The crankshaft is the heart of the engine. Therefore, it must be durable enough to withstand the engine’s dynamic demands, and that means it must handle the specific horsepower and torque loads. It must also cope with the crank speed and the deflection forces imposed by cylinder firing. The main and rod bearing clearances must be correct so the crankshaft’s main journals and rod big ends are supported. Also, the crank must be straight to eliminate rolling resistance and prevent bearing wear, and it must be properly balanced with the rotating and reciprocating assemblies.
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Modern crankshafts are offered in three basic constructions: cast, forged, and billet. Cast cranks are suitable for 300 to maybe 500 hp, depending on the application. Forgedsteel cranks, depending on the grade of steel, are designed to handle up to (and often beyond) 1,000 hp. Billet crankshafts represent the ultimate in strength for high horsepower and are predominantly used in professional-level racing applications.
The first step in the casting process involves pouring a molten mix of iron and other alloys into a two-piece mold. The casting cools and solidifies and is released from the mold. At this stage the finish machining takes place: machining all journals; finalizing counterweights, flanges, and nose; drilling/tapping flywheel bolt holes; and drilling critical oil passages. The casting process creates a random grain structure and the material is relatively porous, so the cast crank is susceptible to fracturing and failure under high stress. A distinct “parting line” from the mold halves identifies cast cranks.
A forged crank begins as a dense, forged chunk of steel. Although specific procedures may vary among aftermarket crankshaft manufacturers, forged crankshafts are commonly made by starting with a steel ingot that is heated to about 2,200 degrees F, placed into its forming die, and press/hammer forged to rough shape. The enormous pressures involved (around 240,000 psi with each hit) compacts the steel molecules into a very dense grain structure, providing increased strength.
Any excess steel that is forced out of the die is then trimmed off, usually in a shearing process. Next the rough forging is heat treated and tempered. This is followed by finish-machining and stress relief. Stress relieving is done to eliminate any internal stresses that may have occurred during machining. Finally, surface hardening is performed. Using dense steel ingots that are forged under heat and pressure results in a much stronger crankshaft that is much more resistance to cracking than is a casting.
The pressure and heat of this process makes a stronger crank with a tighter grain structure. A forged-steel crank may exhibit a wider parting line, but it is not the result of a mold. Rather, this is evidence of the excess material that has been pushed out of the die and then snipped off while still hot. High-quality aftermarket forged cranks are often so finely finished that you may not see any evidence of a parting line.
Manufacturers use one of two methods to forge crankshafts: twist or nontwist. In the twist approach, the die is shaped to orient two pairs of rod throws 180 degrees apart. This eases removal of the raw forging from the die. While the raw forging is still hot, the crankshaft is twisted in order to place the rod throws in the desired clock positions. Although this process creates correct geometry, the twisting procedure tends to interrupt the internal grain structure, which can result in potential stress risers.
A non-twist forging involves a more complex die and eliminates the need to twist the crank in order to place the rod throws in the proper positions.
A non-twist crankshaft is more expensive than a twist-forged crankshaft.
A billet crank starts off as a chunk of dense billet steel with a very uniform grain structure. It is dense steel and is machined to its final shape rather than using a forging press. The true benefit of steel billet crankshaft manufacturing is that, through CNC machining, custom crankshafts can be produced in virtually any configuration without the need for a die. Prices are around $3,000 for these robust and durable crankshafts and they are worth it when you need ultimate strength.
Several processes can be used to increase the crankshaft’s strength, particularly the main/pin fillet area, and each has benefits and drawbacks. The popular processes include induction hardening and nitriding.
The process of induction hardening creates a harder crankshaft journal surface. This involves exposing the crankshaft to an alternating magnetic field, which heats the component to a temperature within or above the steel’s transformation range. Then it is immediately quenched. Induction hardening does not affect the core of the component.
Crank manufacturers don’t widely use induction hardening because it isn’t easily controllable. Thus, the depth of hardening can vary, potentially creating isolated hard spots, which can result in the creation of stress points. Induction hardening can also excessively migrate much deeper into the crank, potentially affecting the strength of the core. The crankshaft also has a variation of material thickness. If the crank is not cooled in a controlled manner, major stress areas are likely to be created. Induction hardening for crankshafts has many detractors. The potential exists for unwanted harmonics and/or stress-related cracking issues.
