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.