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CHAPTER 3

CRANKSHAFTS

Over the years, the evolution of performance crankshafts has experienced a high level of development in terms of materials, manufacturing processes, dimensions, weight reduction, windage concerns, oil delivery, surface finishes, counterweight aerodynamics, and quality control. Gone are the days when a racer is forced to make do by modifying a factory OEM crankshaft.

Crankshaft Stroke

Crankshaft stroke refers to the distance from the crankshaft main centerline to the centerline of the rod journal. Published crankshaft stroke refers to the total sweep of the rod journal from top dead center (TDC) to bottom dead center (BDC). For example, a crankshaft that features a 4.000-inch stroke indicates that the rod journals will move 4.000 inches from TDC to BDC.

However, when we are planning a stroke, rod, and piston combination to determine the crank stroke, rod length, and piston compression distance (CD) relative to the block deck height, we consider only half of the total stroke of the crank. With the rod journal at TDC, the half-stroke distance plus the rod length plus the piston CD will dictate where the piston dome is located relative to the deck at TDC. If our goal is to achieve a zero deck, we refer to the deck height as our target.

This is an example of a Scat lightweight forged crank with a scalloped flywheel flange, fully gun drilled, and with bullnosed and knife-edged counterweights.

Deck height is the distance from the main bore centerline to the block’s head deck surface. If our deck height is 9.000 inches, the combination of half-stroke plus the connecting rod length plus the piston CD must equal the target 9.000-inch deck height. Connecting rod length refers to the distance from the centerline of the rod’s big end to the centerline of the rod’s wrist pin bore. Piston CD refers to the distance between the centerline of the wrist pin bore of the piston to the piston’s top dome edge.

As an example, again referring to a deck height of 9.000 inches, if our crank features a crank stroke of 4.000 inches, we use half of the total stroke, which in this case is 2.000 inches. If our connecting rods feature a center-to-center length of 6.000 inches, our piston CD needs to be 1.000 inch. If our deck height is 9.025 inches, along with a 2.000-inch half stroke and 6.000-inch rod, piston CD would be 1.025.

Factory Stock Crankshaft Stroke

When building a high-performance small-block engine, we typically take advantage of changes to the stroke to obtain increased performance. Simply as a reference, the following tables show the factory-original stroke, rod length, and piston CD found in original Chevy small-block engines.

Popular Aftermarket Bore/Stroke Combinations

When planning a build to deliver increased horsepower and torque, we’re certainly not going to adhere to factory specs. Depending on the limitations of the block at hand, a wide range of cubic inch displacements is possible. Listed on this page are a few examples. Other limiting factors involve the crankshaft strokes available from specific manufacturers.

Aftermarket performance crankshafts for the SBC are available in a dizzying array of strokes, including 3.000, 3.250, 3.335, 3.480, 3.500, 3.562, 3.625, 3.750. 3.800, 3.875, 4.000, 4.125, and 4.250 inches.

Standard deck height for a small-block Chevy block is 9.025 inches. If a specific stroke, rod length, and piston CD combination exceeds stock deck height, a tall-deck aftermarket block is required to accommodate the extended distance from the main bore centerline to the piston dome. While stock deck height is 9.025 inches, aftermarket tall-deck blocks are available, usually with a deck height of 9.325 inches, permitting a longer stroke and longer rods.

Deciding crankshaft stroke involves several factors, including the physical dimensional variables of rod length, piston compression distance, block deck height, and the desired operating characteristics. Speaking in general terms, a longer crankshaft stroke provides increased torque, while a shorter stroke provides the ability for the engine to generate higher engine speed (RPM). For example, a drag racing application may call for a longer stroke, while a road race application may call for a shorter stroke.

Aftermarket Bore/Stroke Combinations
Engine CI	Bore	Stroke
302	4.000	3.000
327	4.000	3.250
346	3.900	3.620
350	4.000	3.480
355	4.030	3.480
364	4.000	3.620
377	4.155	3.480
383	4.030	3.750
406	4.155	3.750
410	4.130	3.800
414	4.125	3.875
427	4.125	4.000
434	4.155	4.000
441	4.125	4.125
447	4.155	4.125
454	4.125	4.250

We also need to consider crankshaft weight. A lightweight crank, due to a decrease in mass, allows the engine to rev quicker, which is an advantage in drag or sprint car applications. However, in racing where endurance plays a major role, a heavier crank that is not highly modified for weight reduction can provide increased stability with less harmonics, providing increased durability for long runs at high engine speeds, especially where engine RPM doesn’t vary a great deal as the engine tends to run at a fairly consistent RPM. As you can see, choosing the crank stroke and weight involves a variety of factors.

In a small-block Chevy build, you have the option of running 350 main journals or the larger 400 main journals. Today’s aftermarket blocks are available with either main bore size. A crankshaft with 350 mains will feature a main journal diameter of 2.450 inches, while a crank with 400 mains will have a main journal diameter of 2.650 inches. Builder preferences differ depending on their experience and opinions. The small 350 main results in a lower bearing speed, which is preferred for better oil delivery to the bearings. The larger 400 main crank, while slightly beefier, results in additional loss of block material to accommodate the larger journals. My preference is the 350 main.

Always keep in mind that increases in the crankshaft stroke decrease the clearances between the connecting rod’s big end and the block and between the rod’s big end and the camshaft. In terms of cam clearance, rod big ends designed for stroker clearance are vital. They often require rods that feature shorter rod cap bolts, which lowers the profile of the big end’s shoulder. This is the reason that aftermarket block makers offer raised cam blocks that position the cam tunnel about 0.300 inch or more, providing additional cam to the rod’s big end clearance.

Just as the dynamic effect of aerodynamics plays a role in how the vehicle cuts through the air at speed, the profile shape of crankshaft counterweights can affect windage drag inside the crankcase. Although the crankshaft counterweights don’t rotate through the sump’s oil bath, oil that drains down from the top back to the oil pan wet sump or even a dry sump’s pan can drain onto or across the counterweights. The counterweights don’t need oil and should be kept as dry as possible to avoid unwanted parasitic oil drag.

Many builders prefer to knife edge the counterweights, which typically involves creating a rounded/radiused nose on the counterweights’ leading edge, tapering down at the trailing edge, very much like the cross section of an airplane wing. This allows parasitic oil that tends to cling to the counterweights to skim over the counterweight and evacuate quicker, theoretically reducing oil cling and drag, providing what you might call a slipstream effect. While not necessary for a street-driven performance engine, profiling the counterweights can often provide an advantage in a high-revving racing crankshaft.

Another method of reducing drag caused by parasitic oil cling is to have the counterweights treated with a slippery specialty coating that prevents oil from sticking to the counterweights. Specialty coating firms, such as Swain Tech Coatings, PolyDyn Performance Coatings, and others, offer these services for racers who are looking for every possible advantage.

Many aftermarket performance crankshaft makers gun drill a hole through the center of the mains to reduce weight. This photo was taken during the manufacturing process at the Callies Performance Products factory.

OEM factory crankshafts were notorious for journal oil holes that featured no chamfering and had sharp edges. Performance builders commonly addressed these ports by softening the hole edges and grinding a chamfer to promote better oil flow. Today’s aftermarket cranks are already prepped and generally have no need for further modification as seen in this main journal example.

While older OEM factory cranks featured a square-cut fillet at the journal-to-counterweight intersection, aftermarket performance cranks feature a radius transition. This provides superior strength, vastly reducing or eliminating the potential for a stress fatigue.

Rod journal oil holes are also lightly chamfered for better oil delivery to the bearings. Addressing and improving oil flow is just one of the features that aftermarket crank makers provide, eliminating modifications normally needed when dealing with yesterday’s factory cranks.

The trailing edge of this counterweight features a knife edge cut. This slows an increased slipstream effect. Coupled with a bullnosed leading edge, this approaches the aerodynamic effect similar to that of an airplane wing and provides less windage drag during revolution and faster evacuation of parasitic oil.

This closeup shows the extreme scalloped profile of the Callies Performance Products Magnum XL. Unneeded counterweight material is removed, providing a drastic weight reduction while maintaining a high strength-to-weight ratio. It is designed for lighter-weight rods and pistons, in terms of balancing accommodation. (Photo Courtesy Callies Performance Products)

Aftermarket cranks commonly feature generous fillet radii that eliminate potential stress risers. (Photo Courtesy Lunati)

Finish-grinding journals is performed with a high degree of precision on state-of-the-art grinders that are constantly checked for calibration.

The leading edge of this counterweight is bullnosed to reduce drag, providing less resistance than a square-cut edge.

Unlike mass-production factory cranks of years gone by, today’s aftermarket crank journals commonly feature very precise and consistently machined journal dimensions. It’s rare to find a crank made by a reputable manufacturer that features out-of-tolerance diameters and taper. While measuring journals and installed bearing dimensions for fit and oil clearance is always necessary to verify, it’s rare that corrections will be needed.

Note the star-cut flange for reduced weight. This is a feature offered by several crank makers. (Photo Courtesy Lunati)

This is an example of a forged steel crankshaft from Scat. Standard-weight cranks feature full-size counterweights, while gun drilling provides a level of weight reduction. Unless extreme lightening is required for certain racing applications, a standard-weight crank is an excellent choice for all high-performance street and a wide range of racing applications as well. The critical elements include precision machining and journal hardness that are offered by leading crank makers.

An example of extreme lightening of a racing crankshaft is shown. The Magnum XL series features lightening profiles where counterweight material has been machined from non-stress areas, greatly reducing parasitic material and weight. This minimizes windage within the crankcase. Oil control is improved through the elimination of disruptive undercuts, resulting in smooth-sided, free-flowing counterweights. Each main and rod journal is gun drilled for additional weight reduction and improved throttle response. Magnum XL cranks are shipped fully balanced to the builder’s exact assembly weight, requiring the builder to supply bobweight information. (Photo Courtesy Callies Performance Products)

Examples of the Lunati Signature Series are shown. They are made from a non-twist 4340 forging and are rated at handling over 1,500 hp. Features include gun-drilled mains, lightened rod journals, micropolished journals, and windage-reducing, contoured-wing counterweights. Signature Series blower applications are also available and are designed for use with Roots-type superchargers. Features include 0.125-inch fillet radii on rods and mains, duel keyways, larger nose bolt threads, and enlarged flexplate flange threads. (Photo Courtesy Lunati)

Bore and Stroke Combinations

Determining a bore and stroke combination for cubic-inch displacement involves a very simple formula:

Bore x Bore x Stroke x 0.7854 x number of cylinders.

Example: Let’s say that the planned cylinder bore size for an 8-cylinder engine is 4.125 inches and your planned crankshaft stroke is 4.000 inches.

4.125 x 4.125 x 4.000 x 0.7854 x 8 = 427.6503

Selecting Crankshaft Stroke

Too many enthusiasts tend to choose the bigger option as the best choice for just about everything—a bigger cam, bigger heads, a bigger carb, etc. Increasing crankshaft stroke provides greater torque down low. Shorter strokes allow the engine to rev higher. It all depends on where you want the peak power and torque. Especially for forced induction engines that utilize supercharging or turbocharging, a longer stroke simply isn’t needed because the increased dynamic compression under boost is making the power.

Consider the relationship of cylinder bore diameter and crankshaft stroke, wherein we refer to the “square” of the engine. If the bore diameter and stroke are equal, for example if bore size is 4.000 inches and stroke is 4.000 inches, the engine is square. If the bore diameter is greater than crank stroke, the engine is referred to as over-square. If the bore size is less than stroke, it’s under-square.

With the bore size as a constant, increasing the crankshaft stroke tends to produce greater torque and low-RPM power but is more limited in engine RPM. Going to a shorter stroke allows increased engine speed and moves power higher in the RPM band. If your goal is to obtain more torque for street driving, building the engine square or under-square is preferable. If you’re planning a road racing or oval track build, moving to an over-square platform is likely the better choice.

There’s no magic formula to determine which stroke and/or bore size is ideal because other factors, such as cylinder head flow, valve size, valve angle, camshaft profile, etc., influence the final outcome. However, speaking in very basic terms, when selecting the crankshaft stroke, longer strokes suit higher-torque requirements, while shorter strokes are better suited for higher engine RPM. Many street builds call for maximum displacement and maximum torque, which is why many opt for the biggest bores and longest strokes that will fit into the confines of a specific block package.

A very generic view of stroke selection is that a longer stroke provides increased torque with the powerband moved toward the lower RPM range, while a shorter stroke provides the capability of higher revs with the powerband moved into the higher RPM range.

Forgings and Billet

Steel forged crankshafts can be made using either a twist or non-twist method. A twist forging takes a raw forged crankshaft and, while heated and malleable, twists the forging to orient the rod throws in the proper clock position. A non-twist forging forges the crank with rod throws already in the proper clock positions. The difference is that a non-twist forging has a more uniform grain structure and is therefore stronger.

High-performance forged cranks intended for extreme applications are made using the non-twist method. This is followed by finish-machining, heat treating for strength and stability, and nitriding for increased surface hardness. Examples of makers include Callies Performance Products, Crower, Scat, Winberg, Eagle, and Lunati.

Billet crankshafts are, as the term implies, CNC machined from a solid blank of high-quality dense steel. Billet crankshafts are, not surprisingly, more expensive due to the material waste and the increased CNC machining time. Billet cranks are available from several manufacturers, including Scat, Callies Performance Products, Winberg, and Bryant Racing, to name a few.

The advantage of choosing a billet crankshaft is twofold: strength and custom application. The molecular structure is consistent because the process begins with a dense forged billet. Depending on the specific application, a crank machined from billet stock may be as strong or stronger than a crank that is forged and finish machined.

One of the real benefits of a billet crank is the ability to create exactly the crank you want. Since it’s being machined from a blank at the outset, all dimensions can be achieved to suit your specific needs. That includes main and rod journal diameter and width, stroke, counterweight shape, snout diameter and length, etc. For a street application or a weekend warrior build, the cost versus function makes a billet crank a bit of overkill. However, for the pro racer who requires maximum durability and custom dimensions, the higher cost is justifiable.

Forged crankshafts are produced with a steel/alloy mix, slug heated to formability, compacted to rough shape in a hydraulic press, machined, and heat treated.

Aftermarket forged cranks are a better choice compared to OEM forged cranks. The reason: OEM forged cranks tend to have a high carbon content, but aftermarket forged cranks tend to have a higher content of chrome and nickel in the formulation, and the higher alloy content provides much superior strength.

Common debates exist in regard to strength. Some note that a forged crank offers superior strength because the grain structure has been moved and compacted, resulting in a more uniform grain structure during manufacturing. Billet cranks begin life as an already-forged chunk of billet steel, which is then machined to shape. However, the grain structure tends to run more parallel to the length of the crank. Depending on who you talk to, you’ll hear that billet is stronger than a forging or that a forging is stronger than billet. We won’t get into the debate here. As far as I’m concerned, a forged or billet crankshaft made by a reputable performance aftermarket manufacturer is suitable. The major difference, in my opinion, is that choosing a billet crankshaft provides increased latitude in terms of creating a custom-dimension crank in those instances where a builder’s request simply can’t be fulfilled by an off-the-shelf forged crank.

Today’s performance aftermarket offers crankshaft features that were unheard of only a few decades ago. Thanks to ongoing development within the aftermarket, we now have a greater selection of stroke dimensions, counterweight shaping, weight reduction, journal diameter choices beyond stock sizes, vastly superior metallurgy, high-precision CNC machining, surface finishes, and more, which translate into availability of performance cranks that contribute to obtaining increased power and torque along with substantially improved durability.

