Ah, memories. It seems like only yesterday when offset grinding a crank for a miniscule bump in cubic inches was the only game in town. Anyone lucky enough to score a 400 small-block crank at a swap meet stashed it away in a safety deposit box. My oh my how times have changed, because unless your Mouse packs 427 cubes these days, forget about getting any respect on cruise night. Even a 540 is merely a modestly sized big-block by today’s standards. You can thank the influx of affordable aftermarket stroker crankshafts, and the market is changing rapidly. While the first wave of affordable cranks inundated the market with tons of stroke lengths for a variety of applications, the focus has shifted since then. Now every new generation of cranks brings improvements in finish machining, heat-treating, mass reduction, oiling, and ease of balancing. To get educated on the latest developments in crank and rod technology, we hit up Shane Pochon of Lunati. The company has been churning out premium quality rotating assemblies for decades, and has recently revamped its product line. Additionally, we asked Shane to delve into some more general crank and rod basics, covering everything from metallurgy to manufacturing techniques to harmonics.
In recent months, Lunati has completely revamped its lineup of crankshafts. Our entry-level Voodoo crank offers high-quality features at a budget-friendly price. These non-twist 4340 steel forgings are nitrided for extra durability, and have micro-polished bearing surfaces for reduced friction and wear. Although they’re priced for budget-conscious engine builders, our Voodoo cranks boast high-end features like drilled crankpin arms to reduce rotating weight. These cranks will handle up to 1,000 hp, making them perfect for street and mild race applications. Lunati’s Signature Series cranks offer many of the same basic features as our Voodoo cranks, but with more bells and whistles. While they’re also forged from 4340 steel, the Signature Series crankshafts are manufactured from the highest-quality aircraft-grade material in terms of cleanliness and purity. These American-made cranks feature gun-drilled main journals, profiled counterweights, and a deep-drilled crank snout to better support balancers and blower pulleys. While that doesn’t make much of a difference in power output, Lunati’s Signature Series crankshafts have proven reliable in engine applications exceeding 1,500 hp.
Cast vs. Forged vs. Billet
Crankshafts can be of cast, forged, and billet construction. A cast crankshaft is the most basic and inexpensive. When making a cast crank, metal is poured into a mold, which results in a casting that very closely resembles the shape of a finished crank. This reduces the labor required to machine the crank into its final shape, in turn reducing costs. Cast cranks often use the cheapest raw material as well. The drawback of this approach is that with a casting, the grain structure of the metal runs all over the place. In contrast, forged cranks start with higher quality material, and the forging process involves compressing the metal into shape in a die. By compressing the metal, the grain structure gets rearranged and strengthens the crank. Just like wood, the metal used to manufacture a crankshaft has a distinct grain structure. With a forging, the grain structure is aligned parallel like wood, which helps hold the crankshaft together.
If you need a custom crank built, it will almost always be a billet crank. The reason for this is very simple. Each of the forging dies needed to build a forged crank cost hundreds of thousands of dollars. Obviously, it’s not cost effective to build one die for a small production run of custom cranks. A much more practical solution is machining a custom crank to any specification you want, starting with a slug of steel. Billet cranks are actually forgings. To make one, you start with a cylindrical slug of forged steel, then slowly whittle it down into shape. There is some debate as to what type of crank is the strongest. Depending on who you talk to, some people say that a forged crank is stronger than a billet crank because its grain structure is more compressed.
All Lunati’s crankshafts and connecting rods are forged from 4340 steel, which is widely regarded as the most durable material for these types of components. Other common grades of steel used in cranks and rods are 4130, 5140, 1045, and 1053. What these numbers represent are the levels of carbon, nickel, and chrome added to the base raw iron to create a certain grade of steel alloy. The numbers are like a recipe—but for steel—and the stronger grades of steel have a higher tensile strength. Materials like 1045 and 1053 are typically used in factory forged cranks. Premium aftermarket cranks are forged from 4340 steel because it’s the strongest, with a tensile strength of roughly 150,000 psi.
Twist vs. Non-Twist
There are two basic ways of pressing a crank into shape on a forging die. A twist forging refers to the process where each crank throw is forged one at a time. After one throw gets forged, the crank gets twisted, and then the next throw gets forged. In contrast, all four crank throws are forged at the same time in a non-twist forging. This requires a much more complex die, but the benefit is a straighter parting line on the throws. This is a good thing, because the parting line is a potential weak spot. Consequently, all Lunati crankshafts are non-twist forgings.
Crankshafts from back in the ’60s and ’70s didn’t receive any kind of heat-treating, but it’s very common today since it increases strength and durability. Every grade of metal requires a different heat-treating method. The process involves heating a crank up to a very high temperature, letting it cool back down, then heating it back up. The entire procedure is computer controlled, and the heat is cycled up and down for 30 hours. Nitriding is the most common method of heat-treating used in aftermarket crankshafts, and all Lunati cranks are nitrided. By depositing nitrogen onto the surface of a crank, nitriding hardens the surface of a crank to increase wear and impact resistance. An alternate method of heat-treating is induction hardening, which is often used by the OEMs. During induction hardening, the crank is heated and then dipped into water. This results in a harder metal surface, but it also makes the metal more brittle. Nitrided crankshafts have more give, which makes the metal less brittle and increases durability. Likewise, while induction hardening only treats the journals, nitriding can treat the entire crank.
Cranks often have lightening holes drilled into them to reduce mass, but whether or not this technique is effective depends on how the lightening is performed. Gun drilling the main journals does reduce mass, but only from the centerline area of the crank. Since the rotating weight of the crank remains the same, there are no horsepower benefits to this type of drilling. On the other hand, drilling lightening holes in the rod journals does indeed remove rotating weight, so it does offer some potential horsepower benefits. A common misconception is that since removing rotating weight is beneficial, cutting down the counterweights can also be beneficial. The truth is that cutting holes into the counterweights is a hack job and can wreak havoc on the balancing process. Since the counterweights offset the mass of the pistons and rods, any weight that’s removed from the counterweights will usually have to get added back in with slugs of Mallory. If you can get away with trimming down the mass of the counterweights, it’s much better to grind them down on a big lathe instead of drilling holes into them. Every hole that’s drilled into the counterweights increases drag as they move through the oil. That’s why very high-end racing cranks have no holes drilled into the counterweights at all.
During the manufacturing process of a crankshaft, it must be transformed from a raw forging into a precise piece of hardware that has very tight tolerances. Although forgings are far stronger than castings, a drawback of the forging process is that it requires a lot of finish machining. Many people would be shocked if they saw what a raw crank forging looked like, because it has lots of extra material on it and doesn’t really resemble a finished crank at all. As such, Lunati has many quality control procedures in place to ensure that tolerances in critical areas, like the journals and bearing surfaces, are kept as precise as possible. The biggest key to precise machining is establishing the centers since everything is based off the crankshaft centerline. Once the centers are established, the counterweights are machined and brought to a rough grind. Most of the process is performed on CNC machines, but the finishing grinding of the main and rod journals are still done manually by hand. Some might ask why you would manually grind the crank journals. The answer is we’ve had cranks that needed two tenths of a thousandth of an inch removed off a crank for clearance, and our guys can do it. To give you an idea of how much labor is involved in building one crank, it takes 30 hours from start to finish. If one person had to walk a crank through the manufacturing process himself it would take four days.
Many hot rodders buy a quality rotating assembly then go cheap on a balancer. However, keeping crankshaft harmonics under control is critical to engine longevity. The combustion process sends vibrations and pulses through the crankshaft on every power stroke. At certain rpm, these frequencies can resonate and the resulting harmonics can rattle an engine apart. Anything you can do to reduce harmonics is beneficial. The most obvious solution is using a high-quality harmonic damper. For high-rpm race motors where regulating harmonics are more important, engine builders can change the firing order around. While this does not eliminate harmonics, it spreads them out more evenly. Furthermore, you can also control harmonics by balancing a crank differently. If you know the rpm range of the motor, you can over- or under-balance a crank. This involves adding or removing weight from the bob weight during balancing to simulate the bending loads placed on a crankshaft.
