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"5140 or 4340 ? Get the Facts and End the Confusion."

Before we can answer the question "which metal do I need in my crankshaft". I think we need to take a moment and review just what each metal is made of and what are the best applications for each. In the following discussion we will see the strengths and weaknesses of each and with this information we will be able to decide which Crankshaft material will best fit our needs.

Starting with the basics, metals containing primarily iron are classified as "ferrous metals". They range from pure iron through exotic high-alloy steels. Stock Crankshafts are made from cast iron, a metallic iron with more than 2 percent dissolved carbon. One preferred variation, ductile or nodular iron has all its carbon contained in the form of tiny spherical graphite nodules uniformly dispersed throughout the metal's matrix. This makes the material more ductile (deformable rather than brittle) and eases casting and machining.

Even the best cast iron has only limited tensile strength. Increasing ductility, hardness, malleability and fatigue resistance requires removing most carbon and at the high end, alloying iron with other elements, creating "steel" an iron with less than 2 percent carbon

The most basic form of this is carbon steel, which contains up to 1.7 percent carbon and minimal additional alloying elements. Carbon steels are designated by a four digit number. The first two digits indicate the basic type, and the last two digits indicate the approximate midpoint of the carbon content. The "10" ID's these alloys as non-resulfurized carbon steel with some manganese (popularly called medium-carbon or mild steel). The second two digits the "45" or "53" means the steel contains about 0.45 or 0.53 percent carbon respectively. Stock forged OEM cranks are usually made from 1045 or 1053 steel. There are exceptions to this, some 350 high performance steel cranks in the sixties were made from 5140 and some manufactures offer 5140 or 4340 in their high performance aftermarket catalogs.

From these mild OEM steels the next step up is Alloy steel. Alloy steels allow for more variations depending on the alloying materials. Over time as manufacturing techniques improved and chemical knowledge grew., metallurgist developed whole families of alloy steels, custom-tailored to make metals stronger, lighter, more durable, more ductile, and harder. Alloy steels are also identified by a four-digit number, with the first two digits indicating the major alloying element or elements, with the last two digits indicating the approximate midpoint of the carbon range.

We will now examine the four most common groups of steel, we will examine their best uses and hopefully come up with a buying criteria for making a decision on our crankshaft purchase. We want our purchase to be based on knowledge of the product and its intended use.

4130 The best known chrome-moly steel. It is a high-strength/high-stress alloy when produced in thin sections (sheet metal and tubing). But 4130 possesses very poor deep heat-treating characteristics which make it a bad choice for machined or forged parts.

4140 A deep-hardening chrome-moly steel , it forges well and has good impact resistance, fatigue strength and general all around toughness.

4340 A nickel-chrome-moly steel, this alloy is used to make premium cranks.4340 has good tensile strength, toughness, and fatigue resistance. Modified 4340 alloys with vanadium and more silicon can make this already good alloy even tougher and more fatigue-resistant. The main drawback is cost.

5140 This chromium alloy increases tensile strength, hardness, toughness, and wear-resistance over carbon steel. It has the same basic elements of 4340 and is made with the same process but is more affordable.

So what can we conclude from this short primer. Our first conclusion is that we don't want to purchase a crank made from 4130. The lack of deep heat treating properties makes it unacceptable for most performance applications. That leaves us with 5140 and 4340. Of the two we feel 5140 is the crankshaft material that suits most clients needs. Reason #1, based on feedback from clients using our cranks the 5140 crankshaft lasts as long as the 4340 when used in all but the most extreme racing conditions. For applications where the engine is putting out 800hp or less and turning 8,000rpm or less, 5140 is the right choice. Reason #2, in engine building you save money where ever you can, if it doesn't effect the performance or durability of the engine and our 5140 crankshafts are priced 30-40% below 4340 crankshafts in cost.

short answer,forged is best, cast steel is significantly stronger than plain cast iron and can be slightly more flexable, unfortunately, as the quality gets better the cost gets higher, and your connecting rods are FAR more likely to fail than the crank in most engine combos below about 6500rpm
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