A properly balanced engine operates more smoothly and with less dynamic strain. The results can include slightly better fuel economy, smoother operation throughout the RPM band, and most important, less life-draining and power-robbing harmonic vibration and avoidable stresses. In other words, a properly balanced engine lasts longer and has the potential to create more horsepower.
A slow-revving big-displacement V-8 may perform fine for common street driving while in a slight state of imbalance. However, today’s smaller V-6 and 4-cylinder inline engines are much more reactive to balance condition. Whether an engine machine shop builds street motors or race motors, or a combination of the two, it’s becoming increasingly important to provide a balancing service to properly recondition both low-RPM street motors and high-revving competition motors. Balancing is not for racers only!
For high-performance and racing engine applications, achieving proper crankshaft balance is an absolute must. For engines that experience high engine speeds and sustained high RPM, out-of-balance dynamic forces not only rob horsepower but can also play havoc on components such as main bearings, the crankshaft, rod bearings, rods, and pistons.
Everything that attaches to the crankshaft has an effect on dynamic balance. This includes connecting rods, pistons, piston pins, piston rings, rod bearings, pin locks, and estimated engine oil that will cling to crankshaft counterweights, rods, and pistons.
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The purpose of balancing, in a nutshell, is to match the crank’s geometric centerline axis to the mass centerline axis. The geometric centerline is an unchanging point of reference. This is the static center of rotation of the crank’s main journals. The mass centerline is the axis that can change, as rotating and reciprocating forces act on the centerline under dynamic conditions.
If the crank’s mass centerline (created by the force of imbalance) is too far out of alignment from the geometric centerline, these two centerlines constantly fight each other. In other words, the crank’s mains are forced to rotate in an eccentric path. This ongoing creation of an offset pressure point can squeeze the oil film out from between bearings and journals. Naturally, the result of a severe imbalance is eventual bearing failure. Don’t think that an engine has to vibrate wildly for this to be a problem. Even small forces, though possibly not felt by the driver, can magnify and, over time, can lead to shortened engine life. Balancing optimizes the operational conditions and is a prime factor in obtaining maximum engine life.
To remedy this serious condition, the crankshaft must be balanced. In the process, the entire rotating assembly must be considered, because of the dynamic effects placed on the crankshaft by the rotating and reciprocating weight (mass) that is moved by the crankshaft. This includes connecting rods, rod bearings, pistons, piston rings, piston pins, and piston pin locks (where applicable). You also need to consider the anticipated weight of oil that clings to the crankshaft counterweights, rods, and pistons.
Rotating weight, or mass, includes the crankshaft and the connecting rod “big” end and the rod’s big end bearings. Reciprocating weight includes pistons, rings, piston pins, pin locks (for full-floating pins), and the “small” end of the connecting rods. By equalizing the weight experienced by the crankshaft, you reduce or eliminate unbalanced reciprocating forces as the crankshaft rotates through its cycles. Engine balancing isn’t required for race engines only. Any engine must be properly balanced to obtain both performance and durability. Performance engines simply require a greater degree of precision balancing to minimize imbalance conditions as much as possible. Consider an imbalance condition of 56 grams (which equals about 2 ounces). At an engine speed of about 4,000 rpm, this results in a force of about 56 pounds of crankshaft deflection. At 8,000 rpm, that force climbs to a scary 224 pounds of imbalance force. The goal is to minimize imbalance as close to zero as practical. It’s important to note that chasing absolute zero is both difficult and unnecessary. Considering the clinging and slinging nature of engine oil, even if you were able to zero balance the crank, it’s not going to run at zero during engine operation. Realistically, balancing a non-high-performance street crank to within less than 4 grams and a performance and/or racing crank to within less than 1 gram is acceptable and practical to achieve with today’s computer balancers.
This is what you need to create your bobweights prior to balancing an internally balanced V-8 crankshaft: the crankshaft, one connecting rod, one piston, one piston pin, one piston’s ring package (including an oil ring support rail, if required for your pistons), pin locks for full-floating pins, and one rod’s pair of bearing shells. This assumes that you’ve already weighed and weight-matched all pistons and weighed and matched all rods. If the crank is externally balanced, you need to add the front dampener/harmonic balancer and the flywheel.
Out-of-Balance Forces on the Crank
An imbalance condition can place stresses on the crankshaft during engine operation. A few sample charts illustrate these forces. In the first example, I consider an imbalance condition of 1 gram at a radius of 3.25 inches (radius referring to the distance of imbalance from the crank centerline). This helps to understand how imbalance in grams results in pound imbalance during engine operation. Impacts per second and impacts per hour refer to the number of times this force impacts the crankshaft per RPM increases. It’s easy to understand how an imbalance condition can cause the crankshaft to deflect at higher RPM, which can cause main bearing oil clearance to diminish, eventually destroying the bearings, with subsequent damage to main journals. This also places undue stress on block main saddles and main caps. The transfer of energy caused by the imbalance can also impact rod-bearing clearances, with accompanying stresses placed on piston pins and piston pin bosses.
It’s important to note that not only are you concerned with the imbalance value in grams, but the radius as well. The radius refers to the distance from the crank centerline where the imbalance occurs or is corrected. The greater the radius (the farther the imbalance is away from the crank centerline), the greater the force that the crank is subjected to.
If the crankshaft requires a reluctor wheel (for monitoring by a crankshaft position sensor), the reluctor, also called a tone wheel, must already be installed prior to spin balancing the crank.
Connecting Rods
The first order of business involves weighing all rods and pistons. Each connecting rod is weighed on a dedicated and extremely sensitive scale along with an adjustable rod scale stand. Connecting rods are weighed in three steps: big-end weight, small-end weight, and overall weight. On a rod-to-rod basis, weights should be within a maximum variance of 1 gram. Always weigh rods with caps installed and rod bolts tightened to spec. If you plan to upgrade to stronger rod bolts, the bolts that you plan to use must be installed before you start weighing rods.
Performance connecting rods are designed with maximum strength while eliminating unnecessary mass. As a result, there isn’t much material available for weight removal. This side view of a Scat LS rod big end’s rod bolt area shows how little material is present in terms of potential weight removal. As mentioned earlier, today’s rods are designed with maximum strength while minimizing mass. Having no excess material in this area presents a problem on a rod that required weight reduction, but considering how well today’s performance rods are manufactured and pre-weight-matched, this simply isn’t an issue.
Old-school OEM rods feature balance weight pads at both the small and big ends. This provides material removal for weight matching.
Notice the raised ribs on this rod’s cap. These are part of the design to provide big end strength and are not considered “weight pads.”
Never attempt to remove material from a connecting rod’s beams. Any scratches, nicks, or material removal from the beam edges can pose a serious stress riser issue and can weaken the rod.
Here’s the small end (piston pin end) of a performance rod. Even if you needed to remove a bit of weight from the small end to obtain a matched set of rods where the pin ends weigh the same, you’d only be able to remove very little material. If by chance you do “kiss” this area for weight removal, make sure that the oil hole is not burred or obstructed.
Performance aftermarket rods feature laser-etched number identification at each side of the cap parting line. This unique number allows you to keep rods and their caps organized. If you’re dealing with rods that are not marked, use either an ink marker or a light-duty electric etching gun to create marks. Never use number or letter stamps and a hammer, since you could very possibly distort the rod big end.
High-precision digital scales can be extremely sensitive, often influenced by a slight movement of air. The digital scale should be located in an area away from open doors or windows. Stand very still and allow the scale reading to settle down. Then tare the scale to confirm that it’s reading zero weight. When weighing a component, stand still to avoid air movement and allow the reading to settle. Some scales are so sensitive that even slight body movement (waving your arm, moving toward or away from the scale) can cause the reading to fluctuate.
This side view of a performance aftermarket rod clearly shows that there is no “extra” material available for weight removal at the rod’s small end. As mentioned earlier, today’s performance rods manufactured by established makers are so well weight matched out of the box that no weight corrections should be needed.
High-performance rods are already equipped with high-strength rod bolts. If you’re dealing with OEM rods or used aftermarket rods and plan to install new rod bolts, do this prior to balancing, as different rod bolt brands/types could differ in weight as compared to the original bolts.
The rod big end is weighed using a precision scale that is specifically designed for engine balancing. You support the big end on a specialty stand that rests on the scale, and the small end is supported on a separate stand that is positioned next to the scale. When properly set up, the rod is positioned horizontally with the big end and small end centers at the same height. Before weighing the rod, first place the rod big end stand on the scale and press the “tare” button. This negates the weight of the stand, zeroing the scale with the stand in place. This prevents the weight of the stand from influencing the weight reading. Then mount the rod with both ends supported on the individual stands. After the scale is zeroed, record the weight of the big end in grams.
Precision digital weighing scales are offered by several manufacturers, in varying levels of sensitivity. In addition to the scale, you must also have a rod support stand that is placed on the scale and a rod support adjacent to the scale. This allows you to weigh rod small ends and big ends while one end of the rod is supported off of the scale. Shown here is a scale set offered by Goodson Tools & Shop Supplies.
When setting up a connecting rod on a digital scale, it must be suspended level so that both the small end and big end bore centerlines are at the same height. Here the small end height is adjusted.
A small bubble level aids in adjusting the rod’s state of level.
The off-scale support is adjustable to allow leveling the rod when weighing either end.
This photo shows the rod’s small end being weighed. Again, the on-scale stand has been zeroed so as not to include the weight of the stand. The small end is placed on the stand, while the big end is supported off-scale. The support bushing that engages into the big end bore is simply swapped onto either stand as needed.
All weigh scales accomplish the same task. Shown here is a model that features an off-scale support stand where the unweighed end of the rod is suspended by an adjustable chain.
An example of weighing a rod big end. A support stand is placed onto the scale platform and zeroed out (by pressing the tare button). The big end is then placed on that stand, while the small end of the rod is suspended on a stand that is not on the scale. The remote support stand is adjustable for height to level the rod centers.
