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Selecting Cranks, Rods and Bearings For Your Next Build

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Racing engines are made for one thing – speed. But speed is a wide target. It is in the sum of parts that are bolted together to make up the package that speed is delivered to your customer. Engine builders know that one of the biggest areas where races are won and lost is in the rotating assembly of an engine – specifically it is the crank, rods and bearings that play a significant role.

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We use the chef comparison quite often because it makes sense. A chef and an engine builder are similar in many ways. There are the home cooks who go to the store and buy everything prepared so they only have to heat it up on the stove or in the microwave. Professional chefs go to culinary school to learn the finer points of mixing a roux, or what spices to use to create a distinct flavor profile. Engine builders have similar choices: you can buy a kit with everything you need or you can get into the finer points and make something unique with your own stamp on it. Either way you choose, you’re still cooking.

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Several manufacturers offer rotating assembly kits that are matched and balanced and ready to go. You don’t have to do anything except bolt it in. It’s a good choice for some builders but for others, it’s more satisfying to put it all together and balance it yourself. Of course sometimes it depends on the level of capability your shop has to do performance work. Do you have a balancer or do you have to farm that out? Other things to consider are how customized your engines are going to be.

If you are building mild street engines then you probably don’t need the equipment with all the bells and whistles, but you can assemble a kit and still come out ahead of the game and make your customers happy. It won’t take as much time or overhead to assemble from a kit but your profit will probably be less than if you did it all. With a kit you’ll save the expense of having to purchase expensive equipment you may or may not use regularly enough to pay for itself.

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Crankshafts

In many cases, a stock OE crankshaft will be fine for mild performance applications. But when the power starts to increase, the stock cast iron crankshaft will no longer be able to hold together. The pounding, twisting forces will wreak havoc on the cast crank until it fails. Most stock cast iron crankshafts are rated to about 80,000-100,000 psi of tensile strength, depending on how much carbon content is in it. The basic cast iron used for making cranks has a tensile strength of around 80,000 psi, but it’s on the brittle side.

A stronger crank material is nodular iron, which has a tensile strength of about 95,000 psi. The higher carbon content in this iron means it has almost twice the elongation characteristics of standard cast iron. Elongation is the ability to stretch and bend and then return back to its original shape without any distortion. This is an important property with higher end cranks because they can withstand the fatigue and pounding forces of combustion without failing.

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Some racing classes, however, require the use of a stock crank, which is usually a cast iron crank. And while you can certainly rebuild a stock cast crank, you never know what you’ve got. Cranks have a life-cycle and when not abused they can last for a long time before any fatigue issues start to set in. It is always a good idea to perform a magnetic particle inspection before going forward with a crank rebuild. You want to find any hidden defects as soon as possible so you don’t end up eating the job later.

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Performance cranks use certain alloying elements to change the physical properties of the component. Usually alloy steel depends on some type of heat treatment to develop specific properties, which primarily means tensile strength and fatigue resistance. Heat treatments in general can raise tensile strength of an alloy steel from as low as 55,000 psi to nearly 300,000 psi in some cases. As far as crankshafts are concerned, 165,000 psi is about the strongest billet steel crank.

Besides heat-treating cranks to provide additional strength and durability, journal surfaces are either hard chromed, nitrided or induction hardened. Some heat-treatments can double the surface hardness and increase fatigue life by up to 25 percent. Many of today’s high end cranks are nitrided, which involves injecting nitrogen into the surface of the crank to a depth of about .010? at a controlled heat. Air is vacuumed out and replaced with a chemical that penetrates the surface of the part. One of the drawbacks of nitriding is that the part has to be re-treated if you do any machine work to the crank.

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The manufacturing techniques used to make crankshafts are either casting, forging and machining from solid billet.  Forged steel crankshafts are widely used in performance applications. Forged cranks are strong and durable and can handle horsepower up to 1,500 hp. Forged crankshafts come in a variety of materials such as 1045 or 1053 with a tensile strength of about 110,000 psi. While this isn’t much more than a cast steel crank these cranks have 22 percent elongation, which is about 16 percent more than a cast crank. Forged cranks have better grain structure than cast as they are pressed into shape. Some of today’s forged cranks are called non-twist, which is considered to have an even better grain structure because the throws are not stretched into shape.

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If you’re building a big horsepower engine 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.

Connecting Rods

In mild street engines, stock connecting rods are good enough to handle modest power increases. Pushing the envelope with stock rods past about 400-450 hp and 6,500 rpm rev limit is asking for trouble, reliability will fall off at that point. When building engines with more horsepower than a mild street build it is recommended that you upgrade the connecting rods to a stronger material than what the stock rod materials can handle. From this point it depends on what you’re building because there are a lot of materials and designs to choose from.

