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."