Diesel Cranks and Connecting Rods - Engine Builder Magazine

Diesel Cranks and Connecting Rods

The demand for mid-size diesel performance products continues to grow.

There tends to be two sides of the mid-size diesel spectrum: on one side there is street performance and on the other there is all-out race. Street performance diesels deal mainly with engine parts that are primarily for bolt-on applications and can make great power gains without sacrificing the integrity of the vehicle as a daily driver.

117363DSC03604a_00000067363All-out racing diesel engine builders typically get more involved in custom made internal engine components in order to withstand the stress from the huge amount of torque they produce. In a typical mid-size diesel engine, for example, replacement parts such as pistons are pretty easily obtainable. But this is not the case with diesel cranks and rods, for the most part.

If something happens to the crank or rod of a daily driver or street performance truck, odds are that you won’t buy a replacement from the manufacturer. The prices are so high that you (and your customer’s budget) might consider buying a used engine instead.

That’s what happened with a friend of mine when the crankshaft broke in his Dodge Cummins work truck. The engine was totally stock and had started knocking. The oil pan was removed to reveal that the crankshaft had broken in half. It was later discovered that there was some sort of error in a batch of crankshafts from the foundry.

If you do some research, you will find that there are no remanufacturers of mid-size diesel crankshafts or connecting rods. There are a few for sale on some auction websites, but how do you know what you are purchasing? But if a used crankshaft or connecting rod is your only option, make sure you have the equipment to repair it or have access to a shop that does.

Unfortunately, there aren’t many machine shops in the crankshaft and connecting rod repair business anymore. Fortunately, however, my local machine facility is equipped to recondition these components. So if you do have a reputable facility to perform these tasks correctly, that’s great. But if you do not, there are some things to think about before sending your parts out for machining.

Will the crankshaft be properly indexed? Will the rods be checked for roundness? Will the parts be balanced? What about magnafluxing and shotpeening? Be sure to ask what services they provide.

As for all-out performance, whether for drag racing or tractor pulling, the aftermarket has stepped up. There are several crankshaft manufacturers that offer custom crankshafts for Power Stroke, Duramax and Cummins engines. Due to the high torque loads and cylinder pressures these engines can generate, a lot of engineering and work goes into making a custom crankshaft for these applications.

For instance, a custom crankshaft for a mid-size diesel can cost around $7,500 for some applications. A very high price to pay, but there are more than 100 processes that some manufacturers put their crankshafts through before the customer receives the final product.

Some aftermarket manufacturers offer custom H-beam and I-beam diesel connecting rods with either a slant cap or straight cap, depending on the application.
Some aftermarket manufacturers offer custom H-beam and I-beam diesel connecting rods with either a slant cap or straight cap, depending on the application.

A stock diesel crankshaft is typically forged, a process in which a chunk of steel, usually made of 4340 or 1020 alloy, is heated to a certain temperature, and pounded into shape with a forging die. This is why forged cranks have such wide parting lines due to the material that oozes out during the process. Forging offers strength compared to casting. A “cast” component, on the other hand, is made when the material is melted and poured into a mold. But it could leave areas where the material is thicker or thinner and is prone to stress cracks.

A custom crankshaft for diesel engines is made from a solid chunk of  billet steel that is literally carved into shape. The steel is usually 4340 alloy, which is the most common for crankshafts due to the fact that it offers high tensile strength (the maximum stress that a material can withstand before failing).

Also, 4340 responds well to heat-treating, which is performed after machining and increases the tensile strength further. During the heat treatment process the crankshaft is subject to extreme heat in order to control the behavior of the material often referred to as its “grain.” This process offers core hardness, and the crankshaft is hardened all the way through, increasing the stiffness of the material.

Some crankshafts are “nitrided,” a process during which nitrogen is diffused into the surface of the metal to create case-hardening. This is used to increase bearing life and durability.

What are some things you need to consider when purchasing a diesel crankshaft? First, what do you want it to weigh? The rule of thumb is if you wish to lighten the crank, you can safely reduce the weight by 5 percent, but no more than 10 percent. The more you lighten the crankshaft, the less durable it will be. But a lighter crankshaft has less rotating mass, so the engine will rev up quicker. But lighter cranks also have less inertia and will drop revs fast as well.