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Experts say higher horsepower engines pound down on the connecting rods while higher revving engines pull the rods apart. The connecting rods typically don’t bend or break under compression but are pulled apart at high rpm and tend to break during the exhaust stroke. Because of this, performance rods are beefed up for compression strength and stiffness for the higher loads, and the higher tensile strength of the upgraded rods help keep them from snapping apart at high rpms.

Usually lighter is better when it comes to rods. In sprint car and drag racing applications this is especially important. In these types of engine builds an aluminum rod or even titanium would be used. The lighter the rods, the faster the acceleration, but aluminum can stretch over time and needs rebuilt or replaced more often than steel rods.

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A discolored con rod is the result of excessive heat. Experts say that a discolored rod is almost always caused by a spun rod bearing. Rod bearings spin due to a breakdown of the oil film between the crank journal and rod bearing. There is a lot of debate as to why this happens, but when discoloration is visible (even a very small amount) the rod has been heated enough to compromise or ruin the heat treatment in the steel. This can have unpredictable effects on the strength of the rod. It is impossible to be sure if the rod can be used reliably once this has happened. The best course of action is to replace the rod.

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Bearings

According to bearing experts, 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.

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 stretching loads generated at the top of each exhaust stroke, and the compression loads at the top of each compression stroke, according to experts. These loads may, over time, create an out-of-round journal and wipe away the oil film, causing eventual seizure.

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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.

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, resulting in 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.

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Coated performance bearings are also gaining more attention now. Coatings are used to help reduce friction and to protect the crank if there’s any metal to metal contact. Experts say it is another layer of insurance for protecting your customer’s expensive engine components.

Metallurgy

Whether in connecting rods or crankshafts, the alloy used in these components and the heat treatment the metal undergoes during the manufacturing process are extremely important for both strength and reliability.

One of the most popular choices for cranks and rods is 4340 steel. This grade of alloy is chosen because of its superior strength and durability. The American Iron and Steel Institute (AISI)  says that 4340 contains 1.65 to 2.0 percent nickel, 0.70 to 0.90 percent Chromium, 0.60 to 0.80 percent manganese, 0.20 to 0.35 percent silicon, and 0.20 to 0.30 percent molybdenum. These elements blended together give the steel its hardness, toughness, ductility and fatigue resistance.

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The ultimate tensile strength, yield strength and hardness of 4340 steel depends on how the steel is forged into blank or billet, and how the steel is heat treated. Component manufacturers have their own ways to temper and quench their parts that can produce very different results. So from manufacturer to manufacturer tensile strength, yield strength and hardness can vary by quite a bit.

Some manufacturers are starting to use 300M alloy. Some experts say that there is not that much difference between 4340 and 300M, however, 300M has a higher level of silicon and a little more carbon and molybdenum for added strength. This little bit extra strength can mean a lighter component as some connecting rod makers say it is good for a 10 to 20 percent weight reduction because they are able to thin out the cross-section areas of the rod.

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When steel is produced from recycled scrap, it’s not as easy to control the all of the alloying elements. It has become a big topic of debate as to whether foreign made products are of a lower quality than U.S. made components. While it is inaccurate to label ALL foreign-made product as inferior, suppliers who are concerned about the quality of their products are testing the forgings to make sure the steel meets specifications. And overseas manufacturing is in some cases on par with U.S. manufacturing. One high-end manufacturer of rods and cranks says it has many of the same capabilities as its U.S. counterparts. This manufacturer has supplied high-end components to top race teams and manufacturers in Asia.

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Certainly there are foreign-made components that don’t meet quality standards, which is why you need to know what you are purchasing and from whom, but that’s good advice no matter where the part is said to come from. Unfortunately, you will always have to be on your guard to watch out for knock-off brand name components. Talking to your suppliers and learning how to identify the authentic product will help you avoid this situation.

Performance engine builders have more choices of materials for rotating assemblies than ever in large part thanks to new manufacturing technologies and an aftermarket that is always finding ways to give you what you want – bigger and better components at an affordable cost. So make sure you do your homework before you select a crank, rods and bearings for your next build.in many cases, a stock oe crankshaft will be fine for mild performance applications. but when the power starts to increase, the stock cast iron crankshaft will no longer be able to hold together. the pounding, twisting forces will wreak havoc on the cast crank until it fails.most manufacturers offer rotating assembly kits that are matched and balanced and ready to go. you don
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