All of a sudden, the ubiquitous 383 is the wee-man on the block. With 400-plus cubic-inch small-blocks becoming more the rule than the exception, yesteryear's stroker combos have developed a bad case of Napoleon syndrome. The 540ci Rat is the new 496, and if you want to build a 632, tracking down the short-block components necessary to fulfill your displacement fantasies-and finding enough cylinder head to feed it-is only a mail order catalog away. What makes it all possible is the massive influx of affordable stroker crankshafts. Gone are the days of offset grinding factory cranks for miniscule displacement gains, and scouring salvage yards for that elusive 400 crank for your small-block build. Today's aftermarket cast-steel cranks can take a serious beating, and modern 4340 alloys can handle as much abuse as you can realistically throw at them.
Even so, there are some very basic questions that need to be addressed when shopping for a new crankshaft. Everyone knows that forgings are stronger than castings, but how much stronger are they, and how much power can they take? Where do billet cranks fall into the hierarchy? What differentiates the various grades of alloys? What are the different heat treating procedures, and how effective are they? How advantageous are lightweight cranks? To find some answers, we contacted the foremost authorities in the crankshaft business including Alan Davis of Eagle, Tom Lieb of Scat, Dwayne Boes of Callies, Tim Langley of Lunati, and Shawn Mendenhall of Coast High Performance. Here's what they had to say.
Cast vs. Forged vs. Billet
Alan Davis: "Understanding the hierarchy of crankshafts can get confusing due to nomenclature. Take a 4340 forged steel crank, for instance. The '4340' refers to the type of metal alloy a crank is made from, and the 'forged' refers to how that steel is manufactured. You could make a cast 4340 crank, but that would make no sense because you'd be putting a very expensive metal alloy through a very basic manufacturing process that doesn't yield the strongest final product. As its name implies, a cast crank starts out as liquid iron or steel, and is poured into a mold that closely resembles the final shape of a crank. The benefit of the casting procedure is that it reduces the finishing machine work required to get the crank from a raw state to a finished state, which cuts down on costs. Likewise, the equipment used to make a cast crank is relatively inexpensive. This explains why most production engines use cast cranks, and why quality aftermarket cast-steel units can be had for as little as $170.
"On the other hand, manufacturing a forged crank is a much more involved process. With a forging, you start with a large ingot of steel alloy, and then pound it into shape with 200-ton press and dies. The heavy-duty presses needed to forge a crank cost at least $100,000, so you have to commit to building a ton of cranks before you can even recoup your investment in equipment. Additionally, the forging process isn't as precise as the casting process as far as the shape of the crank is concerned, so it requires more extensive machine work. That's why forged cranks cost three to four times more than cast cranks, but the trade-off is a dramatic increase in strength. A typical small-block cast crank is good up to 500 hp, while a 4340 forging can take 1,500 hp.
"Billet cranks are similar to forged cranks in that they also start out as a big ingot of steel. The difference is that the billet blanks are already forged, and they're machined into shape instead of being pounded into a die by a hydraulic press. However, the benefit of billet cranks is that they're highly adaptable to short production runs since you don't have to invest in expensive presses and dies. It might cost $3,000, but you can order up a billet crank with any length stroke and any journal diameter you want. You don't have that flexibility with a forged crank. Even if you manufactured a batch of 1,000 cranks, you still wouldn't be able to recover the costs of the dies and presses needed to make it."
Tom Lieb: "Cast crankshafts are manufactured using the most basic production methods. Molten metal is poured into a mold, then machined into the final shape of the crankshaft. This method is the most inexpensive and also the least durable. Forged cranks start out as a round bar of metal, are heated up, and then pressed into shape with hydraulic presses and dies. In a forging, this compressive force squeezes the molecules together and creates one uniform grain flow. On the other hand, the grain structure in a cast crank looks like beach sand. That's why forged cranks are significantly stronger than cast cranks.
"Billet cranks are the strong ones, which is why they're used extensively in NASCAR Sprint Cup and Top Fuel. Visually, billet cranks are indistinguishable from forged cranks, so the difference between them is in the grain's structure and metal. If you're making a forged 4.000-inch-stroke small-block Chevy crank, you start with a round bar of metal that's 4.75 inches in diameter. After the forging process, the total width of the crank ends up being 6.75 inches. That means grain that was flowing parallel to the crank has been forced to negotiate a series of 90-degree turns to make the rod journals and the mains. What was once the centerline of the crank has now been offset by stretching, sharing, and weakening the grain. On the other hand, a billet crank has uniform grain structure than runs parallel throughout the length of the crank. Instead of being pressed into shape, a billet crank is formed from a round bar of highly refined forged steel that's much larger in diameter. Compared to a forged crank, the metal blanks used in a billet crank weigh more than twice as much. To make a 4.000-inch-stroke crank, you'd start with an 8-inch diameter piece of steel, then whittle it down into the shape of a crank. This method eliminates all stress risers, yielding a stronger end product than a forged crank."
