For any engine to operate at its peak, all vital components of the rotating assembly must be balanced. You need to perform the balancing procedure once all components have been verified for use and all primary machining and fitment has been accomplished. Don’t waste time and money by balancing the crank until you’re certain that all machining and test fitting has been performed.
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Discussion about engine balancing is essentially referring to balancing the crankshaft. The factors that affect crankshaft balance include all of the rotating and reciprocating weight, so in order to balance the crank, you also need to balance the pistons and connecting rods. Rotating weight involves the crankshaft and the big ends of the rods as well as and the rod bearings. Reciprocating weight involves the small ends of the rods, the pistons, wrist pins, pin locks, and rings.
When balancing a crankshaft, the common approach is to consider 100 percent of the rotating weight and 50 percent of the reciprocating weight. Overbalancing is an approach that considers a higher percentage of reciprocating weight. Instead of weighing only a single piston, a single rod, etc., take the time to weigh each component in order to weight-match all pistons and all rods.
Follow basic weight-matching guidelines (use the lightest piston as the reference and remove weight from all remaining pistons to match, etc.). Even when performing an internal balance, don’t assume that the zero-balance dampener or flywheel is in fact balanced to zero. Spin balance the dampener and flywheel independently as well, correcting it if needed. If, by chance, the dampener will be painted, spin it again after the paint job to verify that no severe paint-thickness inconsistencies are present that affect balance.
Engine balancing refers to balancing the crankshaft to accommodate the reciprocating and rotating forces that it encounters during operation. Reciprocating force is presented by the forces that go up and down and act upon the crankshaft centerline. This includes the weight provided by the pistons, piston pins, piston rings, and pin locks. The rotating weight attached to the crankshaft includes the rod big ends, rod bearings, and the amount of oil that clings to the rods.
Although the goal is to balance the crankshaft, you must first weight match the components that attach to the crankshaft. This means that each piston and pin must weigh the same, all rod small ends must weigh the same, and all rod big ends must weigh the same. Additional items such as rod bearings, pin locks, and rings are manufactured so consistently that by weighing a sample of each, you can assume that the rest weigh the same.
Once the pistons are weight-matched and the rods are weight-matched, a bobweight can be created, which duplicates the components that attach to the crankshaft. This bobweight is then secured to the crankshaft. The crank is spun on a balancing machine, and weight is then removed from or added to the crankshaft counterweights.
The goal is to eliminate unwanted forces at the crankshaft geometric centerline, creating a rotating and reciprocating package that allows the crankshaft to run as smoothly as possible.
Internal versus External Balance
Crankshafts may be balance corrected internally or externally. In either case the attached parts (pistons, rods, etc.) must first be weight-matched. When a crankshaft has internal balance, balance weight correction is performed on the crankshaft itself, adding or removing weight from its counterweights. This requires a zero-balanced dampener and flywheel. The dampener and flywheel for this example are designed with a zero balance, so neither affects crankshaft balance. This is one advantage of internal balance. The dampener and flywheel may be changed in the future without affecting crank balance.
When a crankshaft is designed for external balance, the offset-weighted dampener (or balancer) and weighted flywheel must be attached to the crankshaft during balance spinning. Corrections are then made by adding or removing weight at the crankshaft’s counterweights.
Two common types of crankshaft dampeners include the rubber/elastomer type and the viscous type. An OEM-style elastomer/rubber dampened balancer is designed to reduce crankshaft harmonics at a pre-engineered frequency range. A viscous dampener has a cavity partially filled with a viscous gel that is designed to maintain balance at all engine speeds.
If you are balancing an externally balanced crankshaft, where you need to include both the flywheel and dampener on the crankshaft during balancing, using a viscous dampener requires a different approach. Since the viscous dampener constantly works to maintain balance, this can mask slight crankshaft imbalance conditions. If the viscous dampener is designed for use on an externally balanced crankshaft, it was likely designed as a two-piece unit. Remove the viscous balancer ring from the hub and mount only the hub to the crankshaft during balancing.
Although some original equipment engines were designed for external balance, it is possible to internally balance any crank. The shop equipment required to perform crankshaft balancing includes: a professional-level digital scale, which is used to weigh individual parts including pistons, pins, rings, locks, rods, and rod bearings; a connecting-rod support stand used in conjunction with the digital scale; an electronic spin balancer designed for crankshaft balancing; an overhead drill press to drill holes into the counterweights; and a set of bobweights, which are used to simulate the weight of the pistons, rods, etc. Add a flywheel adapter hub to allow balance corrections to a flywheel that has been serviced or is in question.
