The significance of this visit by President Obama is that manufacturing is an important part of our nation’s economy. General Electric, which is a known innovator of all types of engines, employs over seven hundred individuals at their Waukesha engine plant. In fact, a machine operator named Reggie Troop had the distinct honor of introducing President Obama to his fellow employees, members of the media and others in attendance.

If you visit the official White House website, you can see a picture of President Obama viewing a crankshaft. The full story on President Obama’s visit to this engine plant may be viewed here.

Although it is great that the President of the United States is touting our nation’s manufacturing capabilities, and seeks to strengthen them, many of the aftermarket crankshafts used in performance applications are forged overseas. For example, many 4340 forged steel crankshafts are produced in China and some are finished here in the United States. It is our hope that the President will look at current trade agreements and evaluate their impact on our manufacturing sector.

Both General Electric and President Obama deserve our praise for their roles in helping individuals enter job training programs that require a great deal of technical knowledge. With nearly one third of the workers at GE’s plant becoming eligible for retirement in the next few years, many employment opportunities will soon exist for young machine operators and machinists that are interested in pursuing a career with an innovative company that produces highly specialized engines.

To understand how pricing crankshaft repairs are billed, it is important that you understand that most automotive machinists have a procedure that they follow to perform repairs. While grinding a crankshaft may have a flat fee, its current condition directly impacts the customer’s total out of pocket cost. For example, no automotive machinist wants to machine a dirty crankshaft in a $50,000.00 crankshaft grinding machine. Before internal engine parts are remanufactured, they must be cleaned, inspected and repaired in a sequence that matches the damage that must be repaired.

Dirty crankshafts are most often cleaned in a hot tank that contains a caustic soda chemical solution to remove grease and oil. This process, which is also often mistakenly called an acid dip, is quite effective when used in conjunction with an agitator. The agitator helps to move the solution around a crankshaft’s counterweights, into the oil galley holes and around the journals. In most cases the fee to “dip” a crankshaft in a hot tank is approximately $30.00.

Crankshafts that are rusty, from being exposed to the weather during storage, are often cleaned with a bake and blast process. Baking a crankshaft allows all the grease and oil to harden, which then is blasted off with very small steel shot peen pellets. Cast iron cleaned with this process often looks like aluminum when completely cleaned. The cost to bake and blast a crankshaft is approximately $100.00.

Many automotive machine shops that grind crankshafts do not have baking and blasting capabilities, so they often rely on glass beading crankshafts that are rusty. To glass bead a crankshaft, it must first be free from dirt, grease and oil so that the glass particles do not stick to the crankshaft being cleaned. Many people erroneously refer to glass beading as sand blasting, although both processes use compressed air and particles to act as an abrasive. The cost to glass bead a crankshaft is approximately $50.00, but also may be combined with hot tanking for a total cleaning fee of $80.00.

When a crankshaft is to be ground, it is first inspected with a micrometer to measure the main and rod journals. If the machinist suspects the crankshaft is bent, they will use a pedestal base dial caliper to measure the runout of the crankshaft’s main journals to determine how much it is bent. Most automotive machine shops do not charge separately to inspect a crankshaft that is to be ground. However, if on inspection the crankshaft is determined to be beyond repair, the bill may cost approximately $40.00 for checking the crankshaft for straightness and the journals for size.

Crankshafts that are slightly bent can often be straightened and are normally billed at an hourly rate. For example, if your shop’s labor rate is $80.00 per hour, a crankshaft that takes fifteen minutes to straighten will cost $20.00. Most passenger car crankshafts can be straightened within fifteen minutes, but large industrial crankshafts often take longer to straighten because of a larger journal radii.

When extreme damage to a crankshaft must be repaired, it may need a thrust or journal to be welded. Welding a single journal or thrust can cost approximately $90.00, and this process must be performed before the crankshaft receives its finish grind. Most automotive machine shops include any straightening and the rough grind needed to perform this operation as part of the total cost of the weld.

Grinding a crankshaft does vary slightly for passenger cars. For example, a V6 crankshaft has more rod journals to grind than a V8. Generally speaking, it costs approximately $130.00 to grind a passenger car crankshaft and considerably more for industrial crankshafts. The reason why industrial crankshafts cost more to grind is that a special wheel often must be used that is better suited to machine large diameter journals and the significantly larger radii.

