Perhaps no part of an engine is hit with as much stress as the connecting rods. Designed to transfer linear motion and energy produced in the combustion chamber into a rotational motion at the crankshaft, connecting rods also serve as a key component in managing those same events and making a difference in an engine’s durability and life cycle.

“The rod package has to be custom tailored to the engine and the customer’s needs,” says Kerry Novak of Crower.

While different materials are used to construct connecting rods, this discussion will focus on steel — particularly billet and forged 4340 steel. For expert advice, we contacted some of the top figures in the rod industry, including Crower’s Novak, David Leach of Lunati, Alan Davis of Eagle Specialty Products and Manley’s Michael Tokarchik. We also reached out to Bryan Neelen at Late Model Engines for additional insight.

Understanding Rod Stresses

Connecting rods are subjected to both compressive and tensile forces during the 720 degrees of the four-stroke combustion cycle. On the compression stroke, pressures inside the cylinder increase, pushing back down against the rod. Depending on your engine’s compression ratio, power adders, etc, that pressure can rise quickly and steeply.

• 
• 

Compression ratio, boost pressure, ignition timing, camshaft overlap, horsepower, torque, engine speed and many other factors influence the stress on connecting rods.

On the combustion side, the rod must endure a sudden and violent direction change in addition to the pressure generated by the burning and expanding combustion gasses. That load on the rod can be calculated by multiplying the area of the bore (bore radius squared multiplied by pi) by the cylinder pressure. For example, a 4-inch bore would have a surface area of 12.566 inches. With a chamber pressure of 1,000 psi, the cumulative pressure on the rod at that point in the combustion would be 12,566 psi. And don’t forget the plug will fire just before the piston reaches top dead center, meaning the rod is still on it’s way up as the combustion mixture ignites, further increasing cylinder pressures that the rod must overcome.

This point in the combustion cycle also brings up the issue of pre-ignition, detonation and misfire. Knowing that the cylinder pressure increases once the air-fuel mixture is ignited, pre-ignition increases the load on the rod earlier, further straining it with compressive force. If the pre-ignition event is violent or frequent enough, the rod may be stressed beyond its limit.

Given the stress of these events, one might assume the exhaust stroke would be easiest on the connecting rod. The objective is simply moving the piston to push the spent gasses through an open exhaust valve. This, in fact, is the most dangerous time in the entire combustion process for a connecting rod. As Manley’s Michael Tokarchik explains, “The reason why is there’s no cylinder pressure buffering during that cycle.” With many camshafts having at least some type of intake and exhaust valve overlap, there is no cushioning pressure to slow the piston down.

As the crank makes the turn again over and past top dead center, inertial forces continuing driving the piston on it’s upward journey. This is the end of the exhaust stroke and beginning of the intake stroke. At this point, the rod is stressed in a tensile fashion. The big end must comply with the crank and begin the journey back in the opposite direction, while the small end wants to stay with the piston and continue upward. According to Tokarchik, this is actually where Manley sees the most failures occur in connecting rods.

During all of these directional changes, both ends of the rod are stressed, which can eventually lead to ovaling the bearing bores or complete failure.

 The Manufacturing Process

There are two manufacturing processes used to make high performance connecting rods today: forging and billet. Both processes have unique pros and cons, and both produce a very strong finished product when quality manufacturing processes and materials are used.

Forging

Forging is a manufacturing process involving tooling dies, extreme heat and pressure. The die is essentially a negative of the rod, similar to a mold. A blank piece of metal is heated to a temperature where it is malleable and then forced into the die using high pressure, often referred to as hammering. The metal takes the shape in the form of a raw connecting rod, which then goes to final machining. This includes cutting and sizing the rod for the end cap, drilling holes for the rod bolts and pressing in bushings. Rods can also be stress-relieved, heat-treated and fine tuned to the proper weight.

• 
• 

Left: A raw forging from Eagle Specialty Products before final machining. Right: A finished, forged Eagle H-beam rod ready for shipping.

Grain alignment is a key factor in the strength of forged rods. “The hot forging process also compresses and correctly aligns the grain structure of the metal, for increased strength,” explains Lunati’s Leach.

“A forged part is pressed in such a manner that the grain of the metal is aligned to best withstand the loads they are put under,” echoes Davis from Eagle, adding that a flow or swirled appearance to the grain around the big end of the rod further increases its overall strength.

Perhaps the biggest disadvantage to forged rods is the initial production cost. The dies can cost tens of thousands of dollars to produce, with a specific die needed for each design. These dies eventually wear out and must be replaced. Changes to a design require either a new die, or altering the final machining process. While forging offers increased strength, it is also best suited for large volume manufacturing for a company to achieve a profitable return on investment.

Billet

Billet connecting rods are built from a single piece of flat forged steel. They are designed using a CAD-type computer program, then individually cut from a billet material using a water jet or other CNC-controlled machine.

