5 Common Causes of Connecting Rod Failures & How to Avoid Them

Connecting rods are key players within internal combustion engines. These components, which convert the up-and-down motion of the pistons into rotating motion at the crankshaft, are typically made from reinforced materials designed to withstand high mechanical stresses and dynamic loads.

Because of the intense pressure and repeated loading they endure with each engine cycle, connecting rods must be carefully selected and properly installed to ensure they deliver optimal, long-lasting performance.  

When a rod fails, it can have serious consequences, including engine seizure, damage to internal parts, and even complete engine failure. Which is why understanding the causes of connecting rod failures–and how to prevent them–is so important to the health and longevity of your engine. Here, we’ll count down five common ways rods can fail, and provide some direction on how to prevent these failures from happening in the first place.

5. Improper Bearing Clearance Resulting in Rod Bearing Failure

One of the main causes of connecting rod failures is improper bearing clearance. Situated between the rod and crankshaft, the bearing consists of an upper half, which is attached to the rod’s bigger end, and a lower half, which attaches to the rod bearing cap. Rod bearings are primarily called upon to reduce friction and support the load from combustion forces.

If the clearance between the bearing and the crankshaft journal is too tight, it may lead to insufficient lubrication and excessive friction. This can cause the rod bearing to make hard contact with the crank journal, causing it to spin and damage the bearing; it can also send massive amounts of heat to the rod’s big end. Either scenario could have potentially catastrophic consequences for the engine.

Conversely, too much clearance can cause the crankshaft to expel excess oil from the area between the bearing and crank journal, leading to a loss of oil pressure. This, too, can result in metal-on-metal contact between the two parts, accelerating wear and potentially causing bearing spin, which can damage or destroy the rod and crank.

Using precise measuring techniques and sticking to the manufacturer’s specifications during engine assembly will go a long way toward ensuring the correct bearing clearances. Regular maintenance and watching for signs of faulty bearings, including rod knock from the engine or dashboard warning lights indicating low oil pressure, can also help prevent failures.

4. Rod Bolts: Improper Torque or Selection for the Application

Rod bolts play a key role in securing the connecting rod cap to the rod itself and maintaining clamping force under extreme engine conditions. In fact, these bolts are often considered the single most important fasteners in the engine, since they experience the greatest stress from a reciprocating load standpoint; they have to withstand tremendous amounts of force created by the piston and connecting rod in motion.

Using the wrong torque specs during assembly can lead to insufficient clamping force, causing the rod bolts to overstretch or loosen over time. Ultimately, the bolts may not be able to maintain their grip on the rod and cap, which can either result in rod cap separation from the rod, or excessive movement. Worst-case scenario, a rod bolt failure can cause the connecting rod to break free and punch a hole through the side of the engine block, resulting in a complete loss of oil pressure, overheating, and seizure of the engine.

Selecting rod bolts that aren’t designed to handle the engine’s power output and operating conditions can compromise their integrity as well. Unlike those found in everyday grocery-getters, high-performance engines generally require stronger, high-strength bolts that can withstand increased loads and vibrations. These bolts are typically made from ultra-strong materials and have specialized coatings to enhance their durability and fatigue resistance. A good rule of thumb: Always follow the manufacturer’s torque specs and choose the appropriate rod bolts based on the engine’s horsepower, RPM, and intended use.

3. Detonation Causing a Shock Load on the Rod, Resulting in Connecting Rod Failure 

Engine detonation, also known as engine knock or pinging, can have catastrophic consequences on connecting rods. Detonation happens when the fuel-air mixture in the combustion chamber ignites before the spark plug fires, resulting in an uncontrolled release of energy. This energy sends a shockwave through the engine, placing extreme pressure and stress on the connecting rods (and other parts).

Detonation can be caused by any number of factors, including lean air-fuel mixtures, high compression ratios, overheating, or the use of low-octane fuel, among others. In racing and performance applications, aggressive ignition timing and insufficient fuel enrichment can result in detonation as well.   

During detonation, the connecting rods are subjected to intense mechanical stress. The resulting rapid pressure spike can lead to rod bending or breakage, as well as bearing failure and crankshaft damage. In addition, the excessive heat generated by detonation can cause piston damage, including melted or cracked rings, which may also contribute to rod failure.

Retarding ignition timing, enriching the fuel mixture, and upgrading to higher-octane fuel can help prevent detonation. The choice of materials used in both the connecting rods and rod bearings also plays a role in helping to withstand the shock and stress of combustion, so keep those factors in mind as well.

2. Hydrolocking Due to Head Gasket Failure or Excessive Fuel

Hydrolocking occurs when a cylinder fills with liquid, preventing the piston from completing its upward stroke. This can happen because of a head gasket failure, where coolant leaks into the cylinder, or a cracked or stuck open fuel injector, which causes excessive fuel to flood the cylinder. Then, when the piston tries to compress the liquid, the connecting rod may bend or fracture due to the sudden increase in resistance.

Hydrolocking puts a ton of stress on the connecting rod and other engine components, potentially causing serious and/or permanent damage if it’s not addressed quickly enough; for example, a broken connecting rod can easily fly off and blow a hole in the engine block. Regularly inspecting your head gaskets and fuel injectors, along with monitoring your coolant levels and engine performance, can help prevent hydrolocking.

1. Choosing the Wrong Style Rod for Your Application

Choosing the wrong connecting rods for your engine can easily put your build in a bind, since engines vary in power output, RPM ranges, and the stress levels they place on rods. For instance, using lightweight rods meant for lower horsepower in a high-performance engine producing 2,500 horsepower and running at 50 psi of boost can easily result in failure. Simply put, lighter-weight rods may not be able to handle the increased stress and vibrations, leading to bending or breaking under pressure.

On the other hand, using overly heavy rods can increase the reciprocating mass, negatively impacting engine response and potentially causing fatigue failure over time. When assembling an engine, it’s always important to match connecting rod specs with the engine’s power output, RPM range, and intended use to maximize both longevity and reliability.

The Right Choice: Why BoostLine Rods Represent the Best of Both Worlds

The most common materials for connecting rods are steel, aluminum, and titanium. Each has its pros and cons, so basing your selection on the engine’s intended purpose, including power goals, is always the way to go.

While aluminum connecting rods offer some inherent benefits–namely flexibility–their longevity is largely unknown. There’s just no ironclad data on how long they can be expected to last. Which makes their failure rate highly unpredictable. Matched up head-to-head, a steel rod offers superior strength and durability over aluminum, as well as a much longer lifespan.

Titanium connecting rods are durable and lightweight, but they’re also very expensive. In fact, a set of titanium rods can cost upwards of four to five times more than steel rods. Plus, they’re known for having a significantly shorter lifespan than steel.

Made from 4340 forged steel and designed to resist bending under huge power loads, BoostLine’s connecting rods are purpose-built for high-horsepower applications–primarily forced induction or nitrous combinations. Equally at home on the street or track, these rods are capable of handling 2,000-plus horsepower right out of the box. 

While their three-pocket design is geared for massive power, BoostLine’s rods have also been developed using a finite element analysis (FEA) process that helps pinpoint stress levels. This advanced, data-driven approach allows BoostLine engineers to better understand how and why a part might fail. The result is more precise, efficient development of a lighter-weight yet highly durable product.

BoostLine’s connecting rods are available for a wide range of engine platforms, including GM’s LS family, Chevy big block and small block, Ford modular V8s, Gen III HEMI, GM and Cummins diesel, Mitsubishi, Honda, Subaru, and more. For more on BoostLine’s growing lineup of best-in-class rods, visit www.boostlineproducts.com.