The fees assessed for polishing and chamfering a crankshaft are often bundled into the cost of the grind itself. The reason for this is that the grinding process does not leave a suitable finish for the engine bearings, and it is customary for most automotive machine shops to include the cost of this process in all crankshafts that they grind. However, some crankshafts that are being reused are in good condition and do not require grinding. When polishing is the only labor operation being performed on the crankshaft’s journals, the fee normally does not exceed $50.00.

Now that you have a better understanding of what it may cost to repair your crankshaft, it is always wise to receive an estimate from your automotive machine shop before they perform any work. Additional labor operations not covered in detail on this page may include repairing a damaged keyway, using a helicoil insert to fix a damaged flexplate bolt hole, balancing and other issues which are not so commonly seen in your average crankshafts. Obtaining an estimate first allows you, the customer, to be fully informed of what the machinist will do to correct the defects in your crankshaft and what the expected total cost will be upon completion.

Whenever machine work is performed on a crankshaft’s journals, these surfaces must be adequately finished before use in an engine. Besides maintaining a sufficient microfinish, it is also important that any burrs are removed. A critical area where burrs are often found are just inside of the journal’s oil passages. The way to remove these burrs is by using a process known as chamfering.

Burrs develop on oil passages from a variety of machining operations. Just grinding the crankshaft .010” can leave a small burr that would restrict the proper flow of oil while the engine is running. This poses a serious concern since engine bearings need to be adequately lubricated at all times. Chamfering is the industry standard in deburring crankshaft oil passages, which is explained in more detail below.

If you look to the image to your right, you will see what the oil passages look like when they have been lightly chamfered. This deburring process was completed with a die grinder, small grinding stone and an automotive machinist’s steady hands. The die grinder rotates the stone at a high RPM and the machinist gently follows the outside of the existing passageway with the stone to make a light cut, while also being careful to deburr the entire circumference around the oil passage outlet.

Generally speaking, chamfering is performed as part of the crankshaft polishing process and must be performed after most significant crankshaft repairs have been made. Just before the crankshaft is polished, it is chamfered in the grinding machine. Because chamfering can also create slight burrs, a polishing belt is all that is needed to remove any burrs left behind from the surface where the stone has met the journal’s outside diameter.

When a crankshaft is welded, it almost always needs to be chamfered. Instead of simply cleaning up the outside of the oil passage after a welded journal has been roughed in, the automotive machinist will often use a long and narrow stone to cleanup the inside of the passageway as well. Because welding penetrates existing steel, a small amount of weld may enter the passageway that is easy cleaned out with the right stone and chamfering process.

When a journal has been welded, some automotive machine shops prefer to instead drill out the oil passage using a special carbide chamfering drill bit. Although this type of chamfering may be needed when an excessive amount of weld has blocked the interior oil passageway, the finish it leaves behind is often less than desirable. Many automotive machinists that must use a chamfering drill bit to clear out an oil passageway will then smooth out the passage with a die grinder and fine stone.

There are some automotive machinist that will not only chamfer the circumference of the outer oil passage hole, but they will add a directional cut with the die grinder’s stone that leads into the journal. The theory behind this is that such an elongated chamfer will aid in the distribution of oil, especially on high performance engines. While this may be true while the engine is running, on initial startup of the engine it will take longer for the oil to completely surround the void between the engine bearings and journals. For this reason, most automotive machinists opt to perform a light chamfering process that does not elongate or extend the size of the oil passages.

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To better understand steel terminology, as is used by SAE and AISI, we can dissect the steel grade to determine some basic information. For example, the number 4340 can be broken down into two sets of digits. The number 43 represents the primary composition of the crankshaft beyond iron, which in this case includes nickel, chromium and molybdenum. The second set of digits represents the amount of carbon present in the steel in hundredths. Therefore, the number 40 represents a .40% carbon content.

4340 Crankshafts

4340 steel crankshafts contain 1.82% nickel, .50-.80% chromium, .25% molybdenum and .40% carbon. The remaining composition is primarily made up of iron (95-96%). This is one of the most common steels used for both aftermarket crankshafts and connecting rods. 4340 steel has a tensile strength that equals or exceeds 108,000 PSI. Because of the composition of this material, it responds well to heat treating and the final piece is quite durable.

