Crankshafts:Stock and Performance
Engine builders have to pay close attention to the condition of the crankshaft because it bears the brunt of the power produced by the engine
By Larry Carley
The crankshaft is the working arm of the engine. All the force generated by combustion and the downward motion of the pistons is focused on the crank throws.
The leverage effect of the force exerted on the crank journals twists the shaft and converts the up-and-down reciprocating motion of the pistons into rotational motion of the crankshaft. This creates torque that the crankshaft uses to turn the flywheel and rest of the drivetrain.
The rotating mass of the crankshaft combined with that of the flywheel and harmonic balancer also creates inertia and momentum that keeps everything spinning. This gives the crankshaft enough energy to push the pistons up on the compression and exhaust strokes, and to pull air and fuel into the cylinders on the intake stroke. It's basic physics in motion and the torque that's produced is what propels every gasoline and diesel-powered vehicle on the road today.
As those vehicles chalk up the miles, though, their crankshafts take constant pounding. Engine builders have to pay close attention to the condition of the crankshaft because it bears the brunt of the power produced by the engine.
Crankshaft wear is a concern as the miles add up. Journals must be carefully inspected and measured for wear, out-of-roundness, taper, hour glass or barrel distortion. If worn beyond specifications (typically more than .001" of specified diameter), the journals must be reground to undersize and polished to restore the bearing surface.
The amount of grinding that can be done on the crankshaft journals is limited by the depth of the case hardening as well as the availability of undersize bearings for the application. Bearings for ten, twenty or even thirty thousandths undersize journals are usually available, but on some passenger car engines ten thousandths may be the limit due to the thin case hardening or the small size of the journals.
When journals are too badly damaged to be reground or are worn beyond a certain limit, the journals can be built back up by hard chroming, metal spraying or welding - if the cost is justified. Welding large diesel and industrial engine cranks often makes sense because of the high cost of replacement cranks, but on most passenger cars and light trucks it's usually much less expensive to replace a worn or damaged crank than to weld and repair the original crank. Welding a crank also requires straightening and redoing the heat treatment, which adds cost. It all boils down to how much a customer is willing to spend on the repairs.
Every piston fires at a different instant in time, which creates vibrations in the crankshaft that grow in magnitude with the number of cylinders, the length of the crankshaft and engine speed. The constant pounding and stress may cause small hairline cracks to develop in and around journals, and particularly around oil holes. That's why crankshafts should always be examined with a magnetic particle inspection machine to check for cracks before they are reused.
Used crankshafts also have to be checked for straightness. The constant loading and pounding combined with wear in the main bearings may result in a bent shaft. Straightness can be checked by placing the ends of the crank in V-blocks and using a dial indicator to measure deflection in the center main bearing as the crank is rotated. If the crank is bent more than about .0015" (specifications will vary depending on bearing clearances), it must be straightened in a hydraulic press or replaced. Forged steel cranks will typically accept more correction than a cast crank. But if the crank is bent beyond the point where it cannot be straightened without weakening or cracking it, it must be replaced.
When To Upgrade
For stock engine rebuilds, a reconditioned stock crankshaft should be more than adequate. Most passenger car and light truck cranks are cast iron and have enough strength to handle the loads produced by a stock or slightly modified engine. If the OEM crank happens to be a forged steel crank (which are used in some high output engines as well supercharged, turbocharged and light diesel engines), so much the better, because forged steel is much stronger and not as brittle as cast iron.
Stock crankshafts are engineered to handle typical loads generated by a stock engine, say 250 to 350 hp in a small block V8, or up to 400 hp in a big block V8 with a redline of 5,000 to 5,500 rpm. But there is a limit on how much additional torque and rpms a stock crank can safely withstand if the engine is modified. Most stock cranks have enough built-in margin of safety to handle your typical aftermarket "bolt-on" modifications such as a street hydraulic cam, a performance intake manifold, larger carburetor, exhaust headers and even aftermarket performance heads.
But when an engine is being built to produce serious power or will be used for racing, a stock crank may not be up to the task. When the loads experienced by a crankshaft start to double or triple over that of a stock crank, all bets are off as far as durability is concerned.
According to most of the performance crankshaft suppliers we interviewed for this article, engine builders should seriously consider upgrading from a stock crank to some type of performance crank when the horsepower output of a smallblock V8 is pushed beyond 400 to 450 hp. For a big block Chevy, Ford or Chrysler motor, a stock crank can usually handle up to 550 hp. But beyond that, a stronger performance crank is needed to handle the additional power.
