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Engine Bearings and Crankshafts, What makes them [not] tick?

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The relationship between the crankshaft and main bearings is an integral one. Any metal-to-metal contact can have catastrophic results, and if the carnage was the result of improper finish or fitment, you’ll likely get to work on your relationship skills again.

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There are many analogies to explain how its parts relate to the engine as a human body. A crankshaft is like the spine of the engine because it controls engine power, albeit indirectly, through its stroke and displacement. The crankshaft is what transfers the up and down motion of the pistons and connecting rods into the more useable rotating motion that transfers power to the drivetrain. The crank also must lug the weight of the rotating assembly and be able to handle combustion loads. The engine bearings, too, must handle the loads from the crank and rotating assembly. So there is a great demand placed on both the crankshaft and engine bearings. We will review some of the things that make these two important components tick (or in some cases, not tick).

  • Selecting a crank

    There are many styles and materials available when choosing an aftermarket crankshaft – whether or not you go for a stock or performance crank will depend on your application. If it is for racing, it will depend on what kind of racing and how much horsepower you have. If it’s a performance street application you may decide to modify the stock crank instead of install a replacement. In most cases, for a stock “grocery getter” engine you will likely replace with OEM quality. Experts say that unless there is a good reason to upgrade it is best to stay with a stock crank.

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    The most common style of crankshaft is a cast iron crank found in many stock passenger car and light truck applications. While good for stock applications, these cranks are not well suited for high horsepower performance applications. Although some experts say you can run a stock cast crank if properly prepared in high performance applications, others ask why take that chance with an expensive high horsepower engine? Ultimately it is up to whatever you and your customer decide.

    The range of materials found in today’s crankshafts goes from nodular cast iron to 4340-alloy steel. The material that is best for you depends on the application. If you are building a stock engine “grocery getter” you won’t need to upgrade to a high performance material, however, if you’re building a performance engine, depending on the application, you may want to upgrade to a stronger more durable material.

    The next step up from cast iron is the forged steel crank made from either 1038 alloy steel or 1053 alloy steel. The 1038 steel is very basic and contains just enough carbon for heat-treating and a little extra durability. A forged alloy steel crank is very durable but it too has its limits. The strongest crank with the best material is a 4340 billet steel crank, but it is not inexpensive. These cranks are primarily used on high-end racing applications.

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    When rpm and horsepower increase a cast iron crank will not be able to withstand the load, because as rpms increase so too does the load carrying capacity of the crank and bearings. So, especially in high-end applications, upgrading to high strength materials is a must.

    The basic differences between a cast, forged or billet crank are its strengths. A cast crank is the easiest, least expensive way to manufacture a crank, and that’s why most stock OEM cranks are made of cast iron. They are strong enough for the daily driver and generally will live forever in that environment, but when you start adding performance modifications there isn’t enough strength to support the extra load. Typically a stock cast crank is rated at about 95,000-psi tensile strength, which is okay but not compared to the higher-end cranks rated up to 165,000 psi.

    A typical stock or mild performance crank that is made from 1053 high-carbon alloy steel is rated at around 100,000-psi tensile strength (resistance to failure under high load). A stock cast crank can generally handle up to about 350-400 hp applications with a 5,000-5,500 rpm limit. A typical upgrade from the 1053 cast iron crank would be a forged 5140 steel shaft that has a tensile strength of around 115,000-psi. According to one expert, the “40” in 5140 represents the carbon level, which is approximately 40 percent in this case. It’s a good upgrade from 1053 because it has less carbon (40% vs. 53%) and it contains chrome and silicone for improved durability.

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    Some experts recommend using a forged crank whenever power and rpm levels increase beyond mild performance levels. Forging increases the density of the metal as it is squeezed and compacted into shape, which results in a stronger core and better fatigue resistance. For use in high performance applications the material must have high tensile strength and high fatigue strength, which simply put, is the ability to resist breaking under load and failure due to repeated bending and twisting.

    If you’re building a big horsepower engine such as a 350-hp-plus small block V8 or a 450-hp-plus big block, you may want to consider upgrading to a forged or billet crank made of 4340 alloy steel, which is the strongest material available with a tensile strength of 140,000-165,000 psi. The 4340 billet steel crank has the highest fatigue strength (around 160,000-165,000 psi). The biggest difference in the various steel crank materials is the grain structure, heat-treating process and the mixture of elements. Cranks made of 4130 or 4340 for example, have higher amounts of chrome and nickel, which makes them stronger.

  • Crankshaft Welding and Reconditioning

    It’s not always a simple decision whether or not to recondition a crankshaft. Crankshaft journals must be carefully inspected and measured for wear, out of round, taper and distortion. Based on these important factors, you must make the decision to polish, regrind and replace bearings with undersize or build-up the crank journals by welding and grinding back to standard size.

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    Some rebuilders we spoke to say this may take too much time and be too expensive (especially with regard to labor) when there are relatively inexpensive replacement cranks readily available. This doesn’t mean you will not have to repair any more crankshafts, however, it may be far easier for you to regrind and polish a crank than to go through all of the necessary steps of welding and grinding the journals and then straightening the crank afterwards.

    It’s an involved process that is as much art as it is science. Therefore experience and cost may be key contributing factors to why some engine builders prefer to send this type of work to specialists. However, a skilled hand at doing this type of work may land a custom engine rebuilder (CER) lots of work in his area.

    Reconditioning large industrial and diesel crankshafts often makes sense because of the high cost of replacement. And many performance cranks are reconditioned; again, because of cost factors it is often times less expensive to repair an otherwise very expensive racing crank. An added benefit to repairing a crank is if you want to add more stroke to the crank you can fairly easily at that point. But on many passenger car and light trucks it’s usually less expensive to replace the worn or damaged crank than it is to repair the original component.

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    No matter what you do, however, some cranks will not be worth reconditioning. It is important to thoroughly inspect every crank before reconditioning so you can assess it. It can be a very fine line between reconditioning a crank and purchasing a replacement. If it’s too far out of spec it may take too much time and labor to repair versus buying a replacement crank.

  • Surface finish

    The crankshaft rides on a very thin oil wedge only about .00005? thick when the engine is running. With tolerances this tight, a properly polished crankshaft is a must. If there are any nodules or burrs poking through the surface of the journal it won’t take much to wipe the oil film and cause a bearing failure.

    Crankshaft grinding is considered by many to be an art form if you are polishing with a manual belt machine. But with today’s OEM finishes being extremely smooth and flat, achieving this level is more and more important. There are some good machines available for polishing and micropolishing, no doubt, but they must be used properly.

    Customarily with an aftermarket crankshaft, rebuilders mic the journals and go through a two- or three-step polishing process. If the crank proves salvageable and it doesn’t have to be ground, some engine builders start with a #400 grit belt, moving to a finer cork belt or other fine grit micropolishing belt for final finish. Other experts say to start with a #320 belt, then go to the #400 before moving on to the finer belt for a few revolutions.

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    Micropolishing machines are the high tech way to achieve OEM-like surface finishes, but costs may put these machines out of reach of smaller shops. According to one manufacturer, micropolishing is the most advanced way to achieve OEM-level surface finishes on cranks today. He cautions rebuilders who believe that using a very fine belt won’t remove material that this idea is a myth. With micropolishing it is possible to consistently remove the peaks and get down closer to the valleys in the surface creating a finer finish with higher load-carrying ability.

  • Heat-Treating

    Steels that contain specified amounts of alloying elements, besides carbon and the acceptable levels of manganese, copper, silicon, sulfur and phosphorous, are known as “alloy steels.” Alloying elements are added to change mechanical or physical properties. Usually alloy steel depends on some type of heat treatment to develop specific properties, which primarily means tensile strength and fatigue resistance. Heat treatments can raise tensile strength in alloy steel from 55,000-psi to nearly 300,000-psi in some cases.

    Most performance cranks are heat-treated and case hardened to provide additional strength and durability, and journal surfaces may be hard chromed, nitrided or induction hardened. Some heat-treatments can double the surface hardness and increase fatigue life by up to 25 percent.

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  • Balancing

    Balancing reduces internal loads and vibrations that stress metal and may eventually lead to component failure. Every engine regardless of the application or its selling price can benefit from balancing. A smoother-running engine is a more powerful engine with less energy wasted by the crank, which in turn produces a bit more usable power at the flywheel. Reducing engine vibration also reduces stresses on other internal and external accessories.

    Though all engines are balanced from the factory, the balance is lost when you replace pistons, connecting rods or the crankshaft itself. The factory balance is based on the reciprocating weight of the OE pistons and rods., so if any replacements or substitutions are made, there’s no guarantee the new or reconditioned parts will match the weights of the original parts closely enough to retain the original balance.

    Most aftermarket replacement parts are “balanced” to the average weight of the OEM parts, which may or may not be close enough to maintain a reasonable degree of balance inside the engine. Aftermarket crank kits can vary considerably in their balance because of variations within engine families.

  • Failures

    According to bearing manufacturers, debris is the number one cause for bearing and crankshaft failures. Debris can become trapped between the journal and the bearing surface, causing all kinds of headaches such as scoring the journal or removing the oil film, which, in turn, will lead to bearing seizure. Misassembly and misalignment are other leading causes of bearing failures.

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    Lack of lubrication is another cause of failure. Oil starvation and dry starts are extremely hard on bearings and journals. Any metal-to-metal contact will destroy both the bearing and crankshaft surface.

    Crankshaft failures often involve severe impact to the rod journal bearing surface. Rod journals show the most amount of wear over time from the loads generated at the top and bottom of each exhaust stroke, according to experts. These loads may, over time, create an out-of-round journal and wipe away the oil film, causing eventual seizure.

    Another cause of crankshaft failure is due to bending fatigue as opposed to torsional twisting. The vibration and flexing that can occur at high rpm and loads can cause cracking to develop in the journal fillets. Cracks may also spread from oil holes in the journals, so chamfering these oil holes helps to relieve stress as well as lubricate the bearings.

  • Selecting bearings

    Eventually, there comes a time in every engine’s life when the crankshaft bearings will need to be replaced and the worn surfaces restored. The type of bearings used in an engine will primarily depend on what the manufacturer used, but depending on what your customer wants you may wish to upgrade to bearings that suit the appropriate load.

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    In production engines, primarily the bearings of choice are aluminum/silicon (bi-metal) and copper/lead/babbit (trimetal). The trimetal bearing has more load carrying capacity at roughly 12,000 psi vs. about 7,000 – 10,000 psi for aluminum, and more resistance to wiping and scoring under heavy load. But aluminum/bi-metal has a higher melting point than copper/lead bearings, which provides protection against localized overheating due to detonation, overloading, misalignment and other such maladies.

    Aftermarket bearing manufacturers generally have to follow what the OEMs do as far as bearing material is concerned. But if an OEM design has a problem, aftermarket manufacturers are sure to react with a solution. In fact two of the main aftermarket bearing suppliers also supply the OEMs and therefore can study the OEMs’ failures and come up with the best solutions. Today, most late-model passenger car/light truck engines use aluminum bearings. Aluminum bi-metal is the bearing material of choice because it tends to be harder along with the silicon and tin, which adds to fatigue and seizure resistance and helps to polish the journals, leaving behind a very smooth surface free from imperfections.

    Traditional main bearing construction is based on three-layers, which include steel backing, copper-lead and a thin overlay of babbit material. Tri-metal bearings have more conformability and are more resistant to high loading pressures; therefore this type of bearing is commonly used in performance and heavy-duty applications.

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    Bearing suppliers tend to take sides in the debate over aluminum vs. trimetal and say one type of construction is better than the other. But both tri-metal and bi-metal (aluminum) are better in certain applications. The embedability of tri-metal and high load carrying ability make it attractive in many applications where there are extreme loads (i.e., high horsepower, high rpm, high cylinder pressure, etc.). Aluminum bi-metal is preferred for reasons such as the ability to select very accurate sizes due to fewer layers that are bonded together and its fatigue and seizure resistance, among others.

    Most standard bearings work fine up to a point. But in high horsepower, high load applications, you will need to upgrade to a performance bearing. Crankshafts used for high performance applications typically have an increased fillet radius for strength. If standard bearings are used in these applications fillet ride may occur, causing crankshaft failure. Check with your supplier for bearings with increased chamfer. Most performance bearings are tri-metal because of the increased load carrying ability and conformability characteristics. Coated performance bearings are also gaining more attention now that two major manufacturers have introduced them in the last year.

  • Coatings

    Coated bearings have been around for a long time, but until recently they were not readily available. Now the two leading bearing manufacturers offer their own line of coated performance bearings. Various types of coatings have been used to help reduce power robbing friction and improve wear resistance and heat management. However, the main purpose of coated bearings is to protect against dry start damage and lost oil pressure. It is just another layer of protection should the thin layer of oil the crankshaft rides on go away.

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    Manufacturers use different blends of polymers for their coated bearings, however most are bonded to the bearing with a very thin spray that adheres to the metal surface. The coating is typically not thick enough to affect fitment so you can run tighter tolerances.

    One manufacturer claims its coatings will extend bearing life up ten times that of an uncoated bearing in racing applications. And there have been many stories of racers running out of oil during a race but continued to run due to the coating on the bearings.

    There are many things to consider when selecting engine bearings and a crankshaft; whether it’s for a high performance application or a daily driver, the right choices will make a big difference in the durability and performance of your engine and ultimately in your customer’s satisfaction.

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