The first order of business is to determine the weight of the pistons and connecting rods. You need to know how much each part weighs, in order to create a bobweight card, and to make sure that all piston and rod weights are identical.
Your digital scale must be absolutely clean. Astute engine builders always keep a dust cover on their scales when not in use. The scale should also be located in an area free of moving air. These scales are so sensitive that a breeze caused by a nearby door opening can easily create a false reading.
After pressing the ZERO button to calibrate the scale, weigh each piston and label with a marker pen. With the entire set of pistons weighed, you then examine the set to look for variations. If weights do not match, the lightest piston becomes the reference point, so you remove material from the remaining pistons in order to match the lightest.
If weight must be removed, do this carefully to avoid compromising the structural integrity of the piston. With the piston secured in a dedicated piston pin vise, material can be removed (by milling) at the underside of the pin boss. However, today’s high-quality high-performance pistons are manufactured with such a high degree of precision that the need to perform weight correction is rare. When all pistons in the set weigh the same, record the weight of a single piston on the bobweight card.
Next note each piston pin weight. Again, it is rare to find a set of quality piston pins that are not already weight-matched. You should not consider modifying a piston pin, but if any slight variation in weight is found with the piston set and the pin set, before attempting to remove weight from any of the pistons, you can try to match pins to pistons in an effort to correct any weight deviation.
By mixing and matching combinations of heaviest and lightest, you may be able to create piston and pin sets that create a closely matched combined weight. If so, then each pin must remain with its piston all the way to final assembly.
Connecting Rod Weight
Next weigh each connecting rod. Place a specialty rod stand that accommodates the rod small end onto the digital scale and TARE (zero) it in order to ignore or negate the weight of this stand. Place another rod stand next to (but not on) the scale. This stand supports the rod big end. Each stand is adjustable for height. Mount the rod to the stands in a horizontal manner and adjust so that the big-end center is level with the small-end center. Note and record the small-end weight.
Then reverse the rod in order to weigh the big end. Again, the rod must be level. Rod stand designs differ; if a different stand or stand component now rests on the scale, the scale must again be TAREd to negate the weight of the stand. Record the big-end weight.
After recording all rod weights, compare and determine if weight correction is needed. Today’s high-quality performance aftermarket forged and billet rods are usually extremely well weight-matched, so chances are no correction is needed. Older-design OEM rods may have weight bosses at the small end and at the cap, allowing material removal. Today’s rods are typically free of these bosses, and you may not have any available excess material for removal.
If you have a set of aftermarket rods that are not weight-matched, you’re better off contacting the manufacturer to discuss the problem, which may mean exchanging for another set. Once all rods have been weighed and recorded, remove the stand from the scale and ZERO the scale.
Weigh and record one piston’s set of pin locks (if your pistons have full-floating pins). Weigh and record the weight of one piston’s full set of rings (including the oil ring support rail, if so equipped). Finally, weigh and record the weight of one rod’s pair of bearing shells.
Bobweights have heavy-duty aluminum V-clamps that fasten together onto the crankshaft rod journals. Each side of the V-clamp has weights. One half of the total bobweight is placed on each side of the journal. Using the information recorded during component scale weighing, the bobweights are assembled on the digital scale to duplicate the determined weight. The total weight of the bobweight must match that of the bobweight card, which includes the V-clamps, adjusted weights, and nuts that secure the bobweight halves together.
The weight is adjusted by adding lead shot to capped barrels that attach to the V-blocks or by adding weighted discs onto threaded rods on the V-clamps (designs differ). To create a bobweight on a V-8 or 90-degree V-6, 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 is factored in, which includes the pistons, small rod ends, rings, pins, and locks. That means both rod big ends and both sets of rod bearings, but only one piston/ pin/ring set, is factored in for the reciprocating weight.
For other engine configurations, different percentages of the reciprocating weight may be required. The balancing-equipment manufacturer usually provides a reference chart, or has this information programmed into its computer software.
Once the total bobweight is determined, the weights are asembled to duplicate the real-life reciprocating mass.