Remove the rod from the stands and, if necessary, install a smaller-diameter bushing on the stand (some scale stands feature two different bushings, one for big ends and one for small ends). If you change the bushing on the stand that rests on the scale, install that bushing and once again tare the weight to zero the scale. Next, install the rod with the small end on the scale stand and the big end supported by the adjacent stand. Adjust the rod so that the small and big end bore centers are on the same horizontal plane (level). Note the weight of the small end and record this in grams.
Perform these steps for each rod, using a felt marker to record the weights on each rod (or carefully organize the rods on a large sheet of paper with the weights recorded next to each rod. If weight deviations are noted that are greater than 1 gram, you may need to remove weight from the heaviest ends to lighten ends that are too heavy, to match the weight of the lightest rod. In practical terms, weight variations under 1 gram are certainly acceptable. Chances are, the only instance that you need to remove weight to equalize rods is when you’re dealing with older original equipment rods that feature weight pads.
Old-design OEM connecting rods likely require weight matching by removing material from all but the lightest rod big ends and small ends. When grinding a rod, follow the “grain” of the metal. Here the sides of a rod big end are being ground to remove a few grams, in the area where the cap meets the rod. Notice that the grinding wheel is contacting the rod in a vertical orientation. If you grind perpendicular to the rod, you can create unwanted stress risers. Even after grinding in the correct plane, be sure to bead blast and polish the ground surfaces to eliminate potential stress risers.
Again, considering today’s high-quality performance aftermarket rods, it is extremely rare to find a set of rods that are not already weight matched. The objective is to obtain a set of rods that are weight matched. When using OEM production rods, although today’s mass production rods are much more accurately matched by weight than in previous decades, it’s not uncommon to run into a set of rods in which a few are under- or overweight. Considering that many OEM rods are made in a mold using a pressure-cast “powdered metal” construction, it may be difficult to weight-match rods by removing material from rods that are too heavy, because there may not be enough material removal availability without weakening the rods. In those cases where you are forced to use OEM rods, to achieve a matched-weight set, you may need to obtain several additional rods, weighing each, to achieve a weight-matched set. The goal is to have all rod small ends match to within .5 gram. The same holds true for rod big ends. Equalizing the weight of rod big ends benefits rotating balance. Equalizing rod small end weight benefits reciprocating balance.
In the “old days,” it was common practice to correct connecting rod and piston weights by removing material from the rod big ends and small ends (most rods featured a “weight” boss at each end where material could be removed on a grinding wheel. By the same token, pistons could be weight matched by carefully removing material from the underside of the pin boss areas.
Thankfully, because of the precision manufacturing techniques of CNC, today’s performance aftermarket rods and pistons rarely require modifications to achieve rod-to-rod or piston-to-piston weight matching.
If you do need to remove weight from rods, this should be done on a grinding wheel. If you’re removing material from a weight boss on the bottom of the cap, or from a weight boss at the top of the small end, you can lay the rod flat on the feed table of a bench disc sander or grinder, and use the flat side of the wheel during material removal.
If you’re removing material from the bolt sides of the rod, the grinding wheel should make contact parallel to the surface (making the grinding scratches run in a direction from the top of the bolt toward the bottom of the bolt area). Don’t grind perpendicular/90 degrees from the length of the rod, as this can create stress risers. After grinding, any disturbed surfaces should be glass bead blasted and polished to remove any scratches. Depending on the design of the rod, you may not have enough material from which to grind. Avoid removing material that could weaken the rod. If removing material from the bolt sides, avoid removing material at the parting line. If the shoulders provide enough material, you may be able to carefully remove material from the upper shoulders.
Again, you want to avoid reducing rod strength. Carefully remove a very minute amount of material and reweigh, repeating until the big end and/or small end matches the weight of the lightest rod in your set. You can easily remove weight but you cannot add weight, so take your time. If you make a mistake and remove too much weight, that rod now becomes your lightest, making it necessary to match the remaining rods to that lightest rod. Grinding any surface of a rod creates scratches, which can result in stress risers. After grinding, you should bead-blast the ground surfaces and/or polish to remove or reduce these scratches.
When grinding material from a rod big end, always keep the rod cap in place and fully torqued to spec. Today’s performance aftermarket rods are designed with minimal material while maximizing strength. As a result, there probably isn’t much material available at either end to remove without weakening the rod. Because quality performance rods are made to such exacting tolerances, as I mentioned earlier, a set of rods is so closely matched in weight that you likely don’t need to touch them.
Performance aftermarket rods are always laser etched with matching numbers on the rod and its cap. Never mix rods and caps. If your rods are not marked, use an etching pen on one side of the rod big end, above and below the cap parting line (for example, 1, 2, 3, 4, etc.). Avoid stamping numbers onto rods with a number stamp and a hammer. It’s too easy to potentially distort a rod. Always etch numbers, if necessary, onto one side of the big end, parallel with the rod bolt. Never stamp or etch onto the rod beam, and don’t use a file to cut notches onto the rod. File cuts can result in potential stress risers.
Rod bearings from the same set match in weight, so there’s no need to scale weigh all bearings. Simply weigh one rod’s pair of bearing shells when making up the bobweight card.
