Scat Crankshafts

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They say that the heart of the engine is the camshaft, since it is one of the key components that dictates the engine's power level, power band, idle quality, and other characteristics. If the cam is the heart, then the crankshaft is the spine. The crankshaft also dictates power and powerband, but in a much more ambivalent way (through its stroke which, along with the bore size, dictates the engine's cubic-inch displacement). The crank is what transfers the up and down reciprocating movement of the piston and rod into the rotating motion required to drive the transmission. It carries the weight of all eight rods and pistons, and must deal with the shock loads of the combustion process. A stock crank does this fine...in a stock engine. But when power levels start to climb, that stock crank will eventually give under the tremendous loads imposed upon it.

Aftermarket crankshafts are hugely popular in the Mustang world, since they are required for stroker kits and are usually necessary when an engine goes from bolt-on status to real power. But not all aftermarket cranks are created equal. There are different materials, different manufacturing processes, and different ways to prep a crank. One of the most respected crankshaft manufacturers is Scat Enterprises, in Redondo Beach, California. Scat has been in business for 35 years, and builds more than 15,000 crankshafts per year. Some of their customers include NHRA Top Fuel, Indycar, and Winston Cup teams, and they also build the cranks sold by Ford Racing Performance Parts in its 347, 393 and 514 stroker kits, among others. You would be shocked to learn how many aftermarket stroker kits use Scat cranks and rods.

Scat makes everything from inexpensive cast cranks, to forgings, to the ultimate gotta-have-it custom billet-steel piece that will withstand more power than you can build in a small-block Ford. We wanted to see what went into building a custom crankshaft, so we spent a day at Scat's 42,000 square-foot facility to follow along as a billet crank is created, from the heavy chunk of steel to the finished beauty. We also chatted with Scat's owner, Tom Lieb, on the different crank types and got some killer information on the subject.

This is how the raw billet
material is delivered to Scat.

The billet material is put into
a CNC machine and cut for the
proper diameter, and also cut
around the main journals
(notice the slots - that's where the
mains are).

The slot-cut piece of billet is then loaded into the huge "pin mill" CNC machine, which cuts the basic shape of the crank.
The magic of big CNC machines still amazes us.

Materials

You've heard of a crankshaft referred to as a casting, a forging, or a billet, but what does that mean, what are the differences between them, and what is the strength comparison? Basically, a cast crank is made by pouring molten iron into a sand mold, letting it cool, than pulling it out and machining it. This is the easiest and least expensive way to make a crank, and that's why the majority of stock cranks are castings. They are strong enough for most stock applications, and will run forever in a daily driver. But bolt a blower or nitrous system on the engine, and start making some power, and a stock cast crank is living on borrowed time. Scat has a line of cast cranks, the 9000 Series, that are available in several different strokes and are stronger than stock. Lieb told us that the tensile strength of a stock cast crank is 95,000 pounds, whereas the 9000 Series cast crank is rated at 105,000 pounds.

A forged crankshaft is made by taking a big piece of iron and forcing it into roughly the correct shape for a crankshaft. This obviously requires mammoth machines, which dramatically drives up the cost of manufacturing. The advantage of a forging over a cast crank is strength. The material used for a forging is generally better to begin with, and by moving the metal instead of melting it, the grain structure of a forging is better. Pushing the metal around does not break the grain structure, it just stretches it into a different shape (think Play-Doh), and that promotes greater strength. Among forgings, there are several different materials used to make cranks.

A production forging is made of 1045 carbon steel, which has a tensile strength of 105,000 pounds (the same as Scat's 9000 Series cast crank). The next step up the ladder is 5140 steel, which has a strength of 115,000 pounds, and 4130 with 120,000 pounds tensile strength. The strongest forging is 4340, which has a strength of 140,000 pounds. The biggest differences in these types of metal is the grain structure, (which is what holds the material together), the heat-treating process, and the actual mixture of the elements. For instance, 4130 and 4340 steel have more chrome and nickel in them, which increases strength.

Crankshaft durability is measured in tensile strength and fatigue strength. Tensile strength is measure when a 1-inch-round piece of the metal is put into a fixture that tries to pull it apart. The force required to break the metal sample is the tensile strength. Fatigue strength can be best described by thinking of a piece of tin. You can bend it back and forth for awhile, but it will eventually break. The crank goes through the same type of bending and twisting, from cylinder pressures, vibration, block distortion, high-rpm clutch launches and stuff like that. As tensile strength goes up, so does fatigue strength.

This is what the crank looks like after it comes out of the pin mill. The basic shape of the counterweights are there, and the main and rod pins are located.	Final shaping of the counterweights is done on another huge, 4-axis CNC machine, the "KT".
		The next step is to drill the lightening holes in the rod and main pins.>
The oil holes are then drilled and chamfered in the rod and main journals. The lightening and oiling holes are all drilled by hand, not on a CNC, due to the set-up time involved to switch back and forth between all the different crank types and strokes. It also allows the machinist to catch any errors that a CNC machine cannot.