4330 Crankshafts

Similar to the 4340 classification, 4330 steel crankshafts primarily consist of 95-98% iron, 1.82% nickel, .50-.80% chromium and .25% molybdenum. However, 4340 steel does have a lower carbon content of .30%. 4330 crankshafts have a high tensile strength that exceeds 125,000 PSI. Although a high tensile strength is desired in high performance applications, 4330 steel is less resilient to fatigue and is more prone to break if the crankshaft flexes.

4130 Crankshafts

4130 steel crankshafts contain .50-.95% chromium, .12-.30% molybdenum, .40-.60% manganese, .28-.33% carbon and an iron content that ranges from 97-98%. With a 66,700 PSI tensile strength, this is one of the weakest alloy steels used in aftermarket crankshafts. However, 4130 is ideal for light performance applications and often appeals to consumers because the cost is considerably less than higher grade steels.

5140 Crankshafts

Although less popular, 5140 steel crankshafts are used in mild performance applications and are classified in the chromium steel category. These crankshafts are comprised of 97-98% iron, .70-.90% manganese, .70-.90% chromium and .38-.43% carbon. 5140 steel crankshafts also contain lesser amounts of silicon, sulfur and phosphorous. 5140 steel has a tensile strength of 82,700 PSI, making it less brittle when compared to 4330 or 4340 steel crankshafts.

So which type of steel is best for your crankshaft? This determination is based on ones horsepower requirements and budget. Generally speaking, we prefer 4340 steel over all other options, particularly if the existing crankshaft can’t be repaired. There are many 4340 steel crankshafts powering engines that exceed 1,000 HP, which have proven to be quite durable for racing applications. Although many automotive enthusiasts like to compare 4330 –vs- 4340 steel, for example, their material compositions are not that dissimilar. How the crankshaft is hardened is more detrimental in obtaining the benefits from the composition of each crankshaft.

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Balancing an engine requires simulating the weights of the pistons, piston pins, rings, connecting rods, engine bearings and even the oil with what is known as a bobweight (set pictured left). The bobweight accepts individual weights and is secured to connecting rod journals before the crankshaft is spun in a balancing machine. To calculate these weights, and create a bobweight, we must first look at the connecting rods and the components that appear on each end. The large end of the connecting rod, which is affixed to the crankshaft with bearings, is the rotating end. The small end of the connecting rod actually uses a reciprocating motion when the engine is operating. In calculating the weights for balancing, the rotating and reciprocating weights are used in the formula.

The rotating weight consists of the weight of the big end of the connecting rod and bearings. The reciprocating weight includes the small end of the connecting rod, piston, pin and piston rings. Because the reciprocating motion impacts an engine’s balance as a percentage, that is taken into account when determining the bobweight for balancing. For example, the formula to determine a bobweight for a ninety degree V8 engine would be rotating weight grams + (reciprocating weight grams x .50) +5 grams. In this formula, 50% of the reciprocating weight was used to determine the bobweight, while five grams was allotted for the weight of the engine’s oil.

An engine that is intentionally over-balanced for performance will ordinarily use a higher percentage for the reciprocating weight. For example, a V8 engine that will operate at high RPMs may benefit from an over-balance of 52%. Automotive machinists typically have their own preference of what percentage they are comfortable balancing engines at. Particularly among those who also dyno engines, while measuring vibration at various RPMs, personal preferences likely result from their own individual experiences and analysis.

Because of the design of many V6 engines, a different percentage is often used in determining the bobweight. For example, many automotive machine shops prefer to balance V6 engines at 36.6%, while all out race engines are balanced at 50%. Once again, this percentage is a personal preference of the machinist who has a thorough understanding of their customer’s application.

Before determining the bobweights for balancing, the automotive machinist must normalize all of the weights. In most cases pistons are manufactured to precise weights, just as the pins, rings, engine bearings and locks are. However, connecting rod weights do vary and these are often lightly machined by the shop doing the balancing so that each connecting rod has a consistent weight. Using a precision scale, like the shadowgraph machine pictured to the right, automotive machinists may determine exact weights. Generally speaking, the connecting rod with the least amount of weight is used as the baseline for all of the other connecting rods weights to match as it is far easier to remove material than add it.