Also, if you plan to run original/stock rods and plan to upgrade the rods with stronger aftermarket high-performance rod bolts, install the new bolts first before weighing the big ends. Just remember that prior to final weighing the individual components and making the bobweights, all modifications to rods and/or pistons must be performed first. Changing the weight of these components after balancing affects crank balance for the worse. Pistons and rods must be in a ready-to-install state before you make the bobweights.
One rod’s bearings are weighed. This weight is recorded on the bobweight card.
High-quality, high-performance forged pistons are often CNC machined to provide maximum strength while removing unnecessary weight to reduce mass. Pistons, such as those made by JE as shown here, optimize strength-to-weight ratio and leave little if any material for removal. Because these pistons are designed to a high degree of precision, they are already weight matched out of the box, with no need to perform any weight corrections.
Pistons
When weighing pistons, there’s no need for a special stand. Make sure that the scale surface is clean and tare the scale before placing a piston onto the scale pad. Each piston and its pin may be weighed together as a package or individually. Weighing each piston along with its pin is more convenient. If minor weight differences are found, mixing and matching pins to pistons may be necessary to obtain equal weight-matched piston and pin combinations. As stated earlier, when you’re dealing with today’s high-quality performance aftermarket pistons, chances are that each piston in a set weighs the same, as does the piston pins that are included with the piston package. However, it’s best to weigh each and every piston and pin simply to verify weight match, rather than assuming that all pistons and all pins weigh the same. After weighing each piston, mark the dome or boss with a felt-tip marker to note its weight.
It’s always a good idea to weigh each piston to verify weight, but if a set of new forged pistons from any of the leading performance piston makers is selected, you shouldn’t need to make any modifications.
If the pistons feature a short compression height that results in the pin bore intersecting the oil ring groove, a support rail is required at the bottom of the ring groove to provide oil ring support in the void areas that are open to the pin bore. When weighing your ring package, be sure to include a support rail when making up the bobweight card.
Before balancing, it’s a good idea to verify valve-to-piston clearance during test fitting, just in case any valve reliefs need to be enlarged or deepened. If you later discover a need for increased relief, you will likely want to rebalance the crank.
If you’re dealing with budget- priced pistons or older-generation OEM pistons, you may very well find differences in weight. When you’re dealing with a set of pistons, always use the lightest piston as the reference goal. Weight can then be removed from heavier pistons to obtain a set that is weight matched. If weight does need to be removed, only remove material from the underside of the pin boss. This can be done by carefully drilling one or more shallow holes in the boss. Don’t get carried away. Start with a relatively small drill bit, perhaps 1/8 inch in diameter, and drill to a maximum depth of about 1/8 inch, and reweigh the piston. This gives you an idea of how much weight removal that specific relief provided. Continue as needed by moving to a larger-diameter drill bit and performing small-increment material removal, weighing after each step until that piston’s weight matches the lightest piston.
An alternative is to chuck the piston in a dedicated piston vice and carefully mill material from the underside of the pin bosses. Remember: it’s easy to remove material, but it is impossible to replace weight. As I mentioned earlier, with today’s high-quality aftermarket performance pistons as offered by leading piston makers such as JE, Mahle, Ross, Icon, Diamond, CP, etc., you don’t need to correct for weight. Ideally, all piston and pin combinations should weigh exactly the same; as long as weight variations are within a range of less than 1 gram, you’re good to go.
If a piston, generally an OEM style, features enough material at the underside of the pin bosses, weight may be removed here by securing the piston in a dedicated piston vice and machining across the pin bosses. Make very small cutting passes and reweigh often to avoid removing too much material. If you go a bit too far, a heavier piston is the lightest piston and you need to start over by correcting the remaining pistons.
Weigh one piston’s ring package. If the piston requires a support rail, make sure to include the support rail as part of the ring package.
If you are unable to equalize rod small end weight and piston weight, experiment by matching heavier pistons to lighter rod small ends, or heavier rod small ends to lighter pistons. If this provides satisfactory weight matching, you need to label each piston and its matching rod so that they stay together during final assembly.
This Tech Tip is From the Full Book, AUTOMOTIVE MACHINING: A GUIDE TO BORING, DECKING, HONING & MORE. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
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If rod pins do not weight match, use the lightest pin as the reference. Before attempting to remove any material from the pins, first try mixing and matching pins to pistons, for example, pairing the lightest pin to the heaviest piston, etc. You may be able to establish a combination for all piston and pin pairs that allows you to create matching-weight piston/pin combinations without the need to remove material from either pistons or pins. If necessary, material may be removed from heavier pins by carefully chucking the pins in a lathe and lightly kiss-chamfer-cutting material from the ends of the inside diameter. Again, if you’re dealing with current-day high-performance aftermarket pistons and pins, this isn’t necessary due to the extremely close tolerances employed during manufacturing. In the past 20 years or so, I’ve never run into a set of pistons or pins made by leading manufacturers such as JE, Diamond, Mahle, Ross, Keith Black, Icon, etc., that are not already weight-matched.
Don’t weigh the piston rings straight out of the box. After the block cylinders have been honed to final size, check ring clearance of all top and second rings. If the rings need to be file-fit to obtain correct ring end gap, the rings must be filed first before weighing them. Granted, the amount of weight removed by filing may be minuscule, but just make sure that the rings are in the condition of properly fitting the cylinders before weighing them, in case any file fitting is involved.