This process involves the absorption of nitrogen into the steel. Prior to nitriding, the crankshaft is machined, stress relieved, and tempered. Nitriding takes place in a heated container where the crankshaft is exposed to ammonia and nitrogen gas. Nitrides are formed as the gas reacts with the carbon on the surface of the steel. This results in a superior surface hardness that is more resistant to abrasion and stress-related failure. Another aspect of nitriding is that the process treats the entire crankshaft surface instead of only journals and fillets, creating a much more uniform surface hardness.
Nitriding process time is also a factor, since the hardening takes place deeper into the steel as nitriding exposure is increased. Typical nitriding treatment takes about 24 hours. The goal is to harden the surface deep enough to offer the required wear and surface strength, but not so deep as to create a potentially brittle crank that might snap under extreme loads. Depending on the manufacturer’s process, hardness depth on a high-performance crankshaft is likely to be about .020 inch.
The depth of hardness dictates how far a crankshaft can be reground in the future. If ground beyond the initial hardness depth, a crankshaft may need to be renitrided after reconditioning.
Reducing Rotating Mass
Building a lightweight race car frees horsepower and can aid in better handling and braking. When you consider the weight of the engine as a system, removing weight benefits overall vehicle performance. When you also consider how heavy specific engine components are, removing weight not only affects vehicle static weight, but engine response and durability as well.
The idea of making something lighter may initially seem attractive, but you must remember that the crankshaft must remain balanced. If excessive weight is removed from the counterweights (exceeding the bobweight factor) some tungsten (heavy metal) balancing weight must then be added to the counterweights. Unfortunately, this defeats the purpose of lightening the crankshaft.
One common method to achieve weight reduction during manufacturing is “gun-drilling” the centerline of the main, which is performed by a qualified machine shop. This entails drilling a hole through the centerline of the main to remove static weight from the engine assembly. Gun drilling can easily remove as much as 3 to 6 pounds (depending on the individual crankshaft), without compromising strength. This can also help to equalize crank case pressure and vacuum, transferring air back and forth to improve scavenging.
Another method to reduce weight involves drilling the centerline of the rod pins. This not only reduces weight, but also helps to balance the cranksharft when lightweight connecting rods and pistons are used. Commonly, the material removed by drilling the rod pins can easily equal the weight of one or two slugs of heavy, metal tungsten.
You can also reduce crankshaft weight by reducing the weight of the counterweights (see Chapter 6 for more details). This can be done by undercutting, a machining process that cuts the counterweights to a thinner width in specific locations. Counterweights can also be reprofiled by knife-edging or bull-nosing their outer edges. This not only reduces weight but theoretically improves the aerodynamics of the counterweights, enabling them to quickly sling-off parasitic oil.
You can spend substantial money for a lightweight crankshaft, but you can easily negate the benefits by installing a large-diameter and heavy crankshaft balancer. For example, an 8-inch balancer might weigh 12 pounds, and a 6-inch balancer might weigh about 8 pounds.
Here’s a formula that you can use for reference:
For example, you have a 12-pound dampener with an 8-inch diameter (4-inch radius). Using the formula:
By substituting a 6-inch-diameter (3-inch-radius) dampener that weighs about 8 pounds, you reduce the rotating mass to 72 pounds.
Light Rods and Pistons
By selecting lightweight connecting rods and pistons, you gain quicker acceleration (the engine revs quicker). In addition, you reduce the dynamic stress on the crankshaft. By reducing the reciprocating mass, less force is exerted on the crankshaft as the rod tries to stop the piston at the end of the exhaust stroke. When you reduce the stress experienced by the crankshaft, you potentially extend crankshaft life and reliability.
A common practice among race engine and some street builders is to use a smaller diameter rod journal on the crank, which allows lighter rods. Common examples include 1.88-inch rod pins on small-block Chevy engines, allowing the use of Honda rod bearings. Big-end rods are available in a variety of lengths from leading rod manufacturers.
Take time to inspect the crankshaft. With a new crankshaft, check for dimensions and runout. With a used crankshaft, check for flaws (cracks). Inspecting the crankshaft before installation verifies its condition and allows you to avoid problems and/or return visits to the machine shop.
Flaws and Cracks
Clean the crankshaft before you perform a thorough inspection. Always use a magnetic particle inspection process to identify any flaws or cracks in the crankshaft. Magnaflux is one brand name for the process; other companies offer similar products to perform the same process.
