Advanced metallurgy, computerized machining, and computerized quality control give connecting rods that are stronger than ever, allowing consistently reliable power levels previously thought impossible. But that strength comes with a weight penalty, which, for certain applications, can mean a slower-revving engine.
Enter lightweight rods. “Because lightweight rods are a compromise of weight and strength, they’re not the best choice for every application,” says Shane Turner, sales and marketing manager for Oliver Racing Parts. Oliver is familiar with lightweight rods as it offers Standard Light and Ultra Light rods for small-block Chevy and Ford, Modular Ford, and Sport Compact (Mitsubishi) applications, and Standard Light for LS Chevys.
If you have a lightweight rod and a lightweight piston, the crank can spin that easier. It’s not having to lug that up and down, and the engine is going to accelerate a lot quicker. – Shane Turner, Oliver Racing Parts
Lighter Rods Equal Easier Revs
Applications using power-adders such as turbos, superchargers, or nitrous oxide will call for a beefier rod, such as Oliver’s Speedway series for small-blocks, or Max or Max-Plus for big-blocks. But, naturally-aspirated engines, which have lower cylinder pressure, don’t demand a rod with as much ultimate strength. Since a lighter reciprocating assembly means the engine can gain RPMs faster, many opt to trade a little absolute strength for a lighter component weight in an N/A application.
As an example, for the Gen-1 small-block Chevy, Oliver’s Standard Light Series rods are designed to be used in naturally-aspirated high-horsepower, high-RPM engines, such as in late-model stock cars, 410-cubic-inch sprint cars, and late-model dirt cars. The Ultra Light Series is typically used in moderate-horsepower applications where 8,200 rpm is common. Those typical applications include 360 Sprint cars and pavement Late Models.
“If you have a lightweight rod and a lightweight piston, the crank can spin that easier. It’s not having to lug that up and down, and the engine is going to accelerate a lot quicker,” explains Turner.
Three Weights, One Forging
Whether it’s an Ultra Light, Standard Light, or Speedway Series rod, each one (within its family, such as a small-block Chevy) starts as the same U.S.-made forged blank, made of 4340 AQ (aircraft quality) steel that meets AMS2301 specs. The rod is heat-treated twice during its multiple machining processes at Oliver’s Charlevoix, Michigan, headquarters. They are stress-relieved after rough-machining, and quenched and tempered for a martensitic grain structure for optimized durability and strength.
It’s the final CNC-machining processes that determine the rod’s weight, with the difference being roughly 50 grams between each small-block Chevy series: the Ultra Light (610 grams for a 6.0-inch-long rod), Standard Light (665 grams for a 6.0-inch-long rod), and Speedway (705 grams for a 6.0-inch-long rod).
Lightweight is a relative concept, and OEM rod weights can vary application-to-application. So, sometimes a Standard Light or even an Ultra Light series rod can tip the scales a little heavier than a factory rod, but with much greater strength.
The Next Level of Lightweight
As he’s called upon customers, Turner has found some builders even take weight-saving measures to the next level, shaving off even more material. “Rod-to-stroke ratio is just one of many considerations engine builders take when deciding to remove weight,” Turner notes. However, he cautions that applications with poor rod-to-stroke ratios suffer from additional stress, both on the rod and the major thrust side of the piston. Those applications would not be a good candidate for such creative carving.
“These builders are really confident in what they’re doing, and they know they have a really good rod-to-stroke ratio, which basically is how comfortable that rod is going up and down,” explains Turner. “Is it going up and down nicely? Or when it comes down to the bottom of the bore, is the big-end of the rod kicked over so far that when it comes back up, it almost wants to push the piston out the side of the block?”
Engine builders pushing the limit perform careful calculations to know where they can save a few grams on the piston, too. “Most of the time, if they’re using an ultra-lightweight rod, they’ve got a very light wrist-pin, they’ve got a light piston, and they’re typically doing a really thin ring pack, something close to 1.0, 1.0, 1.2-mm — they’re doing everything really, really lightweight.”