A 50-percent factor is normally used for most balancing jobs. But for certain racing applications with high compression ratios and/or high cylinder pressures, some builders prefer to slightly overbalance, using a factor of 51 percent. This adds a bit more weight to the crankshaft counterweights, which counteracts high-compression resistance. This theoretically reduces the negative torque created by the compression force, aiding in further compacting the air/fuel charge.
In certain instances, a high-revving race engine may experience a slight out-of-balance vibration at a specific RPM. In this situation, a slight overbalance may be beneficial by increasing the reciprocating weight factor to 51 percent or more. In doing so, the out-of-balance point may be moved to a lower engine speed range that isn’t critical to the race engine’s use. Overbalancing allows you to optimize crankshaft balance within the most important range of engine speed (the sweet spot) for the specific type of racing.
If you are balancing a four-cylinder in-line crankshaft, there is no need to create bobweights. Since the crankshaft has two opposing strokes, the dynamic forces tend to cancel each other. In order to balance this type of crankshaft, weight match all pistons and rods as you normally do for a V-type engine. Without bobweights attached to the crankshaft, spin and balance the crank.
The crankshaft is positioned on the spin balancer’s V-blocks at the crankshaft’s front and rear main journals. The V-blocks have nylon rubbing blocks to protect the main journal surface, which must be lightly oiled before positoning the crankshaft. The bobweights are installed to the crankshaft rod journals, with each journal’s weights set 90 degrees from each other.
The bobweights must also be centered along the width of the journal. Most spin balancers have a drive belt that contacts the center main journal. With the crankshaft fitted with its bobweights, the spin balancer is turned on to spin the crank. The balancing machine displays the existing heavy or light areas of the crankshaft relative to zero, and indicates where weight must be removed or added relative to the front-to-rear counterweghts as well as the radial point of the counterweights.
If weight must be removed, it is done by drilling to remove material from the counterweights. This can be accomplished with an overhead drill press, drilling into the outer edges of the counterweight, or, as determined by the balancing technician, by machining material from the outer faces of the counterweights on a precision lathe.
Depending on the type of balancing machine, information is provided on the display screen regarding the drilled-hole diameter and depth required.
If weight is to be added, the crankshaft counterweight is first drilled to a specific diameter and depth, then filled with a heavy-metal tungsten slug. If the slug is installed to the radial edge of the counterweight, the technician then welds the slug in place to prevent it from slinging out of the counterweight. Most balancing technicians prefer to drill and install a tungsten slug horizontally through the face of the counterweight, eliminating the potential for accidental slug departure due to centrifugal force.
Each time weight is removed or added, the crankshaft is spin checked again to verify the change in balance because performing a weight change at one end of the crank can affect the opposite end. In some cases, as many as six (or even more) balance corrections may be needed as the technician chases the balance.
Before attempting a crankshaft balance, always inspect the crankshaft for runout. If it is bent (excessive runout), the balancing machine displays a substantial difference in weight between the front and rear counterweights. Always check for runout before wasting your time trying to correct balance.
Inspect the crankshaft stroke and verify that each stroke distance (the centerline of the rod journal to the centerline of the main journal) is identical. This is especially important if the crankshaft has been reground. Slight differences in stroke affect balance.
Balancing is the final step in crankshaft preparation (aside from a final polish of the journals), so be sure that all machining that could affect balance has already been performed. This includes crankshaft machining, piston machining, rod bearing selection, etc. It’s important to note that any change in the weight of the reciprocating parts changes the overall balance. If, during a rebuild, you need to replace even one piston, do not assume that the replacement piston weighs the same as the original, even if it is the same size, style, and brand. Whenever any part of the reciprocating assembly is changed, the crankshaft must be rebalanced.
A flywheel or clutch pressure plate for an internally balanced crankshaft should have a zero balance right out of the box. But if the goal is to follow a blueprinting approach, don’t make that assumption. Separately check the flywheel (or flexplate) and the clutch pressure plate and correct the balance if needed. Although an approximate flywheel and/or pressure plate state of balance may be adequate for routine street driving, blueprinting requires trying to eliminate all variables and deviations.
However, is not critical, and often not practical, to achieve a perfect-zero crankshaft balance on the balancing machine. Even the movement and migration of slinging and clinging oil can slightly alter dynamic balance. Balancing the crankshaft to within 2 to 4 grams is acceptable. Just because you might achieve a zero balance on the machine, it doesn’t mean that the crankshaft will experience zero balance during engine operation. Be realistic. You can spend hours in chasing a zero balance, often with no additional benefit.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks