In the early days, a crank didn’t have to be very sophisticated or strong because most engines were an inline design, and the loads and speeds were relatively low. Fast forward to today. The evolution in engine technology, power and performance has led to a whole new generation of performance crankshafts that are a mechanical work of art. CNC machined, polished and balanced to perfection, today’s cranks bear little resemblance to their ancient ancestors.
The cranks in today’s high revving inline and V-block engines all have counterweights to offset the reciprocating mass of the pistons and rods. Without this internal balance, the engines would quickly shake themselves apart. CNC machining allows counterweights to be placed in more ideal locations to improve balance and reduce mass. One of the limitations of forgings is that the crankshaft has to come out of the die after it has been forged into shape. Because of this, the location of the counterweights has to be compromised somewhat, and additional machining is required to remove the excess metal after the crank comes out of the forge.
The metal in today’s forged and billet steel cranks is also much stronger than the cast irons used in everyday passenger car cranks. Many cast cranks are made of 1053 high-carbon alloy steel. This material has a tensile strength of around 100,000 to 110,000 psi, which is good enough for applications up to about 400 to 450 horsepower (depending on the size of the journals). But for higher horsepower street or racing engines, some type of forged or billet crank is a must.
Some less expensive forged cranks are made of 5140 grade steel (which has a tensile strength rating of 115,000 psi), but most forged and billet performance cranks are 4130, 4340 steel or another high grade alloy steel. Crankshaft suppliers may use different alloys in different product lines, depending on the application and strength requirements.
Cranks made of 4130 alloy have a tensile strength rating of 120,000 to 125,000 psi. Cranks made of 4340 and similar alloys may have a tensile strength of 140,000 to 145,000 psi or higher, and a fatigue strength rating of 160,000 to 165,000 psi or more depending on the heat treatment and the quality of the alloy. The magic ingredients in that boost the strength are chromium, nickel and molybdenum.
The percentages of these ingredients must be carefully controlled and kept within certain limits to achieve these numbers, so quality control is absolutely essential for maximum strength and reliability. The American Society for Metals specifies the ingredients and the percentages of those ingredients that are required to meet the criteria for a specific alloy.
Crankshaft manufacturers are rather coy about where they source the metal for their billet cranks and where they get their forgings. Most forgings are now being made in China because of their low labor and tooling costs. Forgings are still being made in the U.S., but not in the quantity as those coming here from China.
According to one source, there are 15 to 20 crankshaft manufacturers in China producing cranks for the U.S. aftermarket. Some of these manufacturers are capable of producing high quality finished products while others are better suited at supplying rough machined forgings that are finished machined here.
Most of the domestic suppliers of high-end racing cranks say they prefer to do their own finish machine work on the forgings whether they source their forgings from China or the U.S. This gives them total control over the accuracy of the dimensions as well as the quality of the work.
The important point here is not where a particular forging came from or where it was machined, but if the crankshaft is accurately finished to close tolerances and meets the strength requirements for the application. The journals on a high quality performance crank should be perfectly round, and flat side-to-side with no taper, or convex or concave curvature. The location of the journals must be accurately indexed for precise valve timing and ignition.
The counterweights must be accurately positioned and sized to offset the reciprocating mass of the pistons and rods. If a crank meets these criteria, it’s a good crank. If it does not, it may require a lot of reworking before it is acceptable to use – and that’s something you have to figure into your engine building costs if you end up having to rework a bargain-priced crank.
It’s also important to make sure a crank has enough strength for the application. A low-priced entry-level crank will not hold up to the rigors of racing like a high end racing crank.
Why Cranks Break
Too much horsepower with a stock cast crank will almost certainly lead to disaster. Once you go beyond 400 to 450 horsepower with a small block cast crank, or 550 horsepower with a big block cast crank, the risk of breakage goes way up. If you’re building an engine that will be blown, boosted or use NOx, you should always upgrade to a forged or billet steel crank.
