Selecting Connecting Rods to Ensure the Success of your Performance Engine

Selecting Connecting Rods to Ensure the Success of your Performance Engine

Connecting Rod Failure: Common Causes and How We Can Prevent Them

Connecting Rods are a critical component in high performance combustion engines, there’s no denying it. Responsible for converting the reciprocating motion of the piston into the rotation of the crankshaft, connecting rods are constantly resisting the forces that want to pull them apart, all of which are magnified by performance modifications.

While your factory connecting rods are strong enough for modest power modifications and minimal types of racing, they will inevitably meet their limits as horsepower, boost, and RPM values begin to rise outside the realms of OEM spec. Unfortunately, meeting these limitations [AKA connecting rod failure] means costly and sometimes irreparable damage. While you may read comments by forum members boasting that they’ve pushed OEM components and ‘been fine,’ there’s considerable risk to take into account:

“You don’t need _______. The factory one will be fine.” Car manufacturers, including BMW, spend millions of dollars each year developing new components and engines to meet the demands of the factory spec. Every component that enters and leaves the factory was specifically engineered to meet these specs within the budget outlined by the manufacturer. 

In other words, the connecting rods that came in your BMW S58 engine were engineered for a 7200 RPM redline, stock boost, with the tune prepared by BMW for…so on and so on. Once you begin to alter these variables, you are changing the conditions of the engine that the original components were designed for. Your OEM components are no longer working in the system they were designed for; the system was changed and modified. Inevitably, each of these components will meet its limits.

For this very reason, 5150 AutoSport has partnered with some of the best names in the industry to develop solutions that improve on OEM connecting rod design that are prepared to exceed factory performance. To understand these improvements, we introduce some of the common causes of connecting rod failure we've seen in modified vehicles and how 5150-partners have developed engineering and manufacturing solutions to these problems.

OEM Connecting Rod Failure Cause #1: Rod Material

Materials matter in high performance engine builds. When taking things like boost, nitrous, fuel selection, and tune into account, your engine is no longer operating in the way it was intended to from the factory. High temperatures, engine speeds, and modified combustion are all factors that make material selection crucial. Cast connecting rods are great for OEM applications, but begin to show their faults as power and torque rise, warping, bending, and breaking under load. 

Performance connecting rods are made from forged steels, which are both stronger and more durable than OEM cast rods. For example, our CP-Carrillo x 5150 AutoSport Pro-Xtreme Connecting Rods are manufactured from aircraft-grade steel; the same materials entrusted to landing gear and celebrated for their ability to handle high stress, high-impact applications. These steel rods are magna fluxed to check for cracks, and shot peened to dissipate surface stresses for durability and strength. According to CP-Carrillo:

By using Magnaflux equipment, we can locate such inherent and processing defects as: inclusions, seams, laminations, shrinks, cracks, hot tears, laps, flakes, heat treat and grinding cracks, machining tears, quenching and straightening cracks but also fatigue or service cracks.

A CP-Carrillo connecting rod undergoing magnaflux inspection [magnetic particle crack detection] to check for cracks and imperfections. 

In addition to the higher quality forgings, our Pro-Xtreme rods were specifically engineered to handle extreme RPM applications; up to 9500 RPM in a BMW, a nearly impossible-to-believe number for BMW engines, which leads us into...

OEM Connecting Rod Failure Cause #2: High RPMs

During each rotation of the crankshaft, your connecting rods are subjected to large repetitive forces, including compression forces from the piston as it moves downwards, tensile forces as the piston is forced upwards, and shear forces caused by the angle between the piston and the crankshaft journals. These forces are proportional to engine speed (RPM) squared. Simply put, the forces acting on your connecting rods are exponentially increased as RPM increases.

As performance drivers and tuners push their vehicles to spend more time in the high RPM range and alter the RPM limits of their tuning, the forces acting on the connecting rod are magnified and multiplied. Stock connecting rods, not designed for high RPM applications, will begin to stretch and deform. This stress is magnified in the area of the connecting rod surrounding the wrist pin, known as the ‘small end.’ In cases of complete and catastrophic failure, the rod will stretch and fail in this area. 

CP-Carrillo x 5150 Pro-Xtreme connecting rod and piston (Left) compared to a stock connecting rod and piston (right). Note the design of the connecting rods at the 'small end,' and the significantly modified design of the Pro-Xtreme rod for high performance applications. Photo courtesy of R44 Performance.

The manufacturers we work with for the engineering, development, and manufacturing of high performance connecting rods take these forces into consideration and design their rods to handle the abuse of high RPM applications. Another crucial difference between performance connecting rods and OEM connecting rods is material selection. The performance connecting rods we offer are manufactured from 4340 steel. 4340 is known for its high tensile strength and resistance to fatigue. Because of the repetitive forces acting on the connecting rod, 4340 becomes necessary for its ability to resist deformation under impact.

OEM Connecting Rod Failure Cause #3: Rod Bolt Failure

Rod bolts are responsible for keeping the connecting rod secured to the crankshaft, thereby ensuring the overall integrity of the engine. While you increase the pressures and temperatures of your engine with performance upgrades, consider that the tensile strength of OEM rod bolts is less than half that of performance rod bolts. All 5150 High Performance Connecting Rods come equipped with upgraded hardware, including ARP rod bolts on Manley Performance Connecting Rods, and CP-Carrillo’s proprietary CARR or WMC rod bolts. And careful installation is key. According to Manley Performance:

The vast majority of all rod failures are due to incorrect fastener installation. The parting line area and threads should be thoroughly cleaned prior to assembly and be sure to seat the rod cap to the rod body evenly, otherwise the cap can become cocked and would result in cross threading of the fastener(s). This is best achieved by alternately tightening the fasteners until the cap is fully seated to the rod body. Apply the supplied lube to threads and under the head of fasteners before assembly.

When tightening the fastener, bolt stretch is the singular most important value to be considered. Torque the fastener within the indicated torque range until specified stretch is achieved. When the desired stretch has been achieved by applying a torque load that falls within the allowable range, the fastener is properly tightened. 

If recommended bolt stretch is only achieved by applying torque that is outside the recommended torque range, there is a problem either with the fastener, the bolt threads, the application of the lube, or the torque wrench.

Bonus Connecting Rod Failure Cause & Prevention: Rod Bearing Failure

While not caused by the connecting rod itself, rod bearing failure will have a direct impact on the connecting rod. In order to operate correctly, the crankshaft journal and rod bearing must have a thin layer of oil between them to lubricate each properly throughout their range of motion. If these surfaces are deprived of lubrication at any point, even briefly, the friction can cause them to overheat and the bearing to subsequently seize.

When a bearing seizes on an active crankshaft, one of a few scenarios will be the final outcome. The first possibility is that the bearing will spin on the journal, causing severe damage to the crankshaft and/or connecting rod bore. The second, and worst possibility, is that if the bearing seizes up entirely around the crank journal, the connecting rod would be forced to snap as it no longer freely pivots on the crankshaft. As a broken rod spins around the interior of the crankcase, it will wreak havoc, causing severe structural damage with the final outcome often including a hole in the engine block.

A broken engine block with damage caused by connecting rod failure.

Luckily, we have worked hard to source and supply a variety of solutions for this problem, including Calico-coated 5150 High Performance Rod Bearings. 5150 Rod bearings feature Calico’s Specialized Dry Film Lubricant Coating, which is designed to reduce friction and abrasive wear. These bearings not only compensate for oil starvation, but they are also a better, safer bearing even during cold starts and the extreme conditions of high performance engines. 5150 High Performance Rod Bearings for BMW S65 and S85 engines also feature greater clearances than factory bearings, one of the notorious failure-points of these engines. 

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