First, mount the crank in a fixture on a work bench. Pass the crank through a large-diameter circular magnet. Under ultraviolet (black) light, look for any cracks or damage. Cracks are readily apparent and appear as white lines. If you discover cracks, your crank needs to be recycled and a new one must be purchased. This should be your first step in the inspection process. So don’t bother taking any measurements until after a flaw and crack inspection.
Next, check crankshaft runout. Place the crankshaft on a pair of level V-blocks and make sure it is level. Rest the crank’s front and rear main journals on the V-block and mount the dial indicator at the center main journal. The indicator’s probe needs to be slightly offset so it does not come in contact with the journal’s oil feed hole. The first step is to preload the indicator by about .050 inch and then the dial needs to be returned to zero. Gradually turn the crankshaft over and at the same time keep an eye on the gauge.
Once the crank has made one complete revolution, write down the maximum runout. For example, the maximum OEM-spec for allowable runout for a given crankshaft may be listed as .000119 inch. A runout limit of .001-inch or less is typically acceptable. If runout exceeds these limits, straighten the crankshaft or get a new one. However, a qualified technician must complete the crankshaft straightening at a machine shop. Since the crankshaft’s material likely has a “memory” in its grain structure, the crankshaft may need to be forced a bit beyond the desired amount to allow for spring-back. For instance, if a crankshaft has .0015- inch runout, the crankshaft may need to be pushed by about .002-inch, anticipating a .0005-inch spring-back.
Use a micrometer to measure the main journal diameter and verify that it’s to spec. In addition, take the measurement at the center of each main journal. Write down your findings and check to make sure it’s within spec for that particular engine. For many crankshafts, the manufacturer provides a tolerance specification of about .001 inch. For example, a typical spec could be 2.558 to 2.559 inches. A reconditioned crankshaft uses main journals that are reground to a smaller diameter so it’s within tolerance. Take the smaller diameter journal into consideration when selecting bearings.
Also measure the taper of each main journal near the front and rear of each journal. The limit of journal taper is usually between .0002 and .0004 inch. When measuring the journal concentricity or roundness, take measurements at many radial locations on each main journal, not just the center. The maximum-allowable out-of-round tolerance is usually about .000110 inch. Each rod journal needs to be measured for diameter, at several radial locations, and check the measurements against spec. The allowable tolerance range is approximately .0007 inch.
Be sure to measure each rod journal width. Take this measurement from the base of the front fillet to the base of the rear fillet on any given journal. Check this against the stacked width of two connecting-rod big ends. If rod journal width is too tight, the rod big ends have insufficient sideplay. Within general parameters, rod sideplay (the distance the rods can move from front to rear on a journal) should be at least .014 inch.
If journal diameter, taper, or width is out of round or beyond tolerance, regrind the crank on a dedicated crankshaft grinding machine. In order to correct journals, you end up moving to an undersize diameter, in which case you can easily purchase a set of undersized-ID main and/or rod bearings.
Also remember that bearing size needs to be uniform. If one main journal must be reground to accept an undersize main bearing, then all of the main journals should be ground to that same size. The same holds true for rod bearings. If even one rod journal needs to be undersized, then all rod journals must be ground to the same diameter. Always check with your bearing supplier to first find out what undersize bearings are available (-.0005, -.005, -.010 inch, -.020 inch, etc.). This determines the diameter of the regrind.
If a used crank passes inspection and you intend to reuse it, each journal can be polished on a crankshaft belt polisher, using 400 grit, then stepped up to 600- grit. Polishing usually eliminates small surface scratches. Different equipment manufacturers may specify different gritgrade abrasives for polishing. The journals should not be “mirror” polished, since microscopic scratches are needed to provide oil cling.
Always check any crank for block clearance, but this is critical when using a stroker crank that has a longer stroke than the original. Test fit the crank to the block. Install the upper main bearings to the block saddles. Make sure the saddles and rear side of the bearings are clean and dry. Install lower bearings in the main caps as well but don’t install the caps just yet. Lube the exposed surfaces of the bearings with clean engine oil or assembly lube. Make sure that the crank is clean. Carefully lay the crank onto the upper bearings, observing the counterweight positions. If you do have a clearance issue between counterweights and the block, you don’t want to crash the crank into the block.
With the crank resting evenly on all of the upper bearings, rock it back and forth on the upper bearings just a bit to make sure that all journals are evenly resting on all upper bearings. Slowly begin to rotate the crank, while observing each counterweight as it approaches the block surfaces. If the crank dead-stops, find the contact point and mark it on the block.
If you can’t rotate the crank 360 degrees due to contact, mark the first contact spots found, then rotate the crank in the opposite direction to find more interference locations. Mark all problem spots where the counterweights are actually interfering with crank rotation.
Remove the crank and the upper bearings. Use a hand grinder (pneumatic or electric) with either a radiused milling bit or abrasive stone, to minimally relieve the contact areas on the block (don’t remove more than needed).
Clean the block and re-install the upper bearings and the crank. As you rotate the crank without the main caps, watch the upper bearings. If a bearing moves a bit, push the protruded end back down with your finger. Once you verify clearance to rotate the crank a full 360 degrees, perform another clearance check at each counterweight. Rotate the crank and observe the counterweight-to-block clearance. Minimum clearance between a counterweight and any area of the block (on the rotation plane) should be around .060 inch. Mark and relieve the block as needed.
After test fitting the crank a second time, if more material needs to be removed, repeat the process. You can always remove more metal, but it’s very difficult to replace. You don’t want to remove more than needed to avoid weakening any area of the block.
After the block has been align-honed and bearing clearances have been measured, verify crank rotation. First, install the upper bearings and the crank. Install the main caps with bearings installed and lubed. Tighten the cap fasteners to manufacturer’s torque spec with oil or moly. Tighten all main cap fasteners in the proper sequence and to the spec’d torque value. Also tighten the main cap fasteners in stages.
For instance, if the spec calls for a final value of 110 ft-lbs, first tighten to 10 ft-lbs, followed by another pass at 25 ft-lbs, followed by another pass at, say, 50 ft-lbs, then 80-, then 100-. After the first tightening phase, gently rotate the crank just a bit to make sure that it isn’t jammed up. After each tightening step, rock the crank just a bit after snugging each main cap. If you run into a bind all of a sudden, you may have a problem with that specific main bore.
It’s important to understand that an OEM aluminum block, such as the GM LS aluminum block (LS1, LS6, LS2, LS7, LS3, or LS9), is very susceptible to distortion. Don’t expect the main bores to arrive at final alignment until all main cap fasteners are tightened to specification, which includes the main-cap side bolts. If the block has main-cap side bolts, tighten the side bolts only after tightening all primary cap bolts to their full spec.
Once the crank has been fully installed in the block, measure crankshaft thrust. Thrust is the amount of movement available for fore/aft movement of the crankshaft in the block.
Securely mount a magnetic-base dial indicator at the front of the block. Use a magnetic indicator base for iron blocks and mount the magnetic base to the face of the number-1 main cap on an aluminum block. Using a long and clean flatblade screwdriver, gently pry between a counterweight and a main cap to move the crank fully rearward. Place the dial indicator’s plunger onto the crank snout face or snout base (a flat surface). Adjust the indicator to slightly preload the gauge about .050 inch. Zero the gauge. Pry the crank backwards again to make sure that it’s still fully rearward. Zero the gauge again if needed. Then pry the crank fully forward while observing the dial gauge.
Refer to the recommended crankshaft thrust specifications for your particular application. Thrust specs vary depending on the engine, but a fore/aft movement of approximately .006 to .007 inch is a commonly acceptable range.
Shot peening is a process that involves pounding the surface of the crankshaft with steel or stainless steel round shot (tiny balls). This is not a blasting process intended for surface cleaning, where abrasive media is used. Shot peening is not abrasive; it’s a material-compression process. Shot peening hammers and work-hardens the surface while compressing the steel under the surface. This increases below-surface density, serving to make the crankshaft stronger and less prone to distortion or cracking.
Shot peening compresses the metal and reduces the chance for surface galling, cracking, or corrosion fatigue. Shot peening, especially in the journal fillet areas (the edges of the journals where they meet the counterweights) can be beneficial in reducing the possibility of stress cracking.
The purpose of any stress-relieving process is to eliminate internal metallurgical stresses in the steel, which reduces the chance for fracturing by allowing the molecules to align more uniformly. Shot peening is not really a stress-relief process. Because it compacts the surface of the metal, it can actually induce additional stress inside the crank. True stress relief should treat the entire crankshaft all the way through, not only the surface.
Stress relieving a crankshaft is performed as part of the manufacturing process, to reduce the internal stresses of the metal that occur during the forging and initial rough-machining steps. This is commonly done by heating the crank in a controlled, heat-treatment oven. Initially grinding journals to rough size with abrasive wheels tends to generate substantial heat, which can induce stress. CNC machining the journals (as opposed to grinding) results in less distortional heat. The crank needs to be stress relieved after either type of rough machining.
When a crankshaft is repaired, by regrinding journals and/or weld repairs, stresses are induced within the crankshaft. Stress relief “relaxes” and realigns the molecules. If a crankshaft has some runout and the crank is straightened but not stress relieved, the memory in the material may allow the crank to once again bend and return to the former runout state. Basically, whenever a crankshaft has been exposed to high heat and/ or mechanical stress during repairs, it should be stress relieved.
Additional benefits of stress relief include the long-term stability of the crankshaft for performance use. Both cryogenics and vibratory stress relief (see Chapter 18 for more details) are treatments that can improve the molecular structure and can improve durability and reduced harmonics.
Crankshaft counterweight modifications, either by design during manufacturing or during later modification, are performed to reduce counterweight mass (weight reduction) or to address oil windage concerns. Specialty lightweight racing crankshafts often have counterweights that are “carved” or “skeletonized” to reduce weight. A lighter crankshaft means less rotating mass, which means faster crankshaft acceleration (quicker revs). However, you can’t simply hack off counterweight material. The crankshaft must still be balanced by matching rod and piston weight.
Counterweight modifications that address windage concerns typically involve bullnosing and/or knife-edging. Altering the profile of the counterweights is primarily done to reduce parasitic oil cling and drag. During engine operation, parasitic oil (oil that slings around and clings onto the counterweights) can result in slight drag and minor imbalance conditions as the oil clings and then is slung off. This is only a concern for racing engines, where the goal is to obtain maximum performance.
Bullnosing involves radiusing the leading edge of the counterweight. By rounding-off the leading edge of the counterweight (eliminating a flat face), any parasitic oil that might cling onto the leading edge is more easily diverted off the leading edge.
Knife-edging involves tapering the trailing edge of the counterweight. By bullnosing and knife-edging, the profile of the counterweight is then shaped similar to an airplane wing. The radiused leading edge theoretically allows oil to slip over the leading edge. The reduced thickness of the trailing edge then causes the oil to more easily be pulled off the counterweight.
Modifying the counterweights to become more aerodynamic may free some power in a high-revving race engine, but it’s really a waste of time and money to perform this alteration on a street-performance crank. As with many modifications, changing the profile of crankshaft counterweights is for the race engine builder who is trying to extract as much power and longevity as possible, where every ounce counts. Any modifications to a crankshaft that involves material removal requires rebalancing.
Don’t do it. Cross drilling is an oldschool approach intended to feed more oil to the rod bearings for high-revving applications. Cross drilling refers to drilling the oil holes completely through the journals, followed by drilling an intersecting hole from the main to the rod journal. The theory is that this provides a more continuous flow of oil to the rod bearings. However, cross drilling can potentially weaken the crank because it introduces more stress riser points. Also, oil then needs help to overcome crankshaft centrifugal force. Even increasing oil pressure may not help.
Today’s high-quality aftermarket performance crank manufacturers don’t offer cross drilling for these reasons. Although some folks stick with this outdated approach, you’re taking a chance with the potential for severe rod bearing damage. Play it safe and just don’t do it.
Chamfering Oil Holes
Oil holes in the main journals and rod journals should be slightly chamfered, eliminating any sharp edges at the perimeter of the hole. Softening these oil-feed hole edges reduces the chance for stress risers and provides a smoother path for oil travel. Some builders get carried away by severely enlarging the chamfer area, which in most cases is not necessary. However, removal of sharp edges aside, additional radiusing/blending on the trailing side of the oil holes can produce a more efficient path of travel for oil to the bearing.
Blending a softy radiused and slightly extended “teardrop” path for oil can promote more efficient oil travel. The blend area should not be wider than the diameter of the oil hole, and should only be about .200 inch long at the most. Teardrop blending is not needed in all applications. Defer to the crankshaft manufacturer’s design or an experienced engine builder for a specific application.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks