They say an engine is only as reliable as its weakest link. The connecting rods that join the pistons with the crank can be a strong link or a weak link depending on the rods that are used. Most late model stock engines use powder metal rods. They are inexpensive to manufacture, require minimal machining to finish and are adequate for stock power levels and normal driving.
One of the drawbacks of powder metal rods is that the rod caps are cracked to separate the cap from the rest of the rod. It’s a fast, easy way to make high volume connecting rods, but it also means the rods can’t be rebuilt if the big end gets out of round. Cracking the cap leaves a slightly irregular mating surface between the rod cap and rod. The cap only fits one way because the parting line is unique to each rod. Consequently, if the bore is out-of-round, you can’t simply grind down the cap, bolt it back together and hone the hole back to size to make it round again. You have to replace the rod.
Another drawback of powder metal rods is a limited fatigue life – about one third that of a forged rod. For normal driving, torque loads and engine speeds, the rods will usually last well over 150,000 miles with no problems (assuming the oil is changed regularly and there are no lubrication issues). But in a modified engine that makes more power and spins at higher RPMs, powder rods won’t hold up. If you’re going over 400 horsepower and/or 6,000 RPM, you’d better replace the stock rods with some type of aftermarket performance rods.
There are a LOT of aftermarket rod suppliers from which to choose. Some make a range of products for street performance to 2,500 horsepower plus racing applications. Others specialize in high-end race-only rods for Top Fuel and Blown Alcohol drag race engines. The point is, there is a rod in every price range and performance rating for almost any application you can think of, from antique restorations to classic muscle cars to late model Mustangs and Corvettes to sprint cars, drag cars and marine engines. You can buy off-the-shelf rods for popular stock stroke and long stroke small block/big block Chevys, Fords, Chryslers and other engines, as well as custom rods (forged or billet) that can be made to order.
Forged steel I-beam, H-beam, A-beam and X-beam rods made from 4140, 4330, 4340, 300M or other proprietary high strength chrome steel alloys are a good upgrade option for many engines. The strength of the rod will depend on the alloy and heat treatment, the design of the rod, its thickness, dimensions and weight. A number of suppliers have lightweight forged steel rods for SB Chevys and Fords that are a good upgrade over stock rods. Lighter rods are good for faster acceleration and higher RPMs – up to a point. Many of these rods are designed for engines under 400 cubic inches and no more than 500 to 550 horsepower that are going into street performance cars, circle track cars and super stock drag cars. They are not intended for higher displacement or higher horsepower motors.
Using finite element analysis software, a rod designer can remove weight in areas that are not critical to maintain a rod’s strength. The rod has to maintain stiffness in the beam section, and have the strength to prevent it from bending under load or pulling apart at high RPM. The rod cap and big end of the rod also have to be designed in such as way that the big hole stays round and doesn’t distort out-of-round at high speed.
H-beam rods are typically stronger than I-beam rods, especially when it comes to resisting twisting loads. The trade-off is that the H-beam rods tend to cost more than comparable I-beams, and are somewhat heavier. H-beam rods can also create a little more windage inside the crankcase in a high revving engine.
When choosing a rod for a particular application, follow the rod supplier’s recommendations as to which of its products is best suited for the engine you want to build. If it says a rod is good for up to 700 horsepower, don’t use it in an engine that’s going to make 1,200 horsepower unless you want your customer to bring back the engine in a basket. Likewise, there’s no need to buy a set of killer race rods that are rated for up to 2,500 horsepower for a street motor that’s only going to make 500 to 600 horsepower.
Rods can fail for any number of reasons. Installer error is a common one. Not torqueing a rod bolt properly may allow the rod bolt to loosen or break, or the rod cap to pull loose. Lubrication issues at either end of the rod can also cause rod failure. If a press-fit wrist pin seizes in the piston bore, the top end of the rod may break. If the rod bearing on the crank is starved for oil, overheats and seizes, it can snap the rod in half.
Rods are much stronger in compression than they are in tension, so over-revving a weak rod may stretch the rod to the breaking point causing the small end or big end to fail. Rarely does a rod fail under compression load unless the rod was too weak for a really high horsepower application or the rod had an undetected defect such as a hairline crack, scratch or nick. Imperfections and sharp edges on the surface of a rod can concentrate stresses that may lead to cracking and failure. That’s why performance rods are usually shot peened and/or polished.
All rods have a certain fatigue life. After zillions of repeated cycles, the metal will eventually fatigue, crack and fail. In a Top Fuel dragster or funny car, the loads on the connecting rods are incredible and most rods only last 8 to 10 runs before they are replaced. One supplier said the goal is to build rods that will go 20 passes (double the life of competitive rods).
Most Top Fuel and Blown Alcohol engines use some type of aluminum rod because aluminum rods are more forgiving than steel in this type of application. Aluminum rods act like a shock absorber between the pistons and crankshaft, says one rod supplier. By flexing slightly, the rod absorbs the shock of combustion and helps prevent the bearings from being hammered flat. The cushioning effect also reduces the shock loading on the crank, which reduces the risk of the crank snapping.
Aluminum rods can be made from forgings, flat billet stock or forged billet stock. A forged aluminum or steel rod develops grain flow that runs lengthwise down the rod for maximum strength. The forging process also wraps the grain structure around the rod cap, which improves strength up to 30 percent. Some rod suppliers say the forging process (aluminum or steel) creates a stronger rod than what you get if you CNC machine a rod out of a solid chunk of metal. Others argue that billet is always stronger because it has a smoother unidirectional grain structure, especially if the billet itself is a forging.
“Everybody wants lighter rods for faster acceleration,” said one rod supplier. “You can take weight out of a rod to reduce the reciprocating mass, but eventually you reach the point where you are sacrificing strength for weight reduction. So if you’re building a high horsepower big block motor, you want a significantly stronger rod, not necessarily a lighter rod.”
Many Pro Stock drag racers have long used aluminum rods because of the weight savings and cushioning effect they provide. Depending on the power level of the engine, a set of aluminum rods might last up to several hundred runs. Some guys will change their aluminum rods after 300 to 1,000 runs. Others may go several seasons before they swap out their old rods for a new set. In recent years, however, some Pro Stock drag racers and other drag racers have switched back to steel rods to reduce their maintenance costs.
Aluminum rods also require a little additional deck clearance because the rods expand more than steel rods when they get hot. Several aluminum rod manufacturers recommend a minimum piston-to-head clearance of .060˝. Some aluminum rods are actually made .010˝ shorter in length for this reason, so when the rod gets hot it will expand to the same length as a stock rod.
Side clearances with aluminum rods also need to be increased .002 to .005˝ compared to steel rods depending on the viscosity of oil used (10W-30 or thicker is recommended with at least 10 PSI of oil pressure for every 1,000 RPM). Wrist pins should be a little tighter with aluminum rods to compensate for thermal expansion, typically .0006 to .0008˝. Rod bearing clearances should be .001 to .002˝ looser than with steel rods because the rod cap parting lines tend to burnish into each other during initial operation, according to one rod manufacturer. In stroker motors, the minimum rod to camshaft clearance should be at least .060˝.
Aluminum rods are probably overkill for a street engine. Some say aluminum rods are ill-suited for the street because of shorter service life (maybe 15,000 to 20,000 miles). Others say aluminum rods made with superior alloys can be just as durable as steel rods on the street.
Lightweight titanium rods are another option – if you have deep pockets. Titanium rods cost several times as much as most aluminum or forged steel rods. Titanium rods are 20 to 30 percent lighter than steel rods of equal strength. Titanium rods typically require some type of surface treatment to prevent galling against each other where two rods share a common crank journal.
Rod Lengths & Rod Ratios
Rods come in various lengths, with 5.700 and 6.000 inches being the most common for SB Chevy, 6.385 inches for BB Chevy, and 6.125 inches for Chevy LS. If a motor has a stroker crank, the location of the wrist pin in the piston has to be raised to maintain the same piston deck height as before. Various rod lengths and piston heights can be used with a stroker crank, so the question becomes do you want to use a shorter rod or a longer rod? A longer rod is a heavier rod but a shorter piston is lighter, so one often offsets the other from a weight standpoint.
There are a lot of opinions about which rod lengths and rod ratios deliver the best performance in a given application. The short answer is that shorter rods tend to provide better low end torque and throttle response across a broader RPM range while longer rods typically produce more high RPM power within in a narrower RPM range. Because of this, longer rods are usually preferred for high revving endurance motors.
Basically, increasing rod length reduces rod angularity, piston rock and peak piston velocity. It also allows the piston to dwell longer at top dead center on the compression and exhaust strokes. This gives combustion pressure more time to build before it starts to shove the piston down. It also shifts the point where piston velocity is greatest a little later on the down stroke. Because of these differences, longer rods work well in high revving drag motors and circle track engines.
The downside of using longer rods is that the pistons dwell for a longer time at top dead center before reversing direction. This reduces the scavenging effect at low RPM when the intake and exhaust valve openings overlap. Shorter rods pull the piston away from TDC more quickly which improves scavenging during valve overlap. The result is better airflow and throttle response at low RPM, which is better for a street performance application.