“You can manufacture the connecting rod to the application, meaning the rods can be custom-tailored to each engine’s specific needs,” says Novak. Due to this flexibility, the sky is literally the limit in what can be designed and produced.

Since the billet-rod manufacturing process does not rely on retooling or new dies, designs can be easily changed to accommodate variations in strength requirements, weight, rod length, crank- and wrist-pin diameter, oiling and more.

“We can take a rod from our Maxi-Light design that can handle 450 horsepower, and using that basic rod as a blueprint, design one that may have the same dimensions custom tailored for applications that make over 2,000 horsepower,” says Novak.

That rapid manufacturing capability allows billet rod manufacturers to manufacture rods for a snowmobile or motorcycle up to a big-rig diesel engine on the same equipment.

The downside to billet when compared to forging is the grain structure in the rod. Since a billet rod is cut from flat steel, the grain doesn’t swirl and flow around the big end of the rod, as in a forged application. With a billet rod, the grain remains straight or vertical throughout the rod.

Since billet rods are often produced in smaller batches or in custom configurations, more time may be needed in creating the design, machine setup and final finishing. Because of the additional labor involved and smaller production runs, billet rods can be more expensive than a forged rod of the same material.

 Materials

Whether forged or billet, rod strength is dictated in large part by the materials used. When it comes to drag racing and street performance, engine builders have made steel the material of choice in most applications.

Why Steel

It used to be that high-rpm engines used aluminum or other exotic materials to give rods high strength and light weight. As costs have risen and engine designs evolved, however, builders moved back to steel.

Bryan Neelen of Late Model Engines (LME) explains: “The weight below the wrist pin is not as big of a concern as the weight above it.” This is just one of the reasons for the move by many racers and engine builders back to steel. Cost, durability, and longevity are some of the others.

Another big factor is clearance. In high-rpm racing engines such as Pro Stock, valvetrain stability becomes increasingly important. Pro Stock rules allow for a larger camshaft bore, and big-bore cams provide higher valve lift in addition to improving rigidity and valvetrain stability. The additional material necessary for aluminum rods will often interfere with the rod-to-camshaft clearances. By using a high-strength steel rod, larger cam bores can be utilized without interference.

 The most common type of steel used for high-performance connecting rods is 4340 chromoly steel. 4340 has a tensile strength of 145,000 psi. Its hardness, ductility and other properties will vary based on the heat treatment applied to it. 4340 may also be referred to as aircraft grade or aircraft quality steel.

The entire steel manufacturing process determines the strength of these materials, as well. A simple designation of 4340 steel does not necessarily mean that two steel suppliers construct the final product to the same standards or with the same processes.

“Not all 4340 alloy steel is the same,” says Leach. “That makes it critical to know the steel mill, exact alloy of the material, and to deal only with the most reputable metal suppliers.”

Heat treat, drawing, hardness, ductility, and grain structure all play a vital role in the quality of the steel, thus affecting the final characteristics of a connecting rod.

Rod Bolts

All rod manufacturers emphasize the importance of rod bolts. No other fastener in a car is under as much stress as the rod bolt. 

“The purpose of the rod bolt is to keep the bore round, and keep a proper amount of pre-load at the body-cap interface — at top dead center during the exhaust stroke,” says Manley’s Tokarchik.

This is the moment where the rod bolt is most stressed and where rod-bolt failures often occur. As explained earlier, the combustion stroke puts stress on the rod bolts, but the inertial events occurring at top dead center during the exhaust stroke can take a greater toll.

Builders should follow the rod-bolt manufacturer’s instructions for installation.

“There are a lot of concoctions out there for rod bolts, and some engine builders have even developed their own. The fact is that you should stick with what the rod bolt manufacturer recommends for lubricant and tightening procedure and not deviate from that,” says Davis.

Rod Selection

Choosing the proper connecting rod for your application is as vital as selecting the correct camshaft. It’s also just as involved of a process, one in which you should know several things about your combination prior to making a decision. Checking with the engine builder and manufacturer is also a good idea.

“When we design a forged part, we want to make it strong enough to handle what we expect our customers to be using. It also has to be light enough to perform in the proper rev range,” says Davis.

• 
• 
• 

There are several factors that those consulted agree should be considered when selecting rods. Aside from the engine’s basics, such as the stroke and displacement, you also need to know the following:

• Piston package weight (piston plus rings)
• Operating rpm
• Horsepower
• Torque
• Block type
• Crank material
• Compression ratio
• Heads
• Cam specs
• Weight of vehicle
• Gear ratio

Rod selection eventually all comes down to relying on the manufacturer and engine builder to deliver the proper package for a particular engine application. Neelen tells us, “At the end of the day, if your components are not up to the power levels that they will see, it doesn’t matter who your engine builder is.” Choosing the proper connecting rod for an engine will help to ensure the best outcomes possible on race day, and should also increase the useful life of that engine.