Eaton Balancing

“An Introduction to Engine Balancing”

By Ted Eaton

Although the terms “blueprinted and balanced” are typically synonymous with any kind of performance buildup of an engine, it must be noted is that these two terms are completely different in relation to their perceived functions and are generally performed independently of each other. Whereas “blueprint” specifically targets an engines fits, tolerances, volumes, and/or settings, the “balance” deals with the physics of achieving a rotating mass that is the most conducive to transmitting as much potential power as possible to an engines flywheel rather than having it wasted within the confines of the block. By eliminating power robbing vibration or harmonics that can be caused by a state of out of balance, power or torque that would normally be dissipated through an engines main bearings and into the block can instead be redirected to the drive end of the crankshaft proportionally by the degree to which the rotating mass is balanced. Simply stated, the better the balance, the more the potential power that can be seen at the flywheel. For the driver of a vehicle with a balanced engine, it’s simply a physically smoother running engine with improved acceleration.

The physics of balancing can be broken down into two types of imbalance, static and dynamic. Static imbalance will manifest itself as a maximum amount of out of balance in a single area or plane along the outer most surface of a rotating axis. On the other hand, a rotating part can be in a perfect state of static balance but can be off significantly when examining its dynamic balance. If a vibration is observed, static imbalance is the force most likely being felt but dynamic out of balance can also be present depending upon the rotating mass’s length. Even when physical vibration is not being felt, dynamic imbalance can be present in severe enough degrees to be quite destructive or power robbing although no shake or vibration is physically evident. Static out of balance can be found in any rotating part regardless of the length along its rotating axis but dynamic out of balance becomes more significant as rotating pieces increase in length along its rotating axis. Very narrow rotating parts such as flywheels will not exhibit much in the way of dynamic out of balance but can be good examples of static imbalance. A longer item like a crankshaft may not show evidence of being out of balance statically but can be off significantly when looking at its dynamic aspects.

Over the years, set rules or best practices for engine balancing have been established for the various engine designs. These practices at their best are a compromise dealing with conflicting forces that try to share the same physical space normally defined as either rotating or reciprocating mass. While rotating mass can be described as that which travels only in a circular motion such as the crankshaft or the connecting rod big ends, reciprocating mass would be those components that are attached to the crankshaft but travel in a predefined non-circular motion such as the pistons or the connecting rod small ends. Getting back to conflicting forces, the connecting rod does not have a distinct line separating the reciprocating mass from the rotating mass due to the fact that the area of the rod between the wrist pin and the crank pin exhibits the traits of both reciprocating and rotating mass. As this in-between area gets closer to the crank pin, it exhibits more rotating mass characteristics than reciprocating; likewise, as this same area gets closer to the wrist pin, it exhibits more reciprocating mass characteristics than rotating. For this reason, the longer the rod and/or the cranks stroke, then the more this ambiguity. This ambiguity and how to compensate for it will be discussed in a later article in this series.

To precision balance an engine, all pieces of the rotating assembly must be considered. A list of these parts will include the crankshaft, harmonic damper or front hub, flywheel, the clutch disk and pressure plate if a manual shift transmission is being used, connecting rods, pistons with their pins and respective locks, compression and oil rings, rod bearings, and the lower timing gear. Basically everything that moves or rotates as part of the engine assembly short of the camshaft and its attached gear. Also worth considering are the belt pulleys and their retaining bolts. All machine work or modifications including rod reconditioning, piston dome or valve relief machining, and general deburring or polishing of any of the internal engine parts must have been already performed prior to having the assembly balanced as doing so after the fact will only nullify the effects of having the rotating assembly precision balanced.

When taking all the necessary parts to your favorite shop for balancing, it’s important to note that the connecting rods or the piston rings must not yet be installed on the pistons. Likewise, the rod bearings do not need to be installed in the rods or the rods installed on the crankshaft. Because the rods must be balanced end for end separately, they are required to be independent or apart from the pistons. Although the rings could technically be on the pistons during the balancing procedure, they also need to remain uninstalled in order to insure that no machining chips are caught in the ring lands during any of the weight reduction operations being performed.

The balancing operation as performed by your machine shop can be separated into two basic operations; weighing and/or match weighing the various components and then spin balancing the crankshaft itself. Depending upon an engines design, there may be a specific order in which these operations are performed. Most engines with opposed or inline cylinders (i.e.. 4 & 6 cylinders) normally do not require a bobweight to be attached to the crankshaft rod journals prior to spin balancing in order to simulate any of the component pieces (rods, pistons, etc.) that are normally attached to the crankshaft assembly. This is due to the nature of the physical forces being applied at equally spaced intervals on these engines and subsequently being equally opposed or counteracting. A “V” type cylinder engine on the other hand does not have these forces being applied at the same opposing intervals and depending upon the angle between the opposing cylinders, does require a bob weight installed on each crank pin for balancing purposes that uses 100% of the rotating mass but only a fraction or a percentage of the reciprocating mass for each piston/rod assembly. A 90° V design, which covers a majority of the common V8 engines, requires a bobweight on each crankpin during balancing that represents not only 100% of the rotating mass but also a standard value of 50% representing the reciprocating mass. Even this standard reciprocating factor is subject to change for a 90° V design engine in special circumstances. More on that in a later article in this series.