Alan Davis: "The most basic material used to manufacture cranks is iron, and numbers such as '4340' refer to what's mixed in with that iron to make steel alloys. Steel is 95 percent iron, and the difference in the various types of alloys is in the remaining five percent. The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have established a set of standards that determines the content of the different grades of metal. As you add carbon to a basic iron crank, you end up with cast steel. Add more carbon, and then you start getting into the different alloys like 1013. The materials added to iron to make alloys isn't expensive, but the labor to mix all the ingredients together is what drives up costs. Generally, 4340 is considered the best alloy to make cranks and rods out of. Next on the list is 4130, followed by 5140. Factory forged cranks were made from alloys like 1013 or 1053. While they're much stronger than iron, they're not nearly as strong as 4340."
Shawn Mendenhall: "It's hard to generalize how much power a crank can handle, because there are other variables besides whether a crank is cast or forged. Bigger main and rod journals will obviously take more abuse in a cast crank. For small-blocks, a safe limit for a cast crank is 500 hp and a maximum of 6,500 rpm. With big-blocks, their bigger mains will handle 650 hp. We've seen cast cranks in turbo motors take over 2,000 hp, but we don't recommend it. These days, 4340 is the standard for forged cranks. Just a few years ago, 5140 and 4130 cranks were sold alongside 4340 cranks. However, they aren't much cheaper, so it makes no sense to use them over a 4340 forging. For 99 percent of people, a 4340 crank is more than they will ever need. If you can break one somehow, then something else was probably wrong with the motor. Once you get into very high rpm and tons of power, then billet may be the best option."
Alan Davis: "Nitriding is the most common method of heat treating a crankshaft. It increases the surface hardness of the journals, and improves wear resistance. Although the process does strengthen the crank a little bit, the real benefit is the improvement in impact and wear resistance, which reduces the potential for cracking. That's very important, because impact and wear are the most common causes of crank failure. Nitriding involves putting a crank in a furnace, and depositing ionized nitrogen onto the surface. The gas penetrates 0.012-0.013 inch into the surface, doubling the surface harness and increasing fatigue life by 25 percent. You can also grind a crank 0.010/0.010 during a rebuild without having to heat treat the crank again."
Dwayne Boes: "Nitriding is a chemical process where nitrogen is absorbed by the surface of crank at high temperature, which hardens the crank. It also improves fatigue strength. By heat treating the bearing surfaces of the crank, you create an extra layer of protection without affecting the rest of the crank. The older method of heat treating is induction hardening, in which the journals are heated up and plunged into water. Induction hardening can be done with cheaper equipment, but if the rate of cooling isn't carefully controlled, it can create stress risers. That's why nitriding is much more common in aftermarket crank manufacturing."
Alan Davis: "A lightweight crankshaft has the same effect as a lightweight flywheel. On a street car, you may notice slightly better acceleration but they're not really for street cars or drag cars for that matter. In circle track and road race applications, when the motor is moving up and down the powerband over and over again and you want as much acceleration as possible when coming out of a corner, a lightweight crank makes more sense. However, in drag applications, a lightweight crank won't determine the winner of the race. You're better off spending that extra money on better cylinder heads. That said, how a crank is lightened is just as important as how much it weighs. Gun-drilling involves drilling lightening holes right down the middle of the crank, but it doesn't affect performance much since it removes weight on the centerline of the crank. If you drill the rod throws, you have to also take material off of the counterweights. This results in a reduction in rotating weight, which can provide slight increases in acceleration."
Tim Langley: "A lot of manufacturers market their cranks as being non-twist forgings. What that refers to is how the crank is forged in the die. With a twist forging, after one crank throw is forged, the crank is twisted before the next throw is forged. This lets you get away with using a simpler die. In a non-twist crank, all four throws are forged at the same time. This requires a much more complex die. The non-twist process reduces internal stresses during the forging process, but it also requires running the procedure at a higher temperature, which can increase grain growth and reduce strength. However, if done properly, then there isn't any difference in strength between one and the other."
