Connecting rods are some of the hardest working parts inside an engine. They direct the downward force of the pistons to the crank throws to create rotational torque. The rods have to be strong enough to withstand the highest combustion loads without bending or buckling under pressure.
The rods also tie the pistons to the crank so the pistons can reciprocate and complete their intake, compression, power and exhaust strokes. In this role, the rods have to resist stretching forces that wants to pull them apart at high engine speeds.
Stock rods are usually strong enough to handle moderate performance modifications as well as some types of racing (where rules prohibit the use of aftermarket rods). But stock rods have their limits. As power levels and RPMs go up, the loads imposed on the rods will eventually reach the point where the rods are no longer strong enough for the job. And if a rod fails, it can destroy an engine in the blink of an eye!
Fortunately, rod failures from severe overloading don’t happen that often. However, they can occur in racing or extreme street performance applications where heavily boosted or nitrous motor is making insane amounts of horsepower. Most rods are fairly strong in compression, so it takes a tremendous amount of pressure to squeeze a rod beyond its yield point.
A more common cause of rod failure due to structural overloading can occur if a rod lacks the tensile strength to handle extreme RPMs. As G-forces multiply the inertia effects of the piston, the small end of the rod may be stretched to the point where it fails at or just below the wrist pin.
High engine speeds place enormous tensile loads on a rod. The inertia of the pistons reciprocating up and down multiplies the effective weight of a piston exponentially as RPMs go up. The force generated by a piston hitting TDC at 1,000 RPM is 50 times its initial weight when the engine is not running. At 10,000 RPM, the effective weight of that same piston is 5,000 times greater! That’s a lot of force stretching the rods 166 times per second.
The tensile load on the rods will depend on the mass of the pistons, rings and wrist pins, the stroke of the crankshaft (longer strokes create more inertia) and the speed of the pistons as they accelerate and decelerate up and down inside the cylinders. The longer the stroke and the heavier the pistons, the greater the inertial forces exerted on the rods. To minimize such forces, you want to use the lightest piston and rod combination that will be strong enough for the application. Or, go with shorter, lighter pistons and slightly heavier and stronger rods.
During the intake stroke, some of the inertia force of the piston is offset as it compresses the air/fuel mixture inside the cylinder. Squeezing the air charge acts like an air spring to reduce the tensile load on the rod when the piston reaches TDC. But there is no such cushioning effect when a piston reaches TDC on its exhaust stroke because all of the exhaust gases have been pushed out the exhaust valve.
The cushioning effect can also be lost during the compression stroke if the throttle suddenly snaps shut and shuts off airflow into the engine. That’s why rod failures most often occur when the driver suddenly lets up on the throttle at the end of a run down a drag strip or when a circle track car exits the straightaway and enters a turn.
In spite of the forces that are working to squeeze and pull apart the rods inside an engine, most rod failures that do occur are often not the rod’s fault. If a valve spring or valve spring keeper fails and the engine sucks a valve into a cylinder, the rod will usually bend, buckle or break when the piston smacks solid metal. Cast rods are more brittle than forged or billet rods and will often break when this happens. Forged and billet rods are more ductile and will usually bend or buckle rather than break. Either way, the engine is going to need a new rod.
Rod Bearing Failure
Another common cause of rod failure is a bearing seizure. Such a failure may or may not be the rod’s fault. If the oil film between the rod bearing and crank journal goes away for any reason, even for an instant, it can overheat and seize the bearing. Once that happens, something has to give. The bearing may spin and chew up the crank journal and/or rod bore, or it may seize so hard that it snaps the rod in two.
A broken rod whirling around inside the crankcase can do a tremendous amount of structural damage in a very short period of time. The end result is often a big hole in the side of the block and oil pan and a totally destroyed engine full of debris.
Bearing failures may occur as a result of extreme overloading of the bearing, overheating the bearing (insufficient oil flow), or oil starvation due to oil aeration, pump cavitation, a pickup obstruction or oil sloshing away from the pickup.
Loss of lubrication may also be from using a thin oil that lacks the shear strength to stay between the rod bearings and crank journal. Using an oil that has too low a viscosity for loose bearing clearances can do the same thing. If you’re going to run a low friction, low viscosity oil, you have to tighten up the bearing clearances to maintain proper oil pressure in the bearings.
Sometimes a rod bearing will seize because the rod bore elongates too much at high RPM and distorts the clearances between the bearings and shaft. The same thing can happen if the rod bolts are not torqued properly to keep the rod cap in place, or if the rod bolts are too weak to withstand the forces of higher RPMs and allow the cap to pull away from the rod.
Better Rod Bolts
One of the most important upgrades for any performance engine regardless of what type of rods you choose to use is to upgrade the rod bolts to stronger aftermarket bolts. Many aftermarket rod bolts have more than double the tensile strength of a stock rod bolt. Yes, new rod bolts can be expensive depending on the grade of bolts that are used. But it many performance applications, you have no choice.
Most aftermarket rods come with some type of performance bolts. To save weight, many performance rod designs are drilled so cap style bolts can be screwed directly into the body of the rod, eliminating the need for nuts on the bolts. This saves weight, allows more clearance for a stroker crank and usually does a better job of holding the rod cap tightly against the rod.
It’s also important to remember that rod bolts have a limited service life in racing applications and can only be reused a limited number of times before replacement is necessary. Many engine builders don’t replace the rods bolts automatically when freshening up an engine and in some forms of racing like Top Fuel drag racing, there is so much load on the rod bolts that they are usually replaced after every run.
Rod failures can also occur as a result of metal fatigue. A small surface blemish, nick, scratch or imperfection on a rod will concentrate stress. Eventually, this can lead to microcracks in the metal and ultimately a fracture that causes the rod to break. Most performance rods have a machined smooth surface to reduce the risk of stress fractures. Shot peening also helps dissipate surface stresses for improved durability and strength.
The type of alloy in a rod as well as how it is designed, manufactured, machined and heat treated all play a key role in determining a rod’s ultimate yield and tensile strength as well as its ability to survive in a demanding application.
Most stock rods are either cast iron or powder metal (PM). The latter is used in most late model engines because it minimizes the amount of machining that has to be done during the manufacturing process. Iron powder is mixed with alloying elements such as copper and phosphorus, then blended with small amounts of graphite and other ingredients before it undergoes shaping and “sintering.”
The PM rod is pressed and hot forged into it final shape, then placed in an oven and heated to a temperature where the individual iron flakes and other ingredients bond together. When the rod comes out of the sintering oven, it is at net final dimensions. The weight of the rods is very consistent from one to the next, which makes balancing easier. The big end rod bore is then cracked apart to create the cap and the wrist pin bushing is installed in the small end of the rod. The cracking process creates a cap that self-aligns much better than a machined cap. The only trade off is that a cracked cap can’t be machined if the rod later needs to be reconditioned. If the bore is out of round, the rod needs to be replaced – unless oversized O.D. bearings are available so the cap can be remachined like a conventional rod cap.
Powder metal rods are a good upgrade over stock cast rods because they are stronger and less prone to cracking and failure. Depending on the alloys used to make a PM rod, they may even be as strong as some forged steel rods. The cost is also reasonable compared to other types of performance rods.
Forged steel rods, usually made of 4340 steel, are by far the most common upgrade for both street and track performance. Available in traditional I-Beam and H-Beam configurations, some are also available in slightly modified versions of the I-beam (such as an A-beam rod with a wider base, or an X-beam with additional cross rib reinforcements where additional strength is needed). Some performance rods will have lightening holes near the big end to reduce weight where metal can be safely removed without compromising strength.
One new rod that was unveiled at the recent PRI Show in Indianapolis uses a unique three-pocket design near the big end to improve bending strength 60 percent over compared to a common H-beam rod. The rod is designed for booted engines that are turbocharged, supercharged or use a big shots of nitrous.
Billet steel rods are also available and typically cost more than forgings because of the extra machine work that is required to cut them from a solid plate of steel. One of the advantages of billet rods over forged rods is that billet rods can be custom made to fit virtually any application, including those where no forgings are available.