Often the weight and profile of a crankshaft is determined by the class rules for the type of racing. In Hooters Cup racing, for example, the rules dictate a stock crankshaft to keep the costs down.
Engine speed is also a critical factor in crankshaft loading. The loads on the crank go up exponentially with rpm. Consequently, if an engine is built with a valvetrain and connecting rods that can push the redline beyond 6,000 rpm, a stronger performance crank will probably be needed. And if the engine will be spinning in excess of 7,000 rpms (like running flat-out at 9,500 rpm lap after lap in a NASCAR race), a performance crankshaft is an absolute must.
Performance cranks do several things for an engine. First and foremost, they improve durability. The improved strength of a performance crankshaft allows it to safely handle higher loads and rpms without twisting, cracking or failing.
Performance cranks are also finished to a much higher standard of accuracy and precision than ordinary cranks. Tolerances are much tighter, journals are polished to perfection, and counterweights are given knife-edges or other special profiles to reduce windage, aerodynamic drag and to improve oil control. Oil holes are cross-drilled and chamfered to improve lubrication. Special slots to direct the oil flow may also be machined into the journal surfaces.
Performance cranks are usually treated to improve strength and durability. Nitriding and nitrocarburizing are two low temperature, low distortion "thermochemical" heat treatments that are appropriate for crankshafts.
Nitriding can be done by heating the crank to 950° F in an oven filled with nitrogen. Nitrocarburizing is accomplished by soaking a crank in a hot "ferric nitrocarburizing" salt bath. Nitrogen reacts with the metal to case harden the crank to a depth of .020" to .030", depending on the duration of the process. Nitriding roughly doubles the hardness of the surface from about 30 to 35 Rockwell C to 60 Rockwell C without affecting the base metal underneath. Nitriding may add about $200 to the cost of the crankshaft.
Special coatings may also be applied to non-journal surfaces so oil doesn't stick to the crank and high rpm and increase drag. Some cranks are also cryogenically treated to relax and improve the grain structure of the steel.
The strength of the crank not only depends on the base alloy (typically 4340 steel) and how it was made (whether it was drop-forged or machined from a solid chunk of billet steel) but also the size and design of the fillets at each journal, the size, shape and placement of the counterweights, how the oil holes are located and drilled, the heat treatment, case hardening and shot peening of the crank surfaces and cryogenic treatment (if used).
Performance cranks are also available with reduced weight. A lightweight crank with cut down counterweights and drilled throws may be as much as 20 lbs. lighter than a comparable stock crank (say a 33 lb. SB Chevy racing crank versus a 54 lb. stock crank). Most crank suppliers say the trend to lighter and lighter racing cranks in recent years may have bottomed out because most of the weight that can be safely taken out has already been removed and there's not much room left for further reductions.
The theory here is that reducing the weight of the crankshaft by machining off any "unnecessary" metal reduces weight and inertia, allowing the crank to change rpm more quickly and rev faster. But the ability to do this depends on the location of the weight with respect to the center axis of the crankshaft. Weight that is closer to the center of the axis of rotation has much less effect on inertia than weight located out on the crank pins or counterweights.
A super light crank makes sense in an application like a sprint car on a short dirt track where the driver is constantly on and off the throttle in the corners. But in a drag motor or a NASCAR motor that runs at full throttle, inertia is much less important than durability. Because of this, many crank suppliers recommend running a heavier crank that has added stiffness and strength in a drag motor or NASCAR motor to keep the bottom end of the motor intact.
Stan Ray of Ohio Crankshaft Co, Greenville, OH, says his company only sells a big block Chevy crank with center counterweights because it's more durable than cranks that lack the center counterweights. "An extra 10 or 12 lbs. of weight in a drag motor will have virtually no effect on inertia but will have a big impact on how long the crank holds up. Most drag racers want durability."
A crankshaft that's too light can also flex and lose horsepower - or fail, which is something to keep in mind, too.
"Most every application is generating more power than ever before," says Duane Boes of Callies Crankshafts, Fostoria, OH. "Stresses from increased rpm and greater gas loads are forcing people to invest in crankshafts made of better material. The Gen III small block engine by General Motors is rapidly gaining popularity. It's yet to be seen which four-cylinder engine will become the future standard of the performance industry. In terms of acceptance and aftermarket support, there may never be another small block Chevy."
Forged Vs. Billet
Is a forged steel crank better than a machined billet crank? Some say yes, some say no - and it depends on what you mean by "better."
Forgings generally produce a flowed grain structure that provides great strength parallel to the lines of flow. The metallurgy of a forging also depends on the alloy used and the heat treatment that is applied to it after it has been shaped. The one drawback of a forging is that it requires a die to shape the metal. Dies and forging presses cost a lot of money, so the availability of forgings for various applications depends on their popularity and how much people are willing to pay for a forged crank. According to Boes, "The vast majority of performance-related cranks in use today are made from forgings, even some that are claiming to be billets."
Brian Adix, of Howards Cams in Oshkosh, WI, says his company sells two basic forged crankshafts, a "Track Smart" forged crank, for SB/BB Chevy (their least expensive cranks), and "Pro Series" cranks that are also 4340 forged steel, have drilled rod throws for lightness and are "Permatuff" heat treated for SB/BB Chevy racing engines.
He says these forgings are done by Trenton Forging, one of the few remaining U.S. forging operations.
Billet crankshafts, by comparison, are CNC machined from a solid chunk of metal. The machining process cuts through the grain structure so there are fewer residual stresses in the metal as there are in a forging.
Proponents of billet cranks say billet steel alloys also tend to be "cleaner" in terms of impurities and inclusions, which improves strength and durability of the finished product.
The main advantage of billet cranks, however, is that CNC machining allows the supplier to custom fabricate a crankshaft for virtually any engine. No mold or forging die is necessary, so there's no limit on the shape or dimensions of the crankshaft.
A one-of-a-kind crankshaft can be created to any specifications. This gives high performance engine builders a great deal of flexibility in choosing stroke lengths, journal diameters, the positioning of the journals and counterweights, and the configuration of the crank itself.
If somebody needs a performance stroker crank for a sport compact engine or an oddball application, a billet crank can be tooled up from scratch. Of course, billet cranks require a LOT more machining than forged or cast cranks. As a result, they cost more. Billet cranks typically fetch anywhere from $1,800 to $3,000 (or more) depending on the amount of work required to make it. A typical street price for a small block Chevy stroker billet crank is around $1,900 to $2,000. A comparable big block Chevy crank would be a couple hundred dollars higher.
Joe Squires of Bryant Racing, Anaheim, CA, explains that 90 percent of the crankshafts his company produces and sells are billet cranks. "Billet gives us the flexibility to control every aspect of production, from the placement of the crank pins and size of the journals to the location and profiles of the counterweights. We use two different alloys depending on the application, either 4340M or EM30B. We also do our own heat treating in-house and cryogenically treat every crankshaft to improve durability."
In recent years, the trend has been to bigger and bigger displacements thanks to the increased availability and affordability of aftermarket blocks. As blocks have grown, increased internal clearances have also allowed for longer stroke crankshafts.
Ohio Crankshaft's Ray says that, a couple of years ago, his number one selling crankshaft was a stroker to create a 383 Chevy small block. Today it's a stroker crank to build a 434 small block - which will be going to a 468 soon. He says it's the same story with big block stroker cranks. The best-selling big block stroker crank used to be for a 496 cid motor. Last year, that increased to a 555 cid motor, and now they're selling stroker cranks for 632 cid motors.
The new IHRA Prostock rules will allow up to 5 inch cylinder bores with a maximum displacement of 820 cubic inches! That's a lot of motor, and it requires a lot of crankshaft to create and handle that much displacement.
Small block and big block Chevy V8s followed by small block and big block Fords, a few Dodge/Chrysler V8s and a handful of Oldsmobile and Pontiac V8s have been the mainstays of the performance aftermarket for many years. But as time marches on, other engines are starting to develop a following and demand for performance replacement parts, including cranks, grows.
"The engine remanufacturing business has changed in recent years," says Tom Lieb of Scat Enterprises, Redondo Beach, CA. "The demand for everyday replacement engines is down, and traditional rebuilders need other opportunities to grow their business.
"One such opportunity is the restoration market: muscle cars and popular cars from the 1950s, 1960s and 1970s. The demand for crankshafts and connecting rods for these engines is up, but the cores are not available," explains Lieb. "We have cast cranks, forged cranks and billet cranks. We've been supplying engine builders with standard cranks and continue to add coverage."
As for performance cranks, Lieb explains that Scat has been building performance cranks since 1966. "We've been doing Volkswagen cranks since day one, so we know what makes performance cranks work."
The technology needed to make a performance crankshaft for a V8 or any of today's sport compact engines is really no different than what has been learned from building VW crankshafts, says Lieb. The basic technology is the same.
"One of the problems with most of the OE crankshafts in many Japanese engines is that the oiling system is not drilled for higher loads or horsepower. A lot of guys are burning up rod bearings in these engines because the bearings aren't getting enough oil. When we do a performance crank, we redo the oiling system so durability isn't an issue."
Lieb says Scat currently offers a forged crank for Honda, as well as forged cranks for small and big block domestic applications, and can make billet cranks for virtually any other sport compact engine.
In many cases, competitive race rules put limitations on what customers are able to install in their engines.
"We're receiving more calls from people who want a 50 lb. small block Chevy crank than a 30 lb. crank," says Wendy Carr of Callies Performance Products, Fostoria, OH. "The rules really dictate what kind of crankshaft our customers buy."
Carr says Callies offers several lines of crankshafts, including Magnum X-L, Magnum, Racemaster, DragonSlayer, SC-4 Sport Compact, Chrysler Hemi and CompStar.
"Our newest offerings are the DragonSlayer 4340 forged cranks, which are made in the U.S.A., and are available with up to a 4" stroke for small block Chevy, and 4" and 4.250" strokes for big block Chevy. We also have a DragonSlayer crank for Mopar 340 engines, too."
Carr says the new lower priced CompStar crank line uses imported 4340 forgings that are final machined by Callies. The CompStar is currently available for small block Chevy only, but will soon be available for big block Chevy as well.
Also new is the SC-4 Compact crank line for Mopar 2.0L and 2.4L engines. The crank is available with a 3.543" stroke. According to Robert Loftus of Eagle Specialty Products, South Haven, MI, there is a growing diversity in the performance market. "A lot of people read that to mean imports or sport compacts. But it has really spread into the less-than common domestic V8's.
"We are seeing a growing interest in imports and sport compacts, of course, but also in Pontiacs, Ford FE's, even other domestic V8 from years gone by that most of the country had forgotten about," Loftus says. "Maybe these enthusiasts have just started coming out of the closet because they are encouraged by the aftermarket's renewed acceptance of products other than a Chevy or Ford small block. This is opening new doors for expansion for our company and new sources of revenue. As we make new products, the customer base grows. This, in turn, encourages new products."
Loftus explains that his company has recently added small block Chrysler cranks in cast steel and forged 4340, and Buick 3.8L V6 in forged 4340. "We are developing cranks for Mitsubishi 4G63 and 4G64, Ford 4.6L and 5.4L modular engines. We are exploring the possibility of making cranks for several other imported 4- and 6-cylinder engines as well as some domestic 4-cylinder, 6-cylinder, and V8s," he says.
Several other companies not currently offering a performance crank or stroker crank for sport compact cars indicated they are looking at this market and/or are developing cranks for these applications. But one supplier said the sport compact car market is a "different" kind of market than the traditional small block/big block Chevy market. For one thing, the stock cranks are fairly strong and seem to hold up well under the rigors of turbocharging and nitrous oxide. Also, the type of engine modifications that owners of these vehicles are typically making are induction bolt-ons and head swaps - not modifications to the block or bottom end. But as power goes up, so does the load on the crankshaft.
Balancing is a must for any high revving performance engine, and with stroker engines it is a must to offset the increased reciprocating mass of longer rods. Stroker cranks often require the addition of heavy metal plugs to the counterweights. On a very light crankshaft, having to add several pounds of heavy metal to a counterweight may also offset much of the weight advantage of the lighter crank. So for many stroker applications, a lighter crank may be more difficult and expensive to balance.
Eagle Specialty Products' Loftus says many engine builders lose a lot of time and money guessing what to charge for a balancing job. "They need to be able to quote the customer a price for balancing beforehand. They do not want to charge too much and lose the sale, but they can't be too cheap and run into a problem and lose money."
Loftus explains that, to help engine builders, his company has started publishing a "target bobweight" for its cranks. This allows the engine builder to estimate how much work he'll have to do to balance the crank. "Our cranks are guaranteed to be within 2 percent of this target bobweight. Customers should not confuse this with actual balancing, though. Our cranks do need to be balanced. This just gives the shop an idea of what they will need to charge before they actually do the job."
Sourcing A Sore Spot
For many crankshaft manufacturers, the world's steel supply has had a direct effect on business. Rising steel prices, high labor rates and a loss of forging facilities in the U.S. have created market conditions that favor importing lower cost raw forgings or even totally finished crankshafts from overseas manufacturers.
Kerry Novak of Crower Cams, San Diego, CA, says steel is a major issue. "China has sucked the U.S. dry of steel. They are buying up all the steel they can, creating a shortage for U.S manufacturers. As a result, our costs to do billet cranks has risen dramatically and we have steel on backorder."
Steve Slavik of Lunati Crankshafts, Memphis, TN, says his company was forced to raise prices in December because of the increase in the cost of steel. "We use a 4340 non-twist forging alloy that is made in the U.S.A. Our Top Line Pro Series cranks are a very precise, zero tolerance design that require extensive machining so they are expensive ($1,998 for a SB Chevy). To compete with the influx of import cranks, we have introduced a 'Sledgehammer' series of performance cranks for small block Chevy (a big block crank is planned) that sell for half the price of our Pro Series cranks."
"The Sledgehammer cranks are still made with a 4340 forging right here in the U.S.A., but we are not doing as much machining to keep the cost down. These cranks don't have a contoured leading edge on the counterweights, for example, and are available in two different strokes,"
Crower's Novak says a growing number of crank suppliers are now importing forgings and even completely finished cranks from overseas suppliers. This has created a lot of price pressure on crank suppliers who still use only U.S. forgings. Some have been forced to raise prices while others have been able to hold the line - at least for now.
Raul Negrete of Cola Performance Crankshafts, Whittier, CA, says this has created some serious issues within the industry.
"The main issue," said Negrete, "is that some people are selling performance crankshafts that they claim are made in the U.S. when, in fact, the cranks were not manufactured here. They are made with imported forgings that are finish ground and polished here. But the cranks are not manufactured here and shouldn't be labeled as such. It's misleading."
Negrete says the lower price of imported crankshafts makes those who are selling only domestically forged cranks look bad because there's no way the domestic cranks can complete on price. "You can buy completely finished cranks for as little as $140 each. Yet for a U.S-made forging, my cost for the raw materials is $240."
Howards Cam's Adix expresses concerns that "A lot of people are bringing in forgings from overseas but selling their cranks as 'Made in U.S.A.' because they do the finish work here. I think that's misleading."
Says Negrete, "California law says any product that claims to be made in the U.S.A. must be totally made in this country, not just part of it or the final machining," Negrete hints that within the next few months legal battles over labeling concerns may bring the subject to the attention of the industry.
Duane Boes says his company avoided the use of off-shore products for as long as it could, but says the company could eventually no longer overlook the needs of its engine building customers. "We do offer an independent line of products that are forged and semi-finished at various offshore locations," says Boes. "Out of respect for our customers and their integrity, Callies will not hide this fact or disguise these products with an American sounding brand."
Pricing-pressure aside, Boes says imported products don't necessarily have to be of lower quality. "Our efforts to evaluate potential suppliers have been extensive. We have personally visited most crankshaft and connecting rod manufacturers in China, Europe and India. To ensure consistent product quality, specialists have been retained to systematically review our suppliers' processes and capabilities.
"Because all of our CompStar components are finished at Callies, we are able to maintain proper geometry while monitoring both metallurgy and design," Boes says.
HEAVY-DUTY CRANKSHAFTS -
PERFORMANCE ON A DIFFERENT SCALE
Crankshafts for big heavy-duty trucks, agricultural and industrial engines represent a significant investment for engine builders. Replacement cranks are expensive so it's often more economical to repair and regrind worn or damaged crankshafts.
Randy Madden of Quality Power Products, Solomon, KS, says some of the newer cranks that John Deere is using are breaking and can't be repaired, which is good for his business because his company supplies cranks for these engines. "They went from a forged crank to a cast crank with rolled fillets in their 300 diesel engines. This crank can only be reground to .010" undersize. If it is worn more than that, you have to replace it."
Madden says there may be a trend among engine manufacturers to switch to less expensive cast cranks in applications that normally require forged cranks. It's a cost-reducing measure for them, but it also limits the ability to service the crank down the road.
"If you have to weld or rechrome a worn crankshaft, it costs so much that it is often less expensive to replace it with a new one - especially when you factor in freight charges to ship a crank both ways and core charges."
Madden says, liket the automotive market, the heavy-duty market is also experiencing an influx of low-priced imported crankshafts. He says these are mostly castings and, from his perspective, the quality can be very inconsistent. "In my opinion, there is no substitute for a quality-made forging."
Madden says his company sells bare crankshafts rather than complete kits, and that all the crankshafts are refinished to OEM specifications.
"On engines where torque-to-yield bolts are used, such as many of the John Deere engines, the torque-to-yield bolts should be replaced. They may fail if reused."