There are some of the balancing operations that can be performed by the novice in his own shop but the spinning of the crankshaft is best done by those with the appropriate equipment and experience. If so inclined and with a nominal purchase in equipment and materials to do so, the pistons and connecting rods can be independently weighed and appropriately lightened by the enthusiast before sending the crankshaft out to be balanced. A scale reading in grams along with a connecting rod holding fixture that can separate the big end and small end weights would be the bare minimum equipment requirements. Then it’s just a simple matter of finding the lightest piston, lightest reciprocating rod end weight, and lightest rotating end weight and making the remaining component pieces match these weights. The equipment used for lightening operations would be wholly dependent upon the quality of the machining that’s being targeted for as well as the initial design of the pieces that must be lightened. Some parts may require special tools or tooling for machining in order to keep from sacrificing or minimizing the strength of the piece being lightened. For those of you that have a grinder, belt sander, drill press, and/or some form of milling machine available, most lightening operations would be well within your reach. Look for more detail on this in the subsequent articles.

These series of articles are hoped to heighten your awareness of how precision balancing not only increases engine efficiency, but ultimately also increases engine life through reduced internal vibration and unwarranted stress. The next article in this series will cover in detail the differences between external and internal balance. Until then, happy motoring.

Originally published in Y-Block Magazine, Jul-Aug 2004, Vol 11, No. 4, Issue #63

“Internal Versus External Balance”

By Ted Eaton

When getting an engine balanced, it’s important to note that there are two different methods in which to have the engine balanced, either internally or externally. As the Ford Y-Block family of engines are all internally balanced as part of the factory design, this is not expected to be an issue but for other engines it’s a subject worth touching base upon here briefly. External balance refers to when the damper, flywheel, and/or other rotating parts outside of the engine block are counterweighted and must be installed as part of the crankshaft assembly to insure the balance of that particular rotating assembly. Whenever possible, the customer should opt for internal balance unless the cost is prohibitive or not effective for the result being desired.

There are two basic reasons an engine becomes externally balanced as part of a factory design. The first is through evolution where an existing engine series was originally designed as internally balanced but has an increase in stroke to the point that the crankshaft counterweights can not be made any larger within the confines of the block in which to compensate for the additional mass requirements. This then requires additional mass to be attached at the flywheel and/or the damper to supplement the existing crankshaft counterweight mass in lieu of a complete new engine crankshaft or block design. The second reason would be in initial engine development and design where the crankshaft can be made lighter which in turn uses less of the costly nodular material typically used to strengthen cast iron. This then allows inexpensive materials to be used at the flywheel and damper to provide for the additional required rotating mass.

There are two inherit flaws in the factory external balance designs. The first would be the stackup of balance variances due to three pieces (flywheel, crankshaft, & damper) that must match up in balance after being independently balanced of each other. Second would be the forced out of balance at high rpm’s this type of balance promotes due to a given amount of non-evenly distributed mass being located in a non-rigid manner outside the confines of the block. This then allows a given amount of flex or twist in the unsupported ends of the crankshaft at that point. The more the counterbalance on the ends, the greater the flex or distortion throughout the crankshaft at a given rpm and the potential for crankshaft deflection or breakage at high rpms.

Although not recommended, there are instances where a balance shop will externally balance an engine that was originally internally balanced. These occurences most often originate from substituting parts that are much heavier than the originals such as heavier connecting rods and/or pistons which causes the crankshaft counterweights to be too light in which to compensate inexpensively. Another instance is where the crankshaft stroke is being increased while the crankshaft counterweights are remaining the same. Although externally balancing as a result of either scenario is performed as a cost savings measure in both time for the shop and expense by the customer, a major problem presented by an externally balanced engine is the inability to change out the modified balancer or flywheel with alternate units at a later date without having the assembly rebalanced. Rather than externally balancing an engine in such a circumstance, there are other options rather than repurchasing lighter components. One such manner is to balance the crankshaft internally through the use of Tungsten, Mallory metal, or other heavy metal in the crankshaft counterweights in order to make them physically heavier. And yet another option depending upon the crankshaft design is to use lightening holes in each of the rod journal throws which will in turn make the crankshaft counterweights artificially heavier without actually having to add weight. Ultimately, cost is typically the deciding factor on what method is used to retain internal balance characteristics in lieu of leaving an engine externally balanced.