Metal fatigue that results from flexing can also cause a crank to break. The more rigid the crank, the stiffer it is and the less it will flex. That’s good. But if the journals are too small or too much metal has been removed from the crank to lighten it, flexing increases along with the risk of breakage.
Cracks often begin in highly stressed areas like the journal fillets, near oil holes or near the snout where there are high loads from the drive belts or a harmonic balancer that may be out of balance. Most performance cranks are machined with a larger radius in the journal fillets (which may require using chamfered rod and main bearings). Cranks with oversized snouts are also available for blower applications or other applications that place unusually heavy belt loads on the crank.
Balancing is absolutely critical in any high revving performance engine. The loads on the crank go up exponentially with rpm. That’s why many engine builders want the rotating assembly balanced to within tenths of a gram.
On V6, V8 and V10 engines, the pistons are moving in different planes. This requires crankshaft counterweights to offset the reciprocating weight of the pistons, rings, wrist pins and upper half of the connecting rods. The counterweights smooth out the vibrations but also add weight to the crank. This, in turn, increases the inertia of the crank. So reducing the size and/or number of counterweights is a trick that’s often used in lightweight racing cranks designed for circle track and road racing applications where instant throttle response is desirable.
With “internally balanced” engines, the counterweights themselves handle the job of offsetting the reciprocating mass of the pistons and rods. In “externally balanced” engines, additional counterweights on the flywheel and/or harmonic damper help the crank maintain balance. Some engines have to be externally balanced because there isn’t enough clearance inside the crankcase to handle counterweights of sufficient size to balance the engine.
This is true of engines with longer strokes and/or large displacements. And some engines, like late model Corvettes with dual-mass flywheels, the engine is partially internally balanced, and externally balanced with movable weights on the flywheel.
If you’re rebuilding an engine that is internally balanced, the flywheel and damper have no effect on engine balance and can be balanced separately. What’s more, the index position of these parts won’t change the internal balance of the engine. Nor will changing flywheels or harmonic balancers (assuming the new parts are zero balanced). But with externally balanced engines, the flywheel and damper must be mounted on the crank prior to balancing. The flywheel and damper must also be indexed to the crank because changing their position will upset the balance.
Replacing a flywheel or harmonic balancer will also require rebalancing the engine. If a customer doesn’t know that and changes a flywheel or balancer, he may create a balance problem that causes unwanted vibrations and ultimately a crankshaft failure!
Everybody wants bigger engines because more cubic inches means more horsepower (assuming the cylinder heads, cam and induction system can flow more air to take full advantage of the increased displacement). We’re seeing bigger engines on the street and bigger engines on the strip. A 450 cubic inch engine was considered a big street engine a decade ago. Now we’re seeing engines over 630 cubic inches being run on the street.
In ProStock drag racing, there seems to be no upper limit on engine displacement. Some of today’s monster motors are as large or even larger than some over-the-road heavy-duty truck engines. We recently saw a new CNC billet aluminum big block at the Race and Performance EXPO that can displace over 1000 cubic inches! That’s a lot of cubes, and a lot of load on the crankshaft.
The displacement of any block is limited by a number of factors. The maximum bore size is limited by the spacing between the cylinder bores and the thickness of the casting or cylinder liners that can be installed. The stroke is limited by the height of the block, the clearance between the pan rails, and the location of the camshaft. Stock blocks can only handle a limited increase in stroke. In the case of a small block Chevy, a 4-inch stroke with 6-inch rods is about the limit. Popular strokers include 383, 406, 412, 420 and 434.
With aftermarket blocks, raising the position of the cam inside the block, making the decks taller to accommodate more piston travel, and opening up the area between the pan rails allows cranks with much longer strokes to be used. Some of these “small blocks” displace over 450 inches.
A stock crank can be made into a stroker by offset grinding the journals (which also reduces their size and strength), or by welding the rod journals and remachining them back to their original size (or undersize) with more offset. Or, you can replace the stock crank with a forged or billet stroker crank that is machined with more offset on the throws. Of course, the length of the rods also has to be changed and/or the wrist pins moved further up in the piston to match the increased stroke, otherwise the pistons will hit the heads.
A number of companies we interviewed for this article indicated they have recently expanded their product lines to offer more stroker cranks for a wider range of engines, including Chrysler, Oldsmobiles, Jeeps, and even flathead V8s. That’s good news because more product availability means you have more choices when building an engine.
Although stroker cranks seem like an easy way to gain displacement and power, there are some tradeoffs. A long stroke is good for low-end torque and off-the-line throttle response, but not as good for high rpm power. Also, the longer the stroke, the more counterweight is needed to offset the greater motion of the pistons and rods. This, in turn, may require using expensive heavy metal to balance the crank. With some long stroke cranks, it may not be possible to achieve internal balance by adding metal to the counterweights. The engine may also have to be externally balanced.
Eventually, the point of diminishing returns is reached. One crank supplier said that engine breathing efficiency really drops off once you go beyond 600 cubic inches of displacement. It becomes harder and harder to fill a bigger hole with a naturally aspirated engine. Of course, you can always use a blower or turbocharger to cram more air into a big engine – but you can do the same thing with a smaller displacement engine, too.
One important point to keep in mind when increasing displacement with a stroker crank is to also increase the port volume of the cylinder heads and intake manifold runners and plenum. More cubic inches won’t produce as much power if the induction system can’t handle the airflow.
Though forgings are available for most popular engines, there are some gaping holes. If no forging is available for an engine you are building, your only alternative is to buy a custom-made billet crank CNC machined from a solid chunk of steel. Billet cranks require a LOT of machining, and typically cost three to six times as much as a forged crank depending on how much work is required. But billet cranks are not limited by the availability of a forging, and the throws and counterweights can be placed virtually anywhere, leaving plenty of room for experimentation.
As for strength, billet cranks are probably the strongest (though it can be argued either way, say some crank manufacturers). Most Top Fuel, Funny Car and Formula One engines run billet cranks. So do most NASCAR engines. Even so, forged cranks can be just as tough, and are winning all kinds of circle track, drag race and off-road competitions.
Forgings have a flowed grain structure, which makes them stronger than an ordinary cast crank. The forging process starts with a slug of metal that is heated to around 2,400 degrees F. At this temperature, metal is glowing bright yellow and is relatively soft. The slug is placed between dies in a press that hammers the metal with up to 250,000 lbs. of force. After several hard whacks, the metal has been reshaped into a rough crankshaft, which is then removed from the press for trimming and rough machining.
The forging process makes the metal denser. But it also distorts the grain structure, creating residual stresses that have to be relieved by heat-treating. With billet cranks, there is no compression or deformation of the grain structure, so there are less residual stresses in the metal.
Lightweight racing cranks are available for a wide range of engines. Lightweight cranks make sense for circle track and road racing applications, but they don’t offer much advantage on the street other than allowing the engine to rev faster (which can also be accomplished by running a lighter flywheel). In drag racing, a heavy crank is better because it helps maintain momentum off the line. In marine applications, a light crank may allow the engine to over-rev when the prop jumps out of the water so a standard weight crank is usually better.
There’s only so much metal that can be removed from a crank before strength and reliability are sacrificed. A stock SB Chevy crank typically weighs 50 to 55 lbs. depending on the stroke, journal size and type of end seal configuration. A comparable lightweight crank for the same application might weigh only 44 to 46 lbs., saving 6 to 11 lbs. of total weight. Some ultra-light cranks may weigh as little as 34 lbs.! The total weight difference is not as important as where the weight was removed. Removing weight from the outer areas of the counterweights and gun-drilling the throws will reduce inertia more than drilling out the mains or removing metal close to the axis of rotation.
In some instances, drilling out the mains offers another benefit: it helps equalize pressure within the crankshaft (a problem which plagued early Chevy LS engines and caused numerous oil leaks).
A couple of beers may be all YOU need for stress relief, but a crankshaft requires a more involved process. Heat-treating increases the temperature of the crank to a point where the grain structure in the metal starts to change. Holding the crank at a certain temperature for a certain length of time will relieve the residual stresses. Quenching (cooling) the crank at a controlled rate afterwards retains the positive changes in grain structure that increase tensile and fatigue strength.
The heating and cooling processes must be carefully controlled to achieve the best possible results. Any mistakes here can result in a weaker crank or a brittle crank. A cheaply-made crank may not have a very good heat treatment, and won’t as strong or as reliable as one from a supplier who knows what they are doing and who keeps a close eye on the heat treating process.
Shot peening the surface of the crank also helps improve strength and reliability by increasing surface hardness and eliminating stress risers that might form cracks. Cryogenic treatment (freezing to 300 degrees below zero in liquid nitrogen) is also said to relieve residual stress and improve durability.
To improve wear resistance, the journals may be induction hardened or nitrided. Most stock cranks are induction hardened because it is a fast, inexpensive way of hardening the journals. A low frequency electromagnetic induction coil is placed around each journal to heat the metal.
After the metal reaches a temperature where the grain structure undergoes a martensitic transformation, and the desired depth of hardening is achieved, the journal is sprayed with oil to quench the steel and preserve the hardened surface layer. This type of heat treatment typically leaves a hard layer up to .030? or more in depth (which allows the crank to be reground without removing all of the hardened layer).
To improve the wear resistance of the journals, most performance cranks are nitrided after they have been heat-treated. Some crank manufacturers use a “plasma nitriding” process that vacuum deposits ionized nitrogen on the surface of the crank inside a high temperature oven. Others use a process called Tufftriding that soaks the crank in a hot “ferric nitrocarburizing” salt bath, or heats the crank to 950 degree F in an oven filled with nitrogen.
Nitriding causes nitrogen atoms to penetrate the surface of the metal and make it harder. Nitriding typically doubles the hardness of the journal surface (from 30 to 35 Rockwell C to 60 Rockwell C). This also increases the fatigue life of the crank up to 25% or more. The depth of the hard surface layer may range from as little as a few thousandths up to .025? inches or more depending on how long the crank was left in the oven or salt bath.
Micropolishing is often done on the crank journals after they have been ground to size to improve the surface finish. Several crank suppliers we interviewed indicated they aim for a surface finish of 5 microinches or less on the main and rod journals.
Some crank suppliers also polish the entire crankshaft. Not only does this produce a cosmetically attractive finish, it also helps reduce the risk of surface cracks forming by eliminating stress risers. A polished surface also helps shed oil at low rpm, reducing windage and drag. Oil shedding coatings may also be used for the same purpose. But as one crank supplier said, at high rpm there won’t be any oil on the crank anyway because it will be flung right off.
Even so, there are power gains to be had from polishing say some crank suppliers. One company who applies a unique finishing process to their cranks that leaves a bright, chrome-like finish says the reduction in friction and oil retention is good for 1 to 3% more horsepower in a SB Chevy with no other changes, and up to a 4% increase in horsepower on a BB Chevy.
High revving racing cranks typically have counterweights that are shaped to cut wind resistance and drag as the crank spins in the crankcase. Drag slows the crank at high speed and robs power that could otherwise go to the wheels. A vacuum sump can do the same thing. But if you can’t run a dry sump to suck oil and air out of the crankcase, a crank with profiled counterweights is a plus. The most aerodynamic shape is a rounded leading edge on the counterweight, with a knife-edge trailing edge. Knife-edging also helps shed oil more quickly. Oil shedding coatings can also help in this respect. But at high rpm, the oil will be flung off the crank anyway.
Obviously, cranks are different than they were in your grandfather’s day.
For more information on specific manufacturers’ offerings, visit the 2011 Engine Builders Buyers Guides for contact information.
Latest posts by Larry Carley (see all)
- Cutting Tools and Abrasives for Engine Work - Jul 16, 2014
- Front-Mounted Oil Pumps: Good or Troublesome? - Jul 2, 2014
- Replacing Crankshafts, Connecting Rods and Bearings - Jun 27, 2014