Also, keep in mind that pistons may or may not feature specialty coatings, such as ceramic thermal barrier coating on domes and antifriction coatings on skirts. If the pistons are already coated, they’re ready to be weighed. If the pistons are not currently coated, but you plan to send them out to be coated, wait until they’re coated and then weigh them.
Pistons can be weighed separately, without the pin and/or locks. If you choose, each piston can be weighed in combination with its pin and pin locks. If this is done, make sure to keep that pin with that piston. If the piston and rod assembly uses a free-floating pin, where both the piston and rod are free to rotate on the pin, pin locks or buttons are required to secure the pin in place during engine operation. If locks are required, they must be weighed and factored into the bobweight data..
These coatings don’t increase weight dramatically, but it just doesn’t make sense to weigh them while bare when you know that these coatings will be added later.
One point that I’m trying to make is that, although high-quality aftermarket performance rods and pistons are likely already weight matched, don’t assume anything. Just to be safe, take the time to weigh each rod and each piston. You’ll probably be wasting your time, but you’ll feel better after you verify this.
Engine Styles
Not all engine balancing requires the use of bobweights. Engines that operate the crankshafts in a single plane such as inline 4- and inline 6-cylinder engines, or opposing-cylinder engines (as found in Porsches), have opposing rod throws that run in only one plane. Balancing these engines simply involves balancing the crank by itself, and then weight-matching rods and pistons/pins, with no need to factor in the weight of the reciprocating assembly when balancing the crank. V-style engines involve multiple planes of force, requiring counterweights to compensate for the reciprocating weight of the rods, pistons, pins, rings, and rod bearings. These engines require the use of bobweights to represent the mass of the rods, pistons, etc., during crank balancing.
Bobweights
To balance a crankshaft, you need to create “bobweights” that attach to each of the crank’s rod journals, to simulate the weight that the crank experiences in operation.
In order to create these bobweights, you first need to create a bobweight card. The card lists the weight (in grams) of the components that will be installed onto the crankshaft during assembly. As noted, this includes connecting rods, rod bearings, pistons, wrist pins, piston rings, piston pin locks, along with the estimated weight of engine oil that will cling to these surfaces. As each component is weighed on a digital balancing scale, you record the weights on the reference, or bobweight, card.
There is no need to weigh each piston ring set or each rod bearing, because these components are always closely matched in weight. Any difference is so negligible that it doesn’t affect balance. That’s why it’s only necessary to weigh one piston’s set of rings and only one rod’s bearings.
After the rods, pistons, and pins have been weighed and recorded, weigh one rod’s pair of bearings and record this on your bobweight card.
A sample bobweight card. You include 100 percent of the rotating mass (both rod big ends and both rod bearings that share one journal) and 50 percent of the reciprocating mass (one rod small end, one piston, one set of rings, one set of pins, one set of pin locks).
When both bobweight halves are verified for weight, weigh the pair and verify that it meets your total bobweight requirement.
Both sides of a bobweight must be of equal weight. If, for example, total bobweight needs to be 2,000 grams, each half of the bobweight must weigh 1,000 grams. Assemble each bobweight half to the appropriate half-weight and weigh each half of the bobweight.
Weigh one piston’s pin locks (if featured) and record. Weigh one piston’s set of rings, which includes the top ring, second ring, and oil ring set. If your pistons feature a fairlyshort compression distance where the pin bore intersects with the oil ring groove, you also have an oil ring support ring for each piston. Weigh the entire set of rings for one piston and record this weight.
The bobweights are composed of two-piece aluminum plates that each feature a “V” notch that attaches to the crank rod journals. These plates accept the bobweight that simulates the weight of the rods, pistons, pins, rings, rod bearings, and anticipated oil-cling that the crank deals with during operation. There are two basic types of bobweight designs, one featuring hollow tubes that are filled with lead or steel shot, and another style that features weighted shim discs that slip over the bobweight studs.
To create a bobweight on a V-8 or 90-degree V-6, you consider 100 percent of the rod throw’s rotating weight (the big end of the rod and the rod bearing) and 50 percent of the reciprocating weight, which includes a piston, rod small end, rings, wrist pin, and locks. For other engine configurations, different percentages of the reciprocating weight may be required for use. The manufacturer of the balancing equipment usually provides a reference chart or has this information programmed into its computer software.
When the total bobweight is determined, the weights are assembled to duplicate the real-life reciprocating mass.
Installing Bobweights
The bobweight must be installed perpendicular to its rod pin. In other words, install the bobweight to simulate an opposing pair of rods 90 degrees from TDC of the rod throw. With the specific rod journal positioned upright at TDC while on the balancing machine’s V-blocks, position a bubble level on the top surface of the bobweight clamp flat surface and adjust the bobweight to level before fully tightening it onto the journal.
Bobweights attach to the crankshaft’s rod journals with aluminum V-block clamps that align together on sliding pins. Because aluminum is softer than the crankshaft’s surface, this does not damage the journal bearing surfaces. Naturally, both the clamps and the journals must be clean prior to installing the bobweights.