A billet crank is the strongest available (assuming the material is high-quality). A billet crank starts out as a big, heavy chunk of steel, and goes through many machining operations until it is a ready-to-rock, finished crank. Obviously, a billet crank requires a lot more work than a cast or forged crank, and that's why a billet crank is typically twice the price of a forging. But for ultimate strength and light weight, billet is the way to go for a serious engine. Scat's billet cranks are made of 6415 billet steel, which has a tensile strength of over 160,000 pounds.

There are other advantages to billet over a forging in the area of strength, as mentioned by Scat's Mark Cafourek. "With a billet, we have much greater control over the material. The SAE spec on forgings is kind of a generic number. There are 10 to 15 different grades of 4340, so the strength really depends on which kind you get. Metal is like chocolate Hershey and Godiva are both chocolates, but Godiva is a much higher quality and is also more expensive." Cafourek also said that the metal is "traumatized" when it goes through the forging process, so even though the grain structure is not broken, it is subject to changes nonetheless. Lieb was quick to point out that "We only buy the best material, and we do not mix and match material. Since all of our billet cranks are made of the same material, the heat-treater never has to change anything they do, which maintains consistency."

Though all of Scat's billet cranks are created equal, the same may not be true of all manufacturers. Lieb said "With a crankshaft, there are several things that are important. Anyone can make a crank, and it'll check out the same whether it's made of balsa wood or the best material in the world. A prudent engine builder relies on the integrity of the of the crank maker. We buy and control all of our steel, and every crankshaft is certified and on file with a serial number. If someone has a question about a crank, we can pull up their records and tell them everything about it. There's a certificate on the heat-treat, and we can trace it all the way back to the guy who drilled the oil hole."

Of course, no matter how strong a crank is, someone will be able to break one. We've all known guys who can blow up a T-5 or a rearend in a 15-second car. Some guys just figure out a way to break an anvil with a rubber hammer! This is the main reason why Scat is so anal about its record keeping. Lieb told us "The metal we use is very predictable and will always do the same thing under the same conditions. From our many years of doing this, we see certain things that happen to a crank, certain types of breakage, and you see the same things over and over. When a crank fails, we do forensics and determine if it was poisoned or shot in the head!" In other words, Scat knows all the different ways that racers have found to break a crankshaft, and they can usually tell you what you did wrong without even seeing the crank in person. Lieb said "The two most common sources of breakage are inappropriate application and a defective damper."

A billet crank is not for everybody. At somewhere around $2000 a pop, they are usually reserved for full-race engines and the builders who can afford them. If you've got a 400 horsepower street car, you don't need a billet crank (though a forging is a very good idea). But when you're talking Hot Street horsepower and beyond, a billet arm will ensure that you aren't driving over counterweights at the top-end of the track.

Here are the five major steps
that a billet crank goes through, from left to right. That chunk of 6415 billet steel on the left (which weighs 265 pounds!) is ordered to the correct length, and is first centered and cut for the rear flange and nose. The next crank shown has been cut for diameter and slot-cut for the mains. Then, the rod throws (pins) are machined and the counterweights are roughly shaped, followed by more
counterweight shaping, and finally detailing and heat- treating.

Each crank is then deburred and
detailed by hand. After this
step, the crank is bead-blasted
for a nice finish, then sent out
for stress relieving.

After stress relieving, the
crank is put in the crank grinder
for final grinding of stroke and
journal diameters. After it is
ground, the crank is once again
sent out for nitriding/heat-
treating.

A word about vibration dampers

In our conversation, Tom Lieb gave us some great (though sure to be controversial) information about a subject directly related to the crankshaft and its survival in a race engine: vibration dampers. He was quick to point out that a very common cause of crankshaft failure is using the wrong damper, and is especially leery of any damper that has moving parts. "A damper is supposed to balance an engine. How can you do that when you have something moving on the damper?" Lieb went on to say "Any damper with moving parts, whether it is fluid or mechanical, is not recommended in a drag race engine. Anything that moves in the damper has inertia. When you quickly accelerate or decelerate the engine, that moving part of the damper slams one way or another and puts a lot of stress on the crankshaft. For an engine that runs in a very narrow rpm band and with very little change in engine speed, like an 18-wheeler or a car that drives on the highway at a constant speed, a damper with moving parts is fine, because it will find the frequency at a constant rpm and dampen the vibrations. But with a drag race engine that accelerates and decelerates fiercely, the engine will only have certain vibrations for a millisecond." For a drag racing engine, Lieb recommends the lightest, smallest damper possible, with an elastomer-type construction. "The rubber will sit there and hum, but it won't change location."


There are several checks and measurements done during the crank's "birth" to verify proper tolerances and dimensions.		Every crankshaft journal is micropolished before shipping.

An option for the billet cranks is undercutting the counterweights, which lessens the overall weight of the crank without overly removing any strength. A lighter crank will allow the engine to accelerate more quickly.	This crate is full of raw forgings getting ready to be machined into forged cranks. If you look closely, you can tell that they have been forced into shape, as opposed to being pulled from a sand mold.	Upstairs in the factory is Scat Engineering, where new crank designs are created and perfected on state-of-the-art CAD CAM machines.