Cast vs. Forged vs. Billet
Manufacturing techniques play a substantial role in the ultimate strength of a crankshaft. Casting and forging are the two most common manufacturing methods, and each has benefits and drawbacks. Cast cranks start life as liquid iron or steel, and are poured into a mold. This allows the raw casting to closely resemble its final shape, which reduces the amount of final machining. Combined with the fact that the equipment necessary to produce castings is relatively inexpensive, it's obvious why cast cranks are the predominant choice of the OEs. Aftermarket cast cranks offer significant improvements in strength, and can be had for as little as $200.
Weighing in at a scant 66... read full caption
Weighing in at a scant 66 lbs, Eagle's lightweight 4340 forged cranks for big-block Chevys are rated at 1,500 hp. They're available in 4.000- and 4.250-inch strokes, and can be had with smaller 2.100-inch rod journals for those looking to minimize friction.In contrast, the forging process requires heavy duty presses and more extensive final machining operations. Forging involves heating up a cylindrical slug of metal to a molten state, then pounding it into shape with presses and dies. It is this compressing action that creates an inherently stronger end product over a casting. "In a casting, the grain structure looks like beach sand," explains Tom Lieb of Scat. "In a forging, the force of the press compresses the grain together so it becomes one uniform grain flow. As the space between the molecules is compressed, each molecule is forced to 'hold hands' with the next molecule." Compared to a cast crank, the drawback of a forging is cost. The heavy duty hydraulic presses used in the forging process are extremely expensive, which leads to a costlier product. Expect prices to start at $500 for more popular engine makes.
Think of billet cranks as an offshoot of forged cranks. Like a forging, a billet crank starts out as a large cylindrical ingot of steel. However, while a forged crank is compressed during the forging process, the steel ingot used in a billet crank is already forged, albeit not quite as compressed as in a forged crank. The key difference between the two is how the ingots are shaped into cranks. "The metal bar used to make a forged 4.000-inch SBC crank measures about 4.75-inches in diameter and the crank's total width ends up being 6.75 inches when the forging process is complete," Lieb says. "The metal bar used in a billet crank of the same stroke is much larger, at roughly 8 inches, weighing 350 lbs compared to 150 lbs in a forged crank. Instead of twisting and pounding the metal in different directions as with a forging, a billet crank is made by whittling away the metal so the grain structure runs parallel throughout the entire length of the crank." Due to the increase in materials and labor over a forged crank, billet cranks are the most expensive of them all. Custom one-offs carry price tags in the neighborhood of $3,000. As for whether a billet crank is stronger than a forging, because there is no consensus in the industry, the various manufacturers will duke it out later in the story and we'll let you make the call.
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Originally Posted by BenFenner 
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