Bobweights must be installed to the crankshaft perpendicular to each of the crankshaft’s rod throws. With a rod throw positioned at twelve o’clock, a bubble level is placed on the bobweight clamp to assist in obtaining a 90-degree location.
An easy and quick way to center the bobweight onto the journal is to temporarily install an aluminum spacer that takes up half of the total width difference between the bobweight clamp and the journal width. Note the spacer installed here. This allows you to sandwich the spacer between the end of the journal and the clamp. After the bobweight clamp is secure, the spacer is removed.
Each bobweight must be centered, front to rear, on each rod journal. Installing bobweights too far forward or rearward affects balance and provides a false reading.
Here’s another view of the centering spacer in place. This saves time as compared to using a ruler or caliper to center the bobweight clamp. Even though the bobweight clamps are aluminum and softer than the steel journal, always make sure that the journals and the clamps are clean prior to installation.
Some balancers feature a strobe light to monitor crank angle. Note the small timing wheel.
Also, and this is often overlooked, the bobweight must be centered on the crank’s rod journal, with equal distance between the front of the bobweight clamp to the adjacent crankshaft face and the rear of the bobweight clamp to its adjacent crank face. An easy way to do this is to make a centering shim that you can slip onto the journal, between one side of the bobweight clamp and the adjacent counterweight. As an example, if the distance between a journal’s boss faces is 1.90 inches, and the bobweight clamp is 1-inch thick, insert a U-shaped shim that is .45-inch thick onto the journal, on either side of the bobweight clamp. Sandwich the bobweight clamp and spacer together and tighten the bobweight clamp to the journal, then remove the shim. Instead of spending time by measuring, a premade shim (for the specific rod pin length of the crank) makes it easy and quick to obtain a centered location for the bobweight. By not centering the bobweight on a journal, you could end up with a difference of a few grams from front to rear of the crank. If the bobweights are installed toward the rear of the journals, you end up overbalancing the rear of the crank and underbalancing the front. The difference may or may not be harmful, but centering the bobweight eliminates this variable.
4-Cylinder Inline Engines
When balancing a 4-cylinder inline engine, you have opposing strokes (two up and two down), so the forces cancel each other out; you don’t need to create bobweights. Simply weight match the reciprocating parts (rods, pistons, etc.). Then spin and balance the crank without adding bobweights. Weigh the pistons and pins separately. You can then match pistons to pins to “even out” the piston/pin set weights, thereby reducing the time needed to machine weight from pistons (match the lightest piston with the heaviest pin, etc.).
Crankshaft
The crank can’t be final-balanced alone. Performing a static balance of the crank means that only the weight is distributed evenly around the center of rotation. The crank must be dynamically balanced to compensate for the rotating force of the rod big ends and the reciprocating forces by the rods, pistons, rings, etc., to correct for centripetal force.
Some crankshaft balancers feature a safety bar (note the black loop next to this crank’s rear flange). This does not contact the crank. It simply provides a fail-safe to prevent the crank from jumping off the balancer in the unlikely event of a severe crankshaft wobble.
Crankshafts are spun via a belt driven by the balancing machine’s electric motor. Always wear safety glasses during spin balancing to protect your eyes from potential airborne particles. If you have drilled the outer edge of a counterweight and have added a slug of heavy metal, spinning the crank before welding the slug presents a possible danger if the slug accidentally leaves the crank. If weight was added to a counterweight outer edge, don’t stand in front of the crank during spinning until the slug is secured with a weld. Some machinists use temporary sticky putty on the counterweight (creating a specific weight on the digital scale) to perform a test spin. Because the putty can sling off, this is another reason to avoid standing directly alongside the crank during the spin.
Centripetal force (not to be confused with centrifugal force) is created by crankshaft imbalance, where the imbalance force tries to deflect the crank back and forth during operation. This back-and-forth stress can result in main bearing damage or even crankshaft fracturing. By dynamically balancing the crank, you lighten the heavy side or add weight to the lifter side to eliminate centripetal force.
A computer balancer, depending on design, can be used to record individual weights from the bobweight card data and display fi nal bobweight requirements.
If the specifi c balancing machine does not provide a drill press, the bobweights must be removed and the crank then placed on a V-block fi xture at an overhead drill press for weight removal. This is followed by moving the crank back to the balancing machine and reinstalling the bobweights to recheck the dynamic balance.
A balancing machine that incorporates a built-in drill press provides convenience and saves time, avoiding the need to relocate the crankshaft to a separate drill press.
Whenever drilling a counterweight edge to remove weight, always place a clean shop rag on the adjacent journals to prevent contamination.
Depending on where weight removal is most appropriate, instead of drilling to remove all or part of the weight required, the outer faces of the counterweights can be machined on a precision lathe. If the balancing machine indicates that weight should be removed from a larger radius, lathe cutting can save some time.
Handy reference charts are available that tell you what diameter and depth of hole is needed to remove a specific amount of weight, pretty much eliminating guesswork. The balancer tells you how much weight needs to be removed and at what location on a counterweight.