Once the bobweight has been established, and the weights attached to the journals, the crankshaft may be spun in a balancing machine. This machine looks for vibration and helps the operator determine where weight should be taken off at. It is important to note that internally balanced engines, often referred to as being neutral balanced, do not need the assistance of a harmonic balancer or flywheel to balance the rotating assembly. However, externally balanced rotating assemblies must have the harmonic balancer and flexplate or flywheel attached to the crankshaft while it is being spun in the machine. As the balancing machine highlights those areas where weight needs to be removed from, the operator takes note and removes the crankshaft from the machine so he or she may remove material.

Removing weight from the crankshaft is often performed on the crankshaft’s counterweights using either a grinder or Bridgeport milling machine. For those crankshafts that have been knife edged, this can make balancing even more of a challenge since the counterweights have been narrowed. Regardless, weight can be taken off as needed to balance the crankshaft. There are some cases where adding weight to the crankshaft is the best option. Using mallory metal slugs, which is often referred to as heavy metal, are often installed into the crankshaft’s counterweights after holes have been machined to accept this material with a press fit. Because of the cost of mallory metal, and the labor needed to install it, most automotive machinists try to avoid using this material at all costs.

Once the crankshaft has been balanced, the bobweights are removed and the crankshaft is then polished to remove any marks caused by spinning the main journals in the balancing machine. After this is completed, the customer should be provided with a copy of their balance card so that they may have access to this data in the future as needed. Especially in the case of performance engines, being able to replace a piston or connecting rod with the same weight can often save the customer hundreds of dollars and reduce the time it takes to make repairs to the engine.

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Those who own performance engines, and want to gain the most horsepower possible, often seek the services of an automotive machine shop that provides a crankshaft knife edging service. Normally reserved for racing applications, a knife edged crankshaft can help the engine produce more horsepower. Below we will look at how the knife edging process is completed and the benefits it offers.

As you can tell by the picture to your left, a knife edged crankshaft refers to the shape of the crankshaft’s counterweights. Removing this material and bringing it to a sharp edge does a couple of things. First, knife edging can often remove pounds of weight from a V8 crankshaft. Even some V6 crankshafts will realize a significant weight reduction when the counterweights have been knife edged. Less weight equates to a rotating assembly that spins with less resistance, and this in itself is responsible for some gains in horsepower. Knife edging also reduces friction, which is explained below.

The counterweights on most crankshafts are flat on the ends. As the crankshaft rotates, these counterweights come into direct contact with the engine’s oil. This contact creates friction and splashes oil all over the bottom end of the engine. A knifed edged crankshaft actually glides through the oil much easier and encounters far less friction. Anytime friction can be reduced in an engine’s rotating assembly or valvetrain, additional horsepower gains are realized.

There are many automotive machine shops that provide crankshaft knife edging services. However, how these shops perform the machining process does vary. Most machine shops will use a Bridgeport milling machine to rough in the counterweights, whereas some shops will simply grind the counterweights to a point. Regardless of which technique is used to perform the knife edge service, there are some steps that the machinist must take to protect the crankshaft prior to the machining operation.

Ideally, it is best to knife edge a crankshaft before any finish machining operations are performed. For example, if the crank needs to be ground or polished, these processes should be performed after the knife edging tasks are completed. The reason for this is that there is the potential for a grinding stone or tool to slip and come into contact with a journal. Although rare, particularly among experienced machinists who have knife edged many crankshafts, it is best to err on the side of caution.

Because of the material being removed from the crankshaft’s counterweights during a knife edging process, the rotating assembly will need to be balanced when the crankshaft is finished. Often lighter rods or pistons can help compensate for the crankshaft’s reduced weight. By properly planning for the reduced weight of a knife edged crankshaft, these variables can be considered first so that the need for heavy metal during balancing is greatly reduced.

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First, many novice engine builders believe if their crankshaft is gouged that all it needs is to be polished. In most cases this line of thinking is wrong. Gouges, caused by material that may have been embedded in engine bearings, can damage a journal. Polishing can’t take these defects out of journals; at least with any degree of accuracy. If there are a lot of deep gouges, lines or other marks in a journal, chances are grinding the crankshaft first is the best course of action.

Generally speaking, crankshaft polishing is the last step in the crankshaft repair process. Normally the crankshaft has been ground first, and the polishing process is used to take off the microscopic peaks that the grinding wheel leaves behind on the finished journals. Many automotive machinists also choose to chamfer the oil holes just before polishing the crankshaft, which aids in removing burrs and helps the engine oil leave the oil passages when the engine is running. Regardless, polishing removes the peaks on journals and seal surface areas so that a smooth microfinish is created. This helps to reduce bearing and seal wear so that rebuilt engines will last for many miles under normal operating conditions.

Although there are dedicated machines to polish crankshafts, like the one pictured to the left, most crankshaft polishing operations occur right in the grinding machine after the last journal has been ground. The reason for this is simple; with the crankshaft already in the grinding machine, there is no setup time required to use a different piece of equipment. To polish the crankshaft in a grinding machine, the automotive machinist will simply need a portable polisher that has been designed specifically for use on crankshafts.

A portable crankshaft polisher, like the one pictured to the right, operates on a standard 120 volt outlet. When not in use, the polisher can be easily unplugged and stored in a safe location that is out of the operators work area. This polisher uses a special type of belt that is designed to come in direct contact with the crankshaft journals and seal surface areas as the crankshaft is slowly rotating within the grinding machine.

Crankshaft polishing belts, like the one pictured to the left, are available in a variety of lengths, widths and grits. Depending on the type of portable polisher used, which is adjustable, the length of the polishing belt can vary from 60” to 72” in length. The width of the belt being used to polish crankshafts is determined by the crankshaft journal or seal surface width. For V8 crankshafts, a one inch belt is normally used while V6 crankshafts normally require the use of a ¾” belt that fits between the narrow counterweights.

To polish crankshaft journals, a 400 grit belt is almost always used. When a new belt is put into production, it is often first used on a piece of steel to take the sharp edges off of the belt before it is used on any journals or seal surfaces. The concept of polishing is to finish a crankshaft with a microfinish that is appropriate for engine bearings and seals. Using a coarse belt will not only leave an undesirable finish, but may also take off too much material and throw the journals out of tolerance. By using the appropriate belt and operation, polishing should remove no more than .0002” of material.

It is important to note that some less experienced engine builders attempt to cleanup their crankshaft’s journals with a 200 grit belt. Although these belts can take out minor gouges and lines in journals, the end result almost always consists of journals that dip in the center. This dip, and out of tolerance condition, exists because polishing belts are not ridged on their ends. The absolute center of a polishing belt has the highest capacity to remove material. Polishing should never be used to repair visual defects on crankshafts. Crankshaft grinding is the only process that can correct visually damaged journals.

Polishing Belt Materials and Design

Most quality crankshaft polishing belts are designed with an aluminum oxide abrasive that is bonded with either glue or resin. Although the most common types of belts have straight edges, others have scalloped edging that is designed to polish into the radii. Though paper based belts are widely used for most crankshaft polishing operations, abrasive cork belts are also available for super fine finishes. Regardless of what type of belts an automotive machine shop uses, the belts lose their effectiveness over time as the bonding agent deteriorates and as the belts become loaded with material.

Since there is no way to dress a belt, proper belt maintenance includes blowing of the belt with compressed air. This process helps to dislodge some of the material that is embedded between the abrasives. Should the embedded material on the belt become too great, it should be discarded and a new one placed into production.

Polishing belts are normally sold in quantities of ten per box. Depending on which type of belt is purchased, the average cost can range from $2.50 to $4.00 per belt. Ordinarily, a polishing belt that has been maintained properly can be used to polish fifty crankshafts or more.

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Since crankshaft grinding wheels are extremely hard and abrasive, a special tool is required to dress the wheel. This tool, which is commonly called the wheel dresser, utilizes an industrial diamond for cutting the wheel’s stone. Not surprisingly, there are various sizes of diamonds that are used and categorized according to their carat weight.

When dressing any crankshaft grinding wheel, it is important that the diamond be set between ten and fifteen degrees of the surface being dressed. Also, coolant must be used on the diamond at all times during the dressing process. This will help to extend the life of the industrial diamond and aids in producing a quality finish on crankshaft journals.

A crankshaft grinding wheel dresser is capable of dressing all five surface points on the grinding wheel. The dresser also has a slide gauge on it to set the diamond for dressing the radii and a pivoting head so that the machinist can accurately replicate the desired radius that is being cut into the wheel. Most wheel dressers use quick releases to make the task of dressing a crankshaft grinding wheel even easier for the operator.

