I'll try to break it down somewhat for ya. All good cranks are really made up of alloy steels. The first two digits in any four-numbered series represents the approximate middle of the carbon range. A 5140 crank for example has a range of say 38 to 42 percent and it takes into account variations in manganese, phosphorous or sulphur,etc. with the same carbon range. The first two digits represents the type of alloy it is. 51 is Chromium @ .80, .88, .93 .95 or 1.00 percent, which increases tensile strength, hardness, hardenability, toughness, resistance to wear and abrasion, resistant to corrosion and scaling at high temps, too. 41 is also Chromium but @ .50, .80 or .95 percent with Molybdenum @ .12, .20 or .30 percent. 43 is Nickel @ 1.83 percent which increases strength and hardness without sacrificing ductility/toughness. It also increases resistance to corrosion/scaling at high temps. when joined with adequate levels of high chromium (stainless) steels. This type of alloy also includes Chromium @ .50or .80 percent and Molybdenum @ .25 percent. Clearly, for any high perf. application, a 5140 alloy is the best way to go but a 4340 alloy steel is the most popular and available. If you need any help or suggestions on cranks, drop me a line at
[email protected] - I own my own shop so I can help you with whatever you may need. -Jabin