Crankshaft, Cast Iron, Forged steel or Billet steel?

A crankshaft makes it possible to transform the reciprocating motion of the pistons into a rotating motion.
The Starting Grid prefers following well-known aftermarket and performance brands:
Arrow precision
K1-technologies
Molnar technologies
And more especially for application-specific one-offs.

Billet steel
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Billet steel crankshafts are at the top of the ladder. Available from many different manufacturers and price ranges. Two of the most common materials you will find in the specifications are; 4130 and 4340. Carbon adds to the hardening ability of the steel, 4130 will have 30% of carbon and 4340 will have 40%. Molybdenum adds toughness to the steel, 4130 has 20% molybdenum, while 4340 has 25%. In addition to more carbon and more molybdenum, the 4340 is also alloyed with nickel, nickel assures deep and uniform hardness in the crankshaft. Clearly, 4340 is the better material, but it does cost more. Save your money in the long run and choose the top quality material, as you’re already choosing the "billet path".
You should consider which crankshaft weight to purchase. The lighter the more it will cost. Most manufacturers start with the same product for their standard and lightweight crankshafts, unless you need to save weight, the standard crankshaft is the way to go, strength and durability will be the same.

Forged steel
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Forged Steel is the second best on the crankshaft ladder. Such a factory crankshaft is usually a good compromise of cost and strength, especially when found used. These crankshafts will hold in more demanding applications than cast crankshafts, but still need some preparation work, similar to a cast iron crankshaft before being used in performance applications. Light weighing is another option, drilling a hole in the rod journals parallel to the crank axis. A 20-millimeter hole makes the rod journal hollow and much lighter, with a negligible loss in strength. Before having the crankshaft ground one could choose to have it shot peened. With this process, the steel relieves stress and creates a more stable crankshaft. A forged crankshaft prepared this way will survive many demanding conditions. Again, one can choose the degree of preparation according to the application and budget.

Cast iron
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Cast Iron is the most common and economical material, that is found in many stock engines. People will say that a cast iron crankshaft is no good for performance use, with the right preparation these crankshafts will withstand a lot of abuse. For restoration, stock rebuilds or mild street tuning one should do some non-destructive testing" like "Magnaflux" and regrind them. Further improvements can be done by removing/polishing all of the casting imperfections, this eliminates stress risers which are a start for cracks. Cross drilling the oil holes will provide better rod bearing lubrication. The most important thing to do is not done to the cast iron crankshaft itself. It is very important to use connecting rods and pistons that are as lightweight as possible, so they don’t tear the rod journal off of the crank when it tried to decelerate and accelerate them back down the cylinder at top dead center.