The first step in dressing a crankshaft grinding wheel involves setting a zero or home point. To accomplish this, the diamond is set in the gauge for the appropriate radius by using the sliding scale. The operator then secures the swing head so that the potential for vibration is eliminated and the machine’s coolant is turned on to cool the diamond as it comes into contact with the wheel. The face of the wheel can now be dressed. Once the face is dressed, the machine’s plunge feed dial is set to zero. The wheel dresser is now appropriately set and the radii is ready to be dressed.

With the dresser safely away from the spinning grinding wheel, the operator may now release the swing head on the wheel dresser and secure it in a ninety degree position to cut the first radius. Once secured, the operator brings the diamond slowly into the side of the wheel (horizontally) until it just barely touches. Once the industrial diamond touches the grinding wheel, the plunge feed on the machine is backed off around one hundred thousandths of an inch. The operator may now slowly move the diamond from a ninety degree position back to its stop, which is in line with the face of the wheel. The operator will then bring the plunge feed in even closer to zero, while taking sweeps with the swing head on the dresser as to avoid a heavy cut that could potentially damage the industrial diamond. Once within a few thousandths of zero on the plunge feed, the operator may then replicate the same process on the other side of the wheel to attain a consistent radius on both sides.

To dress the side of the wheel, which is needed to grind crankshaft thrusts, the diamond is often removed from the dressing fixture and placed in a position that allows the operator to perform dressing operations in a fixed ninety degree position. The operator will secure the dressing tool and bring in the grinding wheel using the power feed. Manual adjustments by the operator may be necessary to dress the section of the wheel that is used to grind thrusts. Once this position has been found, the operator will slowly bring the diamond into the side of the wheel, turn the coolant on and feed the diamond into the wheel about .005”. The power feed is then used to take the grinding wheel back to its home position and results in the initial cut. The wheel then may be power fed back in, diamond adjusted and an additional cut taken. This process is replicated on the other side of the crankshaft grinding wheel to maintain a consistent surface for grinding the thrusts on crankshafts.

The reason why a crankshaft grinding wheel may need to be dressed is because the stone eventually gets loaded with material. Loading in the wheel may be increased if the operator takes heavy cuts when grinding crankshafts or from roughing in a welded crankshaft. Also newly balanced wheels must be dressed before they are put into service. Under typical grinding conditions, a wheel will ordinarily remain sharp and last long enough to grind three or four crankshafts. Welded journals, on the other hand, will dull a crankshaft grinding wheel rather quickly and must be dressed before a finish grind is performed.

As with any machining operation, the speed at which a wheel is dressed will help to determine the quality of the microfinish it produces. For example, dressing with a fast feed often results in a stone that would produce a rough finish. Although a rough finish is not desired on finished crankshafts, it can be rather useful when roughing in welded journals. If the operator has multiple welded journals to grind, or possibly an under-grinding process in preparation for a weld, a rough finish is actually helpful. Regardless of the desired microfinish, the rate at which the operator feeds the stone should always remain as humanly steady as possible to maintain consistency across the cutting surface.

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Cast Iron Crankshafts

Cast iron crankshafts are most commonly found in ordinary passenger car engines and are favored by automobile manufacturers because they are inexpensive to produce. A cast iron crankshaft is made from molten iron or steel and simply poured into a mold. The raw casting is then rough machined so that it can be ground to its finish dimensions and then balanced. These types of crankshafts are relatively inexpensive and can be purchased new for $200 or even less. Since cast iron crankshafts contain flakes of graphite flakes, these crankshafts often have a grey visual appearance.

Nodular Iron Crankshafts

Nodular iron crankshafts are manufactured in the same way as cast iron crankshafts, but the composition of the iron is slightly different. The inclusion of graphite nodules, instead of flakes, adds strength to this component. Cerium and magnesium are also added to further strengthen the crankshaft. Nodular iron crankshafts typically have a carbon content of 3.3% to 3.4%. Many aftermarket cast crankshafts are composed of nodular iron materials for added strength. Although nodular iron crankshafts cost slightly more than a cast iron crankshaft, the added expense is well worth the increased tensile and fatigue strength of the crankshaft.

4340 Forged Steel Crankshafts

Unlike cast steel, forged crankshafts are produced by using hydraulic presses to compress molten steel into its final shape using dies instead of molds. Unlike a casting process, which leaves a sandy grain in the crankshaft, forged steel crankshafts, have a uniform grain structure that adds a significant amount of strength to the final component. Most forged crankshafts are made from 4340 steel, which is a low alloy steel that containins nickel, chromium and molybdenum. 4340 steel can also be heat treated and does maintain an acceptable fatigue strength for moderate to high performance applications.