Grinding and Polishing
Alan Davis: "The surface finish on a crank's main and rod journals is very important. It should be smooth, and polished to a mirror finish to reduce friction. The last thing you want are big variations from the high points to the low points. These peaks and valleys will create stress risers, and that's where cracks will form. However, there's a point of diminishing returns, since you can spend an extra five hours finishing a crank and it won't strengthen it at all. When machining a crank, all that's needed is some grinding, shot-peening, and finishing and polishing of the bearing surfaces."
Dwayne Boes: "Knife-edging the counterweights doesn't really make much of a difference in a street motor. In fact, a knife edge isn't even the most efficient design for a leading edge anyways. You want oil to land on the nose of counterweight and flow off smoothly to the sides. With a knife edge, oil gets thrown around all over the place. In truth, knife-edging was developed more for ease of balancing than for horsepower. A bullnose-rounded leading edge is actually the most efficient. Just like the bow of a ship moves through the water, it allows the counterweights to cut through the oil."
Alan Davis: "How you drill oiling holes into a crank is the subject of lots of controversy. Factory cranks feature standard oiling, which means that the holes are drilled straight across the main and rod journals. The other method is cross-drilling, and there are two different ways of doing it. The first method is to drill across from the main journal to the rod throw, making a '7-shaped' passage where the hole intersects the crankshaft centerline. This works well at low rpm, but not at high rpm, as centrifugal force pushes oil away from the rod throws. The second, more effective method of cross-drilling is to drill a hole from the rod journal and stopping at the main journal. This T-shaped passage provides more consistent oil flow to the rods at high rpm. Cross-drilling isn't necessarily bad, but it depends on how you do it. Today at Eagle, we do standard, straight-shot oiling. Cross-drilled cranks have gotten such a bad rep, that we don't offer it anymore, but it does have benefits if done the right way."
Hardness vs. Ductility
Dwayne Boes: "Designing a durable crank is an exercise in striking a balance between hardness and ductility. Increased hardness can lead to a stronger crank, but it still has to have some give in it so it can bend without cracking, which is referred to as ductility. A good way to explain ductility is to compare glass to rubber. Glass is hard, very hard, but it cracks easily, so it's not ductile. Rubber bends easily so it is very ductile, but not hard. Like a fishing pole, you want the crank to give a bit under load, but snap back into shape without being permanently deformed. Cranks do in fact flex under load, and in a motor with an aluminum block, they can bend as much as 0.200-inch. Where premium forged cranks shine is in their ability to be extremely hard while still maintaining ductility. The ideal crank is one that can be very hard and maintain its shape to spread bearing loads evenly throughout the crank while still having enough ductility to prevent cracking. Generally, as a crank's hardness increases, so does its tensile strength. Having higher carbon content in steel increases hardness, but sacrifices ductility in the process. That's why you don't want too much carbon content in a crank. Nodular iron is the least ductile material used to build cranks, as you go up the scale, you can increase hardness without sacrificing ductility."
Alan Davis: "When balancing a rotating assembly for a street motor, the goal is to equalize the rotating mass and the reciprocating mass. However, in race motors it's not uncommon to overbalance the crank. A balancer generally spins a crank 500-750 rpm, and for obvious safety reason, you can't replicate the actual rpm the crank will experience in a running engine. However, if you spin a motor at very high rpm, say 7,000-8,000, parts can stretch and move around. Aluminum rods might stretch as much as 0.030-inch. This stretch increases load on the crank and bends it, making the pistons and rods behave as if they're heavier than they really are due to dynamic inertial effects. To the crank, the pistons feel heavier. So if you have a rotating assembly that calls for a bobweight of 1,800 grams, a motor may run more smoothly if you overbalance the crank by two percent. In other ways, you'd compensate for the inertial loads the crank endures at high rpm by balancing the rotating assembly to a bobweight of 1,836 grams instead of 1,800 grams. The balancer will indicate that the crank isn't balanced, but the bearings will actually look better when you tear the motor down. On a 6,500 rpm street motor there's no need to overbalance. It's more for race engines than run 7,000 to 8,000 rpm all day."
Dwayne Boes: "Crank overlap is something you should always pay attention to. If you were to stand a crank up vertically, overlap can easily be seen as the portions of the main and rod journals that overlap each other. The more overlap you have, the stronger the crank. To keep production costs down back in the '60s, the OEs used cheap casting materials in their cranks, but compensated for it by making the main and rod journals very large, which increased overlap. With stroker cranks, however, moving the rod throws farther away from the mains inherently weakens the crank by reducing overlap. Likewise, high-end race motors often reduce the rod journal diameters of a crank to reduce bearing speed and friction. This also reduces crank overlap as well. Fortunately, with today's quality aftermarket forgings overlap isn't as big of an issue as it used to be."