Minimum Rod Length
When piecing a rotating assembly together out of a catalog or off the Internet, there are some important pitfalls to avoid. Crankshaft catalogs are often published with a minimum connecting rod length. Some hot rodders don’t see how a crankshaft would affect rod selection, so they ignore this specification only to pay the price later. However, this is a very important specification because it tells you how long a rod must be to clear the piston. For instance, if you have a 3.750-inch crank listed with a 6-inch minimum rod length, that means the crankshaft counterweights will clear the pistons with rods that are 6 inches or longer. If you use the same crank but with 5.700-inch rods, the crank will still work but the counterweights will hit the pistons.
If you have access to a large lathe, another option is cutting down the counterweights for additional clearance. If you have a crank with a 6-inch minimum rod length, but want to use a 5.850-inch rod, cutting the counterweights down 0.15 inch to make it work isn’t a problem. Lunati offers these services in-house if a customer lets us know what they want when placing their order. Keep in mind that the purpose of the counterweights is to counter the mass of the rods and pistons, so the more metal you remove from them, the more Mallory you may have to add back to crank to get the rotating assembly balanced. However, if you have super-lightweight rods and pistons, then it might not be necessary to use heavy metal in the crank.
Supercharged motors place stress on some very specific areas of a crankshaft that must be addressed for maximum durability. In high-horsepower motors, a supercharger can require several hundreds of horsepower to spin, which puts into perspective how much a blower can stress a crank. Lunati’s Pro Blower Series crankshafts for small- and big-block Chevys are designed specifically for supercharged motors. Since driving the blower with a belt puts lots of strain on the crank snout, the Pro Blower Series small-block Chevy cranks incorporate a larger big-block Chevy snout. All Pro Blower Series cranks also feature dual keyways and larger 3/4-inch threads for the crank bolt to ensure that the blower drive pulley doesn’t spin on the snout. Another benefit of the longer threads is that they allow the bolt to distribute clamping pressure to the front main cap. Furthermore, while Number 2, 3, and 4 rod journals still get gun-drilled like on all Lunati cranks, the Number 1 rod pin is not drilled. This strengthens the front end of the crank to deal with stress imparted by the blower. Lastly, to better endure the massive power capable with a blower motor, Lunati’s blower cranks have larger 1/2-inch flywheel bolts on the rear flange.
Lunati offers three different lines of connecting rods. The Voodoo H-beam rods are our entry line and offer a great balance of strength and affordability. They’re forged from 4340 aircraft-grade steel, heat-treated, stress-relieved, and shot-peened. For the typical naturally aspirated, bracket race or street/strip combination, the Voodoo rods are a great choice. For power applications, Lunati recommend our Racer Series I-beam rods. They offer many of the same features as our Voodoo rods, but in a unique I-beam design that can easily handle well over 1,000 hp. The next step up is our Signature Series I-beam rods. They’re similar to the Racer Series rods, but come fully deburred and feature back-cut and radiused boltholes. Since the rods bolts are among the most highly stressed parts in an engine, Lunati includes premium ARP 2000 rod bolts with all of our connecting rods.
Some people say that H-beam rods are stronger than I-beams, while others say the exact opposite. Lunati manufactures both types of rods, so we’re very familiar with the pros and cons of each design. Generally, I-beams are lighter and stronger in compression, while H-beams are heavier and stronger when stretched. That said it’s not accurate to generalize that one design is stronger than the other. It really boils down to the overall design of the rod, and strength is independent of whether a rod is an I-beam or an H-beam in shape. A rod’s strength and durability comes from the material used, overall design, and the quality of the CNC program used to machine it.
Some people don’t mind piecing together a rotating assembly, but Lunati offers convenient rotating assembly packages for both small-block Chevy, big-block Chevy, and LS applications. The kits are compromised of either Voodoo or Signature Series components, and include a crankshaft, connecting rods, pistons, rings, and bearings. A cam, timing set, and lifters can be added as optional items. Although we try to offer a diverse range of displacement options, piston compression heights, and compression ratio, we can also mix and match components to customize a package to a customer’s needs. Just call us and tell us what you need.