Billet Steel Crankshafts (4330 Steel)

Billet steel crankshafts are by far the strongest crankshaft available and are used for extreme high performance applications such as competitive racing. A billet steel crankshaft starts out as a large round ingot of forged steel and is machined to produce a parallel grain that is consistent. The grain structure of a billet steel crankshaft makes it more durable than any other crankshaft option currently available. However, this strength comes at a cost that can exceed $2,500.00 per crankshaft. It is important to note that the steel used to produce billet crankshafts also commonly differs from forged crankshafts. 4330 steel, for example, is a nickel, chromium and molybdenum alloy steel. The carbon content in this grade of steel is on average .30%; making it quite durable for engines that are designed exclusively for race applications.

Please note that specific steel designations, such as 4330 and 4340, refer to SAE International’s specifications regarding the grades of steels, their composition and metallurgic qualities. While there are various types of materials used to produce crankshafts, of varying fatigue and tensile strengths, they are typically hardened to produce a final crankshaft that is even stronger.

For performance applications, crankshaft flex is not uncommon at high RPMs. Forged and billet steels are better equipped to flex and return to their normal state. Cast and nodular iron cranks, on the other hand, are extremely brittle and more susceptible to cracks.

Identifying cast and nodular iron crankshafts is quite easy when you look at the casting lines. Thin casting lines, as in the image to the left, indicate that the crankshaft has been produced using a casting process. Steel forged crankshafts, on the other hand, tend to have wide forging lines. Billet steel crankshafts are typically smooth and will not have any visible casting or forging lines.

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Apart from a micrometer, one of the most important measuring instruments an automotive machinist uses when he or she grinds a crankshaft is an Arnold gauge. Unlike any other device, the Arnold gauge allows the operator of a crankshaft grinding machine to precisely read how much material is being taken off of the journal in real time. This ability, to see how much material is being ground off of a journal, allows the operator to grind crankshafts to extremely close tolerances.

The Arnold gauge, which is pictured left, is not only used in automotive machine shops. Many different industries, such as those involved with manufacturing aerospace and aviation components, rely on an Arnold gauge for a variety of O.D. grinding operations. With an accuracy of .0001”, it is quite easy to see why so many machinists are pleased with how well this gauge performs. Besides being accurate, an Arnold Gauge is also very easy for an operator to use.

Being attached to the crankshaft grinding machine, with a spring loaded support arm, allows a crankshaft grinding machine operator to effortlessly attach the gauge on and off of a journal at will. The entire gauge assembly includes a precision dial indicator, durable frame and an adjustable caliper. In most cases, the operator will use a standard caliper that adjusts from 1” to 3” in diameter. This is sufficient for grinding most passenger car and light truck crankshafts. For industrial crankshafts, which tend to have large journals, the caliper can be easily changed to accommodate a journal that is up to 12” in diameter. In the video below, you can view an automotive machinist using his Arnold gauge while grinding a crankshaft.

In the video you will notice just how easy the Arnold gauge can be used. When it is attached to the journal being ground, two round tungsten carbide contact points allow it to ride on the journal and a plunger on top takes the measurements. Although the plunger spring tension can be adjusted, it is not uncommon for these gauges to work well for many years and without the need of factory calibration. However, operator errors can occur that may damage an Arnold gauge.

One of the greatest hazards the Arnold gauge can come into contact with is an oil passage on the crankshaft’s journal. With the gauge riding on three points, including the plunger, any one of these contact points can ride over and into an oil passage. Even the most experienced machinist will have the occasional mishap and catch part of an oil passage, especially when the steady rest is also riding on a very narrow V6 rod journal. Thankfully this gauge is quite durable and can handle minor mishaps. Crankshaft counterweights, on the other hand, can actually catch the side of this gauge and pull it into the journal; thus causing major damage to not only the Arnold gauge, but to the crankshaft being ground and possibly the machine as well. Therefore, an operator must always be alert when using the gauge to measure the material being ground off of a crankshaft.

The Arnold gauge is a standard measuring device used by most within the automotive machining industry to repair crankshafts. These gauges are found on Berco, RMC, Storm Vulcan, Winona Van Norman and many other crankshaft grinding machines throughout the world.

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