Shown here is Lunati’s Voodoo lightweight crank with a substantial amount of material removed to reduce rotating weight while maintaining rigidity. The 430 is a non-twist forging and is nitride heat treated with lightening holes in the rod journals. Note the undercut counterweights for further weight reduction. (Photo Courtesy Lunati)

Crankshaft Durability Treatments

In an effort to make a crankshaft more durable, more resistant to fatigue, and to provide a hard bearing surface, a process of nitriding is commonly employed. This creates a nitrogen-infused surface treatment that creates a several thousandths of an inch thick hardness increase that allows the crank to better withstand high bearing loads. An ion plasma nitriding process produces a deep case that enhances strength while creating an extremely hard bearing wear surface.

This is not to be confused with cryogenics, which offers its own benefits. Cryogenics involves subjecting the crankshaft to sub-zero temperatures as low as -400°F and warming it back up to ambient temperature in a controlled time process. This compacts the steel/alloy steel material into a tighter, more uniform molecular “grain” to offer higher resistance to fatigue.

Another process that provides a more uniform molecular structure is vibratory stress relief, which is a non-destructive method of subjecting the part to computer-controlled harmonic frequencies that vibrate the molecules, producing a more uniform structure. Vibratory stress relief is referred to as non-destructive because the part cannot be damaged during the process, unlike cryogenics, where strict protocols must be followed to avoid making the part too brittle. Improving the grain structure and/or surface hardening a crankshaft won’t produce additional horsepower, but these processes contribute to improving the durability of the crankshaft during extreme loads and speeds.

An example of a billet crankshaft by Bryant Racing is shown. Starting with a several-hundred-pound dense steel billet, the entire crankshaft is CNC machined to finished state, followed by REM isotropic finishing. (Photo Courtesy Bryant Crankshaft)

REM Finishing

REM’s Isotropic Finishing process (ISF) has been in use in various industries for decades but has been more commonly used in performance and racing applications in recent years. In combination with a proprietary chemical treatment and a vibratory polishing process, an REM-finished crankshaft’s appearance is extremely polished and smooth. Basically, it looks as though it’s been highly polished and chrome plated. With the appearance set aside, the performance benefits are what count.

Benefits include friction reduction, increased efficiency, a horsepower increase due to reduction of parasitic friction and oil cling, lower operating temperatures, reduced lubrication requirements, and increased component durability as sharp potential stress risers are reduced. Applications for REM finishing include not only crankshafts but also connecting rods, camshafts, lifters, valve springs, rocker arms, ring and pinion assemblies, mechanical oil pumps, rack and pinion steering components, transmission gears, universal joints, etc.

The REM ISF process results in a non-directional, low-Ra surface finish, which means that the surface is extremely smooth with reduced microscopic peaks. Ra stands for roughness average. The lower the Ra number, the smoother the finish. Think of it this way: consider the difference between sanding a metal surface with 80-grit sandpaper compared to using 2000-grit paper.

As an example (and a good one at that), an accomplished engine builder and close friend recounted a tale of a customer’s race engine. The engine was built using an REM ISF treated crankshaft. My friend’s shop performed all of the machine work, and the customer assembled the engine. He installed an adjustable-height distributor that featured a slip collar. The owner of the engine forgot to tighten the slip collar. As a result, during a race, the distributor began to climb out of its location to the point where it lost contact with the oil pump drive shaft, quickly resulting in zero oil pressure. He ran the engine for another lap before returning to the pits.

After the race, he brought the engine to my friend’s shop for a teardown and inspection. Incredibly, while the crank showed signs of extreme overheating and the bearings were toast, the crank’s rod and main journals were scratch-free, looking like the day the crank was finished.

The typical cost for REM ISF processing is about $600 for a crank and a set of eight rods. When you consider what’s at stake, that’s not a bad price to pay for added peace of mind.