Beefing Up the Big End
Rods typically fail not in the beam, but in the big end, so some builders spec one of several available high-strength bolts. These include the industry-standard ARP 2000, an L19 bolt, or the king of the steels, the CA625 bolt. The “CA” stands for “Custom-Age,” the strongest alloy of steel in Oliver’s arsenal.
“If you do the lightweight rod, now you have a faster travel speed,” Turner continues. “But with the stronger bolt, the risk of tearing that thing apart at extremely-high RPM is greatly reduced.”
Different applications demand their own unique solutions to their challenges. A naturally-aspirated drag car using a delay box has a wide RPM range from launch to finish line. A turbocharged drag car has to contend with high cylinder-pressure from start to finish. A boat may run at 5,000 rpm all day long, but may also occasionally hop out of the water and spike the RPM. All these engines operate under different conditions and have different rod-bolt needs.
There are many bolt options available. In fact, calls to the tech line by professional engine builders are often about which bolt to use for a particular application and bolt torque specifications. “We ask the builder, ‘How much overall power are you making? How long is the rod, and how heavy is that piston? What’s the block deck-height? What is the rod-to-stroke ratio? How many RPM will it run?’” explains Turner. “We go through a whole list of scenarios to come up with what they need for their application.”
Picking the Right Weight
Professional engine builders rarely call the tech line to pick Oliver experts’ brains about rod length and weight selection, as they’ve done their homework for that, Turner revealed. Instead, they generally want to know what the power level recommendation is for a particular rod series.
“Even when we answer that question, we are fairly conservative, because we don’t know who’s going to drive that engine, who’s going to tune that engine, what fuel they are going to use, and how they are going to take care of it,” explains Turner of the complex question.
For less-experienced builders who ask Oliver for recommendations, Oliver’s tech staff will often suggest a rod that is a step above the expected horsepower of the application to give a little added insurance. “We don’t want to give them the bare minimum, because we don’t know how they’re going to tune it, or if they’re going to run into a problem they can’t hear or something like that,” says Turner.
Combining Strength and Light Weight
Turner pointed to Oliver’s design exclusives as enhancing durability for a greater power-to-weight ratio, such as the Parabolic Beam I-beam design that reduces beam stress. By relying on design geometry for strength instead of material volume, a lighter-weight product with the same strength is achieved.
At the parting line between the rod and the cap, Oliver flanges that area to give it a greater mating-surface area. “With a little more thickness under the bolt head on the cap, the connecting rod is more tolerant of assembly errors, such as an incorrect torque process or failing to use the recommended assembly lube under the head of the bolt, which causes the cap to start to gall,” Turner explains.
Quality Control Ensures Consistency
Whether it’s 85 degrees outside or 5-below, Oliver’s northern Michigan facility is temperature-controlled for consistency. “If step one of production started at 85 degrees, and by the time you got to step seven, it was then 75 degrees, the product itself has shrunk,” Turner says. “In order to make our cutting or measuring exact all the way down the line, step-one has to measure the same as step 15. Otherwise, it’s not going to come in true.”
Oliver’s rods are quality-checked throughout each of the 15 machining processes using equipment such as the Zeiss Coordinate Measuring Machine and the Mahr-Federal Dimensionair air gauge to ensure accurate, high-volume manufacturing processes.
“If we get to step seven and something’s not right, it’s ‘Sorry, start over,’” says Turner of the company’s strong commitment to quality parts.
Making the Decision
While making the decision on which rods to use is a complicated one, Oliver is using technology to assist its customers — whether they are an advanced engine builder or a curious beginner. As of this writing, Oliver’s website is set to be overhauled with some awesome new features.
Rods are now being laser-etched with QR codes, which will eventually be searchable on the new website. Customers and end-users will not only be able to verify the product they are using but it will eliminate cases of “mystery rod” when someone buys second-hand components. Robust technical information on the site should answer many questions currently fielded by tech line staff, who will still be available to help an engine builder’s unique needs, including manufacturing custom rods.
Oliver isn’t just an off-the-shelf application company, and many of its customers specify a custom-length for their builds to accommodate their desired piston compression height or deck clearance, as evidenced by the custom order form on Oliver’s website. And, if a customer has a question about which Standard Light or Ultra Light rod series is the best fit for their application, Oliver experts will be available to assist.