Ideally, the engine is best balanced internally which allows replacement dampers and flywheels to be balanced as separate units at a later date to a zero state and placed back on the previously balanced crankshaft with a minimum amount of upset to the crankshafts previous state of balance. The next article in this series will cover in detail how the piston set is match weighed and machined by your local balance shop.

Here’s a Ford Y-Blk damper that’s been externally balanced!!!  This should be avoided as relacement of the damper without rebalancing the crank becomes extremely difficult.

All these SBF standard transmission flywheels are externally balanced to the 28.4 oz Ford specification.

All three of these flywheels will fit either a SBF or 300 Ford Six but all three exhibit different states of balance depending upon the engine and its application.

This SBF flywheel has been balanced to the Ford 50 ounce external balance specification.

Originally published in Y-Block Magazine, Apr-May 2004, Vol 11, No. 2, Issue #62

“Piston Match Weighing”

By Ted Eaton

Match weighing the piston set is just one of the steps that’s performed as part of having an engine balanced. When balancing a V style engine, this operation must be performed before the crankshaft can be spin balanced due to the piston weight being required as part of the bobweight calculation. Although piston weight matching appears to be a relatively simple and straight forward operation, the machining method in which it is actually performed can jeopardize piston strength or its integrity if not done correctly.

In order to weight match a piston set, essentially the lightest piston in a set is identified and the remaining pistons are lightened through various machining operations so that all pistons end up weighing the same. Piston pins can be either weighed with the piston that it’s going to be used with or all balanced independently so that the pins themselves all weigh the same. Whether the pins are weighed with the pistons or done separately is typically determined by the shops preference. Regardless, either method will not affect the final state of balance if performed with tight tolerances that keep weight variation and any subsequent stackups to a minimum. The typical tolerance for this operation is ½ gram for the whole piston set with the pins in their respective pistons but the closer to zero, the better. Merely lightening the heavier pistons so that they match the weight of the lightest is not the only machining that can be performed at this point. Depending upon the piston design, the potential for additional piston weight reduction can be of benefit in that a lighter piston will subject the connecting rod, connecting rod bolts, and the bearings to less stress as well as reduce the total amount of reciprocating mass. This is conducive to a rotating assembly that can accelerate or increase in rpm’s at a quicker rate due to a subsequently lighter crankshaft which ends up having less stored or kinetic energy in which to release. Additional piston lightening at this point could also make the difference in removing weight at the crankshaft counterweights as opposed to actually having to add additional mass to the counterweights if only match weighing the piston set. Still, the emphasis here is to only lighten the pistons to the point where overall strength is not jeopardized.

Where to take weight out of the pistons depends particularly upon its design. If taking mass out of the piston’s inside deck portion, then rule of thumb dictates leaving a minimum of 0.200″ thickness. While some pistons have as much as 0.600″ deck thickness and give adequate material to work with, other piston designs are already at the 0.200″ thickness value leaving no room in this area for material removal. Blown applications will typically require much more than 0.200″ material in the deck surfaces so this is yet another consideration. Special care must also be taken in the deck areas under the valve reliefs to insure that sufficient material is remaining under the reliefs after machining. Other areas inside the piston in which to work at for weight removal is in the pin boss, both above and below the pin as well as inside the skirt area or behind the piston ring lands. In extreme cases, the whole of the vertical sides of the pin boss can be machined. All these different options are dependant upon the piston design and exactly how much material is available to work with. Some piston designs create quite a challenge depending upon how much weight must be removed in order to merely weight match. To know how much material is available for removal, a gauge or measuring device is necessary in which to know in advance what the thickness is in the area being contemplated for weight removal. The apparatus in Fig. A shows a homemade fixture that holds a dial caliper in which to perform this operation. A dial indicator gauge can also be retrofitted into a similar fixture to do the same measuring operation.

The tools or equipment in which to actually remove material from the pistons for weight reduction purposes can be quite varied. A piston vise or other fixture that will hold the piston in an inverted position while removing weight from it would be a prerequisite at this point so that machining operations can be duplicated within the piston set. The preferred piece of equipment for actual piston material removal would be a milling machine with a moderately sized cutter. The larger the cutter, then the greater the amount of surface that can be removed with a minimum amount of depth. This gives a very good ratio of minimum depth to maximum weight being removed. The use of drill bits for weight removal is discouraged both from the standpoint that not much overall weight is being removed simply by the diameter of the drill bit being used, but also that the drill point hole that is left behind leaves a potential stress riser in the piston for piston failure to originate from. There are also those instances where a lathe can be used for piston lightening depending upon the material available to work from within the piston.

After all the pistons have been weight matched, the final weight is then recorded on a balance card or work sheet for future reference. The piston set can then be cleaned of machining debris at this point and reboxed until actually engine assembly takes place. The next article in this series will cover connecting rod balancing. Until then, happy motoring.

Originally published in Y-Block Magazine, Sep-Oct 2004, Vol 11, No. 5,  Issue #64