Here an aftermarket crank for a big-block Mopar application is “kissed” on a lathe. The alternative is to drill a series of shallow holes along the edge of the counterweight.
A dynamic balance involves balancing the crankshaft in two planes (as opposed to static balance in a single plane). A dual-plane balance involves both ends of the crankshaft, where balance correction at one end directly affects balance at the opposite end.
Most crankshaft dynamic balancing machines spin the crankshaft at about 750 to 1,000 rpm, at a speed that is sufficient to obtain imbalance data.
If you’re chasing dynamic balance and trying to get picky, there are times when only a very small amount of material removal is desired, which can be handled by hand grinding an area of a counterweight. If using a handheld grinder, cover the adjacent journal with a clean shop rag, and be very careful. One slip could nick the journal.
In this case, the rear counterweight required a heavy weight placed close to the crank centerline at a small radius. To drill the hole, a hole was required at the rear flange to access the counterweight. This was done with the crank anchored vertically on a drill press.
Here a slug of heavy metal is carefully interference fit into a counterweight. The pass-through hole, required for drilling the affected counterweight and to install the heavy slug, is then filled or another hole drilled to offset this first hole.
A close-up view of the installed slug of heavy metal. Installing tungsten through a counterweight enables you to work closer to the crank centerline and eliminates the potential for the weight slug to sling off of the crank during engine operation. If a tight interference fit isn’t possible, the weight is then tack welded to the crank to make sure that it doesn’t dislodge.
The crankshaft is typically spun at a speed of approximately 750 to 1,000 rpm, sufficient to develop forces that are measurable.
Setting up the crankshaft on the spin balancer involves entering the radius from the crank’s centerline to the outer radius of the counterweights, as well as the distance between the centerline of counterweights. A computer balancing machine then determines the balance condition at each front and rear counterweight and provides the operator with the necessary information regarding the amount of weight that needs to be added or removed, and the location of that modification.
Heavy Metal
When weight must be added to a crankshaft, a special “heavy metal” slug is used. This material is also referred to as “Mallory” metal. In essence, this material is composed of a heavy tungsten alloy, which is a little more than twice the weight of steel of the same dimensions. Because this material is heavier than steel, it allows you to use a smaller-dimension slug to provide the needed weight. Unlike lead, tungsten is weldable, environmentally friendly, and withstands shocks and heat.
When you select an aftermarket performance crankshaft, you need to know the “target” bobweight. This is the approximate bobweight that the particular crank likely requires. If the target weight isn’t included on a spec sheet with the crank, consider calling the maker and asking. If your required bobweight is heavier than the crank’s target weight, you likely need to add a bit of a heavy metal. In that case, make sure that you have slugs of tungsten on hand. If your bobweight is the same or lighter than the crank’s target weight, you likely only need to remove material to balance the crank.
Under- versus Over-Balancing
For various racing applications, a common balancing “trick” is to underbalance or overbalance the crankshaft to best suit a specific engine RPM range. Although not suitable for a street performance application, where engine balance needs to provide suitable state of reduced harmonics from idle through occasional high-speed operation, under- or overbalancing allows the machinist to minimize harmonics in a predetermined engine RPM range. This is applicable for engines that typically run only within a specific RPM range (for example, due to the nature of the racing series, the engine may operate only within 6,000 to 8,000 rpm). A slight vibration or harmonic at idle or low-RPM is acceptable, since the vehicle doesn’t run at these lower engine speeds on the track.
Under- or overbalancing may or may not be beneficial on all race engines built for the same purpose, due to variations in valvespring stiffness, timing chain, or timing gear harmonics, etc. Basically, in the case of certain race engine applications, you simply don’t care about vibrations at engine speeds that aren’t used on the track, as long as you can dial-in the balance to optimize engine operation at a very specific engine speed that the engine experiences during competition.
Some race engine builders prefer overbalancing by 1 or 2 percent for a high-revving engine and are convinced that this produces more power. In theory, overbalancing can serve to dampen valvetrain harmonics at a specific engine speed. To overbalance, instead of using 50 percent of reciprocating weight when making the bobweights, you might use 52 percent of reciprocating weight. If underbalancing is desired, the bobweight’s reciprocating weight might be 48 percent. Again, under- or overbalancing is often a trial-and-error approach, unless an engine builder has developed a specific routine in his builds (valvetrain geometry, spring pressures, timing system, camshaft, etc.) where under- or overbalancing has proven to be successful.
External versus Internal Balancing
A crankshaft, depending on engine design, may be internally or externally balanced. Internal balance refers to a crankshaft where all balancing (removal or addition of weight) occurs on the crankshaft counterweights and requires the use of zero-balanced crank dampener and flywheel. Of course, this assumes that the pistons, rods, etc., are already weight matched. When an internally balanced crankshaft is placed on the crank balancing machine, there is no need to attach either a front dampener or flywheel, because an internally balanced crankshaft requires the use of a zero-balanced dampener and a zero-balanced flywheel.
An externally balanced crankshaft uses a weighted dampener and weighted flywheel (where each features a balancing weight or pad). In this case, the dampener and flywheel must be attached to the crankshaft during spin balancing of the crank.