What bearing sizes should you consider for the rods and the mains? Everything seems to be getting smaller. NASCAR engines use the same size bearings today as a Honda, but does this mean that smaller is better? The point of smaller rods and mains is to reduce the bearing surface and lower the bearing speed. How do we know what the bearing speed is – and can we measure this on a dyno? We know it has made some impact on power or else it would not be used, but how much did this information cost?

This is all a part of the balancing act. The larger the diameter of the rod journal, the weaker the connecting rod, and the smaller the main journal, the weaker the backbone of the crank will be. So it’s a wise idea if you can increase the size of the crankshaft without sacrificing material for the connecting rod. The more inertia from the weight of the crankshaft will offset the bearing speed.

The counterweights of the crankshaft are also something to consider. Some shops do what is known as knife-edging. This is where the counterweights of the crankshaft are machined to look like a knife blade rather than just being square cut from the factory. The theory behind this is to help the crankshaft “cut” through the oil as the counterweight rotates in the oil pan. This is used to gain better oil control and create less windage.

The method now has shifted from knife-edging to more sophisticated, aerodynamically engineered counterweights. Some counterweights now resemble raindrops, which create less oil splash and deflection. Custom crankshafts are also offered with what is known as micropolishing.

This is performed usually two different ways. One is with chemical etching and the other is with some sort of media. When finished, the entire crankshaft literally looks like a mirror. The primary reason for this process is to allow the crankshaft to shed oil. This, in turn, creates less aeration of the oil and helps lower oil temperature.

Connecting Rods
Engine builders who want to make big diesel power, will also need to consider the integrity of their connecting rods. Most mid-size diesel engines up until the late ’90s to early 2000s were forged. Then, manufacturers started using powered metal material for their connecting rods. Powered metal was fine for stock applications, but as simple bolt-ons were added to increase performance, these parts began to fail. This is something to consider when looking for power gains out of newer diesel engines.

Connecting rods in a diesel engine can see a lot of stress because of the higher compression ratios. Just as with crankshafts, aftermarket companies have answered the call for producing performance connecting rods. Some companies are offering stock lengths and bearing sizes, while others are offering custom sizes for specific applications. Most aftermarket stock replacement rods are made of forged material such as 4340 with an addition of chrome, nickel, moly and vanadium.

This improves grain flow, strength and durability. When purchasing a set of aftermarket forged rods, manufacturers often offer reconditioning services when engines are in need of freshening. These reconditioning services can include:
• Visual inspection;
• Straightness and alignment;
• Magnaflux for cracks;
• Bushing replacement;
• Hone big and small ends;
• Resize big and small end;
• Check bolts and change if needed;
• Shotpeen; and
• Balance if necessary.

Forged aftermarket rods are tough and can take some serious abuse. Connecting rods of this caliber should offer a lifetime of use and can be reconditioned at a fraction of the cost.

If you’re building big diesel power, you’ll need a custom crank made from billet or aircraft-grade material to handle the load.
If you’re building big diesel power, you’ll need a custom crank made from billet or aircraft-grade material to handle the load.

For more serious power, certain manufacturers now offer fully machined billet for custom applications. These billet rods are offered in an “H” or “I” beam design with a choice of a straight cap or slanted cap and your choice of fasteners. Slant cap designs are often used for cam clearance or small bore applications. Most custom billet rods are fitted with bushings on the small end for the wrist pins but can be machined for press-fit wrist pins if the they are coated with diamond-like coating.

When using billet connecting rods, typical factory bearing clearances work well unless you’re using a higher viscosity oil. Clearances may need to be increased depending on application. For some diesel applications, aluminum connecting rods are also being manufactured and used with great success.

Whether you’re in need of a custom crankshaft or connecting rod, or just want to upgrade your existing package, aftermarket manufacturers are tailoring their products for your needs. And many aftermarket manufacturers are using quality materials made here in the USA, machined to exact tolerances and specifications to meet the demands of your performance diesel package.

Special thanks to Carrillo, R&R Connecting Rods and Winberg Crankshafts for their help with this article. For a complete list of diesel crankshaft and rod suppliers, click on our online Buyers Guide.

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