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How to Pick a Crankshaft

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How to Tell if It's a Forged Crank
Many engines were produced with both forged-steel and cast cranks over their production lives. It's surprising how many of us have a hard time telling a forged crank from a cast crank while looking right at them. It's easy to tell them apart, if you know what to look for. Here are some quick clues.

Crank Materials
In addition to cast or forged, material quality also separates the different grades of crankshafts. What we're interested in is raw strength and durability. Various steels and irons are rated on the basis of strength, toughness, and ductility. Any given alloy can cover quite a wide range of strengths depending upon the metallurgical processes involved, so it is difficult to put an exact number on the strength of various steels and irons. However, to provide a general guide, we'll include some relative tensile strength numbers here. At the bottom of the list is a standard cast-iron production crankshaft, which typically has a tensile strength of 65-80,000 psi, and is fairly brittle with an elongation rating of some 3 percent. Some OE iron cranks were made from nodular (ductile) iron, an improved cast iron that ups the tensile strength into the 100,000-psi range, but more importantly improves ductility to about 5-6 percent elongation before failure. With the popularity of low-cost aftermarket cast cranks, we asked Scat Crankshafts about the material in its budget cast cranks. These cranks are cast with a 9000-series cast-steel material with a tensile strength of 105,000 psi and 6 percent elongation--pretty impressive numbers for a cast piece.

Forged cranks also come in a wide range of materials with varying degrees of strength. A factory-forged crank is typically manufactured from plain carbon steel, such as 1053 or 1045. These steels have a tensile strength in the 110,000-psi range, which by appearances doesn't seem like that much more than a good cast crank. But tensile strength is only one part of the picture. A forged-steel crank has an elongation rating of 20-22 percent before failure--far greater than cast iron, so ductility is the forged crank's real advantage over cast iron. From the OE-grade carbon steels, forged-crank materials go up the scale in strength with 5140 chromium steel being the next common grade at 115,000-psi tensile strength. Chromium molybdenum alloy steels are next up the list, with 4130/4140 steels commonly used for more serious performance crankshafts, at about 120-125,000-psi tensile strength. Premium forged-steel race cranks are manufactured from 4340 nickel chromium molybdenum alloy with about 140,000 psi on the tensile scale.

Billet Cranks
Billet crankshafts are at the top end of the high-performance crankshaft scale. A billet crank starts as a solid chunk of high-grade steel bar (typically 4340 material), and then everything that doesn't look like a crankshaft is whittled away through a series of machining processes. The advantage of billet is primarily in the grain structure. The rolled bar that forms the billet has a uniform longitudinal grain structure. In forging, the crank throws are literally pounded, pressed, and twisted in the dies, mashing the metal into the rough shape of the crank. The brutality of this process adversely affects the grain structure of the metal. In a billet crank, the grain structure of the original bar isn't distorted, and remains much more uniform and intact. This makes for a stronger, stiffer, more durable finished product.

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