The important point to remember here is that rod length affects airflow and the RPM range where peak power and torque are produced. Because of this relationship, rod lengths have to be matched to the cam grind (duration and overlap) and intake runner volumes in the cylinder heads to optimize performance.
One performance engine builder says longer rods do not require as much intake runner volume as shorter rods. Shorter rods pull the pistons down faster, which improves initial airflow at low valve lift. Shorter rods also allow the intake valve to be held open longer (more duration) with less reversion.
Rod ratio is a number that also creates some differences of opinion. Rod ratio is rod length to stroke. In a stock 350 Chevy with 5.700 inch rods and a 3.48 inch stroke crank, the ratio is 1.64. Changing to 6.000 inch rods increases the ratio to 1.72, which some say is better for high RPM power and faster acceleration.
For serious endurance racing, higher rod ratios are typically preferred. Many racing venues have rules that limit maximum engine displacement, bore and stroke. But within those limits, there is some degree of freedom to play around with different bore and stroke combinations and rod ratios. NASCAR engines cannot exceed 358 cubic inches, so most NASCAR teams run big bore, short stroke engines with longer rods and rod ratios in the 1.9 range. In Formula One racing, engine displacement is only 2.4 liters but the redline is twice has high as a Cup car (18,000 vs 9,000 RPM). Formula One engines run very short strokes (58 mm or 2.285 inches) with high rod ratios (2.2).
Another area where an engine builder can save some weight and pick up a few extra horsepower is at the bottom end of the rod. A crankshaft with smaller rod journal diameters weighs slightly less than one with larger rod journals. If the crank journals are smaller, the big end of the connecting rod can also be slightly smaller and lighter. This saves weight and also reduces friction in the rod journals. That’s why smaller diameter Honda rods with 1.88 inch main journals are often used in high revving SB Chevys. One rod supplier says switching to Honda rods is good for six additional horsepower.
A connecting rod is only as good as the rod bolts that hold it together. Stock rod bolts are okay for stock applications, but for performance engines stronger rod bolts are a must. The type of rod bolt that’s required for a performance application depends on engine speed and the reciprocating weight of the rods and pistons. Higher speeds and/or heavier components require stronger bolts. The tensile strength ratings of performance rod bolts range from 220,000 to 260,000 pounds or higher.
To create and maintain the proper clamp load, rod bolts have to be carefully torqued to achieve a certain amount of stretch. If you don’t apply enough torque, the bolt may not clamp the rod cap tightly enough allowing it to move or loosen up. If the rod bolts are tightened excessively or unevenly, it increases the risk of a bolt failing.
The most accurate method of tightening rod bolts is to use a gauge to measure bolt stretch after the bolts have been torqued down using an accurately calibrated torque wrench. The amount of stretch that is optimal will depend on the size and length of the bolt as well as the alloy from which it is made. Aluminum rods also require different clamping loads than forged steel rods. Rod bolt suppliers have charts that list the recommended stretch for most applications, which can range from .004 to .008˝.
It’s important to note that the type of lubricant used on the rod bolt threads has a big effect on the actual load that is applied by the bolt when it is tightened. Reducing friction with some type of lubricant increases the load on the bolt, as does loosening and retightening the bolts. Motor oil and various greases all have different friction characteristics and are not interchangeable. Most vehicle manufacturers recommend using engine oil as a lubricant for the rod bolts. Aftermarket rod and bolt suppliers recommend a variety of different lubricants including different oil viscosities (10W-30, straight 30 or 50) to moly grease to specialty thread lubricants designed for rod bolts. The specialty thread lube is often preferred because it gives much more consistent clamp loads than motor oil or ordinary moly grease.
The recommended procedure for achieving the optimum clamp load on the rod bolts is to lubricate the underside of the bolt head and threads with the specified lubricant, measure the length of the rod bolts, tighten the bolts to the specified torque, measure bolt stretch with a gauge to make sure it is within the recommended range, check the rod bearing clearances to make sure they are within specifications, then disassemble the rods and repeat.
Special thanks to the following companies for contributions to this article: ARP Fasteners (www.arp-bolts.com); Bill Miller Engineering (www.bmeltd.com); Callies Performance (www.callies.com); CP-Carrillo (www.cp-carrillo.com); Crower Cams (www.crower.com); Eagle Specialty Products (www.eaglerod.com); GRP Connecting Rods (www.grpconrods.com); Lunati (www.lunatipower.com); Manley Performance (www.manleyperformance.com); Oliver Racing Parts (www.oliverconnectingrods.com); R&R Racing Products (www.rrconnectingrods.com); and Scat Crankshafts (www.scatcranks.com).
Courtesy Engine Builder.