Forged and billet aluminum rods are another upgrade option for serious competition. Aluminum rods are very popular with drag racing because of their lighter weight. You’ll find aluminum rods in most of the Alcohol and Nitro Funny Cars and Top Fuel dragsters. They’re also popular in Pro Mod drag racing, circle track racing (where rules permit) and even street performance.
As we said earlier, one way to reduce the inertia forces pulling on a rod when the piston approaches TDC is to reduce the weight of the rod. Aluminum rods can reduce the mass of the piston and rod assembly significantly and provide the most benefits in high revving engines. The rods can be forged from various alloys, or CNC machined from flat plate aluminum or forged billet aluminum (the latter is stronger because of the directional grain structure in a forging).
Titanium rods are another option. Though pricey, titanium offers even more weight savings.
One thing you have to keep in mind about aluminum rods is that they have a higher coefficient of thermal expansion than steel rods, so the rod has to be somewhat shorter or you have to allow more deck height to accommodate thermal expansion when the rods get hot. Aluminum rods may also have a limited life due to stretching that occurs over time, so rod replacement is often recommended after so many hours of racing.
Rod Installation Tips
One of the first things you should always do prior to assembling any parts is to carefully inspect each and every connecting rod. Check for any obvious surface nicks, burrs, scratches or other damage that might create stress points that could lead to cracking and failure. If grinding is needed to debur or clean up a casting, do so lengthwise on the rod, never sideways.
Next, steel rods for both stock and performance applications should always be Magnafluxed to check for cracks. Use penetrating dye to check aluminum rods. Discard any rods that are cracked.
Rods (including brand new ones) should also be checked for dimensional accuracy. Measure the I.D. of the big and small end bores. Measure the rod’s overall length. And check the rod for straightness. Any bending or twisting between the big end rod bore and the wrist pin bore needs to be corrected.
Performance forged steel rods should also be checked for proper hardness. Heat can cause a loss of temper in a rod, so if the rods show any signs of heat discoloration be sure to check their hardness. If hardness is less than specifications, replace the rod. For many forged steel rods, minimum hardness is Rc 42 to 43.
For a stock rebuild, reusing the original equipment rod bolts may be okay provided all of the bolts are in good condition and are not stretched. Torque-to-yield (TTY) rod bolts should not be reused doe to the risk of breaking or stretching.
For performance applications, don’t even think about using or reusing stock rod bolts. Replace them with stronger aftermarket bolts.
Installing the rod bolts correctly is essential to preventing rod failures. A torque wrench is not that accurate of an instrument for tightening rod bolts, nor is an angle gauge with TTY bolts. The most accurate way to make sure the rod bolts are providing the recommended clamping force is to measure how far they stretch when tightened with a dial indicator. For a set of aftermarket performance rod bolts, the recommended stretch for a Chevy rod bolt may be .0055 to .0075˝ depending on the application and the bolts used. Follow the bolt supplier’s recommendations.
Torque wrenches are not that accurate for tightening fasteners because the wrench only tells you how much force is being applied to overcome friction. The condition of the threads, the number, length and pitch of the threads and the type of lubricant on the threads all affect the reading.
Moly-based lubes act differently than ordinary assembly lubes or motor oil. Different viscosity motor oils behave differently. Some thread lubes provided by aftermarket bolt suppliers are more consistent than other lubricants. Even so, many rod suppliers recommend using a dial indicator to measure stretch so you can have the right amount of clamp load regardless of how the threads are lubricated.
Also critical to connecting rod longevity is checking and double-checking rod bearing clearances and wrist pin fit. Too tight or too loose can cause major problems at either end of the rod. Also, make sure rod bearings have the proper crush fit. The backs of the rod bearings should be installed dry, while the face of the bearings are coated with assembly lube.
Finally, when everything is together, recheck all rod bolts to make sure they have been properly tightened and none have been overlooked. Dumb assembly mistakes happen, but hopefully such mistakes won’t happen to you.