With an internal balance, because the dampener and flywheel are zero balanced and do not affect crank balance, the dampener and/ or flywheel may be replaced during servicing at any time without affecting crankshaft balance (provided you always install a zero-balanced dampener or flywheel).
Late-model engines commonly feature a toothed reluctor wheel that provides a crankshaft position reference for electronic engine management. If the wheel was removed for any reason or a new crank did not feature an already-installed wheel, make sure that the wheel is installed onto the crank prior to balancing. Adding a wheel after balancing could result in an imbalance condition. If the crank is designed to use a reluctor wheel, it must be in place before attempting to balance the crank.
Dampeners and Flywheels
If the crank is internally balanced, the crank dampener and the flywheel must be zero balanced. This can be done on the balancing machine with special arbor fixtures to secure the units separately. However, a convenient approach is to mount them to the crank, after the crankshaft has been balance corrected. If the dampener and/or flywheel aren’t precisely zero balanced already, any imbalance is then discovered and corrected on the dampener and on the flywheel, without any further correction to the crank itself. If this approach is taken, the dampener and flywheel must be reinstalled in exactly the same clock positions used during balancing. If the crank snout is keyed, the dampener only installs in one location anyway (examples of exceptions are OEM LS engine crank dampeners that are not keyed). The flywheel may or may not be indexed to only bolt onto the crank in one location only. If not, place a permanent matchmark on the flywheel and crank flange.
If the crankshaft harmonic balancer features an offset weight pad, this clearly indicates that the crankshaft is to be externally balanced. Unlike an internally balanced crank where the front dampener and flywheel do not need to be installed to the crank during balancing, a weighted dampener must be installed to the crank. If the dampener is replaced down the road, the crank should be rebalanced with the new dampener in place.
A flywheel/flexplate that features welded-on weight was intended for an external balance. For example, if you purchased a rotating kit with crank, rods, pistons, rings, bearings, and flywheel, the weight on the flywheel was added to externally balance the crank. If you’re performing an internal balance job, be sure to obtain a flywheel that is already zero balanced.
If you purchase zero-balanced dampeners and flywheels from reputable makers, checking them probably isn’t necessary, but if you really want to be picky and to verify balance, it’s worth considering.
Viscous Dampeners
A viscous-type harmonic dampener is different than an OEM-type elastomer-dampened unit. And as such, you need to be aware of the following. A viscous dampener (ATI, Fluidampr, etc.) takes advantage of the centrifugal force created by a captive fluid inside the dampener ring. Although an elastomer-ring type dampener is designed to only cancel a crankshaft’s harmonic vibrations at a predetermined frequency range, a viscous-type dampener is designed to cancel harmonic vibrations at any RPM.
As far as crankshaft balancing is concerned, here’s what you need to know: if the engine is an internally balanced design (where all of the crankshaft balancing occurs at the counterweights and the dampener and flywheel are zero-balanced on their own), the viscous dampener itself is already balanced, so there’s no need to perform any balancing work on the dampener at any time. If the engine is an externally balanced design (where the front dampener and the flywheel are integral components of crank balance), the viscous dampener consists of two parts: an outer dampener ring and a center hub. Disassemble the dampener to separate the ring from the hub. Mount only the hub to the crank snout (along with the flywheel at the rear crank flange) for crankshaft balancing. Do not attach the viscous dampener ring to the hub for balancing.
In short, never install a viscous-type dampener or dampener ring to a crank for spin-balancing because the centrifugal internal action of the dampener’s fluid serves to absorb and mask some of the crank’s harmonic disturbances and results in a false spin-balance reading.
Even if the crank is to be internally balanced, and even if you trust the dampener hub to be zero balanced, installing the dampener’s hub to the crank during the balancing setup can’t hurt, and you are able to compensate for any slight deviation of hub balance with the hub in place. If the hub is not keyed to the crank snout and relies only on an interference fit, be sure to place matchmarks on the hub and crank before removing the hub from the crank snout, so that the hub and dampener assembly is final-installed in the same clock position during final engine assembly.
Clutch pressure plates can be checked for balance by mounting the pressure plate to an already-zero-balanced flywheel on a balance arbor. Any weight correction is made by welding weight onto the pressure plate cover. Again, quality pressure plates should already be zero balanced from the factory, but checking, and correcting if necessary, eliminates a potential customer’s complaint of vibration.
If a torque converter is to be checked or corrected for balance, the fluid must be drained first. If the converter does not feature a drain plug, you may drill and tap a hole and install a plug, but if you do that, you should install an identical plug 180 degrees from the first plug to negate the weight of the first plug. If fluid remains in the converter, your spin balance readings aren’t repeatable as the fluid slings around inside. Quality torque converters should already be balanced, so you really shouldn’t need to spend time trying to balance them.
A viscous dampener features an internal cavity with a special silicone fluid that slings around inside and offsets harmonics. Never mount a viscous dampener onto the crankshaft during spin balancing, as this type of dampener may mask vibrational harmonics in the crank.
Written by Mike Mavrigian and republished with permission of CarTech Inc
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