can suffer from a number of faults. The most common is fatigue, as with high revs comes increased stretching and rods in cars with more miles can snap. This causes a lot of damage in a very short time frame. A cracked connecting rod will completely destroy an engine. 

Loss of lubrication and using oil with too low viscosity can lead to conrod bearing seizures, again leading to major damage. Seized bearings result from contaminated oil, with loss in oil pressure. In addition, bearing caps can burst, especially in high revving, mainly due to the inferior quality of bolts and nuts. In performance cars, these are replaced with high-end cap style bolts which are directly screwed on the body of the rods.  

Buying Replacement Conrods 

In engine upgrades, existing conrods are replaced with complete new sets for the engine type. This takes into account the existing pistons and the new sets need to align perfectly. There is also the option of purchasing pistons and connecting rod combos with strengthened bolts to ensure that everything works smoothly.