Compare that spinning mass of metal inside your engine block to the human spine and you get an idea of how important a strong one is. As the backbone of your engine, the crankshaft performs an important task: transferring reciprocating motion into rotation. In other words its job is to guide the pistons up and down the cylinder bores, in the correct sequence, without breaking.
For stock engines, the crank is a component that rarely gets stressed, but in our high-performance and racing world, crankshafts have to be made above and beyond the OEM standards. Companies like Scat Enterprises, Eagle Specialties, Lunati, and Callies Crankshafts all offer an array of choices; everything for torque-happy street powerplants and show car engines to all-out race pieces, and unlike a short time ago, even superstrong cranks are within financial reach of the average enthusiast.
Custom strokes, different journal diameters, materials, and custom machining are just a few things to consider when choosing a crank and thankfully, a phone call or website click can get you an engine spine that can handle any level of horsepower. With many materials, stroke lengths, and other options such as surface treatments (like nitriding and cryogenics) at the top level, limitless horsepower is obtainable with the right recipe and wallet thickness. But what average schmoes like us get to enjoy is the trickle effect. It starts at the top levels of racing and as technology in components gets more commonly available, options that were only available to race teams get utilized in the grassroots and street-performance realm.
For example, there was a time when a 4340 chrome-moly crankshaft was something only "race" engines would run due to cost and availability, but today a chrome-moly crank is an attainable piece even for your street engine. As far as we can tell, our desire to go faster is never ending, so components will have to keep up in terms of strength and longevity. But by the look of what's available in 2013, it's safe to say the bar will continue to be raised each year so long as enthusiasts demand more horsepower from their Chevys.
Cast, Forged, and Billet
There are actually a few different metals used to create a crankshaft. Starting at the lowest level of performance, there is cast ductile and nodular iron. Cast iron (not steel) is the most common among stock cranks, but they are prone to failure in high-powered applications. However, while cast iron by itself tends to be a brittle metal, companies like Scat offer aftermarket cast cranks with a mixture of graphite that allows for more impact and fatigue resistance, which is ideal in a performance application. These performance cast cranks are an affordable option for those wanting some power, but don't need a 4340 crank. With forged alloys, such as 1045 carbon steel stock cranks and 5140 and 4340 chrome-moly cranks, overall strength is increased drastically. The most common in the high-performance world is 4340 chrome-moly and is considered strongest because of the nickel and chrome content. At the highest level, what you'd see in most heads-up race cars and wild power adder applications, is billet 4340 steel, which is considered to be the strongest and most expensive. Billet cranks are literally carved from a solid chunk of steel.
Machining & Processes
For those looking for every amount of acceleration possible, engine builders will sometimes call for special machining processes or treatments to lighten and/or improve oil windage. A crankshaft that's been pendulum cut is one where the counterweights have been slotted to reduce weight. In the same vein, companies offer cranks with a star flange: lightening of the back of the crank where it is contour-machined around the flexplate flange boltholes. To lighten the crank even further, gun drilling is an option where the crank is drilled all the way through the center. Knife edging is another option where the counterweights of the crank feature a sharpened end that reduces windage and weight, as well. Other non-machining processes transform the metal in various ways, all in an effort to ensure your crank's longevity in high-load conditions, such as racing. Cryogenic treatment, for example, is a super-freezing process (-300 degrees) that modifies the microstructure (outer layer of metal) of the crank. This is also common on stock cranks, as it relives any internal stress risers in the metal. It modifies it enough that a piece that's been cryo-treated will machine differently. Eagle Specialties offers a unique process of their cranks called ESP Armor, which is a slick finish that they claim improves power by reducing windage. Nitriding is a treatment that uses ammonia and nitrogen and intense heat to modify the carbon of the crank, again, to ensure longevity at the most intense levels of output.
SBC stroke lengths & cubic inches
|Cubic inches||Bore & stroke*|
|350||4.000 x 3.48|
|346||3.900 x 3.62|
|364||4.000 x 3.62|
|383||4.000 x 3.80|
|400||4.125 x 3.75|
|377||4.155 x 3.48|
|383||4.030 x 3.75|
|*Measured in inches|
BBC stroke lengths & cubic inches
|Cubic inches||Bore & stroke|
|396||4.096 x 3.76|
|402||4.125 x 3.76|
|427||4.250 x 3.76|
|454||4.250 x 4.00|
|502||4.466 x 4.00|
|540||4.500 x 4.250|
|632||4.600 x 4.750|
|*Measured in inches|
Scat's Billet Crank
Scat's billet cranks are cut from solid chunks of chrome-moly steel using advanced CNC machines that can cut any stroke or journal size an engine builder could ask for. These cranks are used at all of the highest levels of racing and are the most expensive. We were at Scat Enterprises in Redondo Beach, California, last month and got to see some of these gems being made, and dreamed of 40 pounds of boost and drag racing glory.
Here’s an easy way to tell if you have a forging or a casting: the forging (on the left) will have a wide, raised band on the end of the journal, whereas a casting will have a small parting line (above). However, that’s not always the case as a lot of aftermarket companies have this machined down since it’s considered to be a stress riser. A stress riser is an abrupt change in the shape of matter, which can lead to cracking in high-horsepower applications where severe conditions are seen. Think of stress risers not only as surface imperfections, they can also be present inside the metal itself.
Knife edging a crank refers to the "sharpened" edges of the counterweights. The idea is less weight and quicker rpm acceleration.
The wider the fillet of the journal is, the stronger the crank. On stock cranks this radii is very sharp, unlike the 0.125-inch or even 0.140-inch-wide fillet on most performance aftermarket pieces. This area, however, does determine what bearings you need, and typically performance builds call for a chamfered bearing. Crank material will also determine the type of bearing needed.
Here are some examples of performance rod bearings used in various applications. Notice how much wider the bearing is on the far left? That’s for a stock crank with a narrow fillet. Oftentimes a performance crank will call for a narrower bearing in order to clear the fillet. The bearing in the middle is a typical performance piece, while the small one on the left is a coated piece for racing applications. The coating provides an additional barrier of protection in case of a low oil situation.
If you’re building a top-tier LS in the near future, check out Lunati’s line of Pro Series LS crankshafts. These 4340 cranks are American machined, and the shaft surface finish is graded 5 RMS or better, while journal radii are ground to large 0.140 inch and roundness of each journal is 0.0001 inch or less. Each rod journal is drilled with 7/8- or 3/4-inch lightening holes to reduce the inertia weight of the crankshaft. Lunati claimed these crankshafts have been successfully used in 1,500-plus horsepower applications.
Here’s an example of what happens when you run a standard bearing with a crank that has a wide fillet. The edge of the bearing will rub until it gets burned and will eventually lead to failure—not ideal for a performance build.
Here you can see what a crank with a “star flange” looks like. Notice how the extra material around the holes has been removed and contoured? This is to make the crank as light as possible without giving up strength.
On full-effort engines, engine builders sometimes order cranks with Honda-sized rod journals (1.888 inches), which experience less friction and require less oil pressure to keep the bearing separated from the crank. However, they are not ideal on long stroke applications since it reduces the cross-sectional area and strength.
Howards Cams also offers a line of 4340 cranks called the Track Smart line. These are CNC’d, nitrided, micro-polished, and the No. 1 and 4 rod journals feature a lightening hole.
You’ll notice on most performance crankshafts the oil holes are thoroughly chamfered to help promote oil distribution on the bearing.
Callies Crankshafts out of Fostoria, Ohio, also offers billet crankshafts for high-horsepower applications: the Ultra Billet line. These high-end cranks are machined from premium-grade Timken alloy steel. According to Callies, this high-quality specialty steel has been formulated to create a level of toughness that is unmatched in the crankshaft industry. These cranks are typically custom-made so any main journal, rod journal, post configuration, stroke, or other specialized characteristic that you may require can be provided.