Understanding Crankshaft Balancing

Ever wonder what you are actually getting for your money or if you even need to get your crank "balanced" or an engine "balance job"?  We decided to explore the dark and shrouded world of crankshaft or engine balancing and shed some light on it! There are many different "balancing machines" most are based on turn of the century technology with bouncing analog gauges like the speedo in an old Pinto... that was fine with model t's, but has since been replaced with digital PC based technology! We use that technology to pass on the best balanced product to our customers. If you are considering having your local shop balance your assembly purchased from us, ask them if they can supply a "final imbalance" or results in grams front and rear to verify you received a quality 'in spec' job. All our Balance work is completed in less than 1gram front and rear with printed results balance card! Weigh a dollar bill sometime... it weighs 1 gram... to give you an idea of the quality service you will receive! Enjoy our Guide.

Since different rods and different pistons are different weights, it is impossible to make a crankshaft that is balanced "right out of the box" for any rod and piston combination. All crankshafts must be balanced to your specific rod and piston combination.

The first step in understanding crankshaft balancing is to understand the purpose of the counterweights. The counterweights are designed to offset the weight of the rod and pistons. You have the weight of the crankshaft and the pistons and rods. At any point in the assembly's rotation, the sum of all of the forces are roughly equal to zero. If the counterweights are the correct weight to offset the weight of the rods and pistons, the crankshaft is balanced. If the counterweights are too heavy, material must be removed by drilling or milling the counterweights. If the counterweights are too light, weight must be added to the counterweights. This is usually done by drilling a hole in the counterweight and filling the hole with "heavy metal" or "mallory". This filler metal is denser and heaver than steel so the weight of the counterweight will increase as a result.

ALWAYS CHECK BALANCE

One thing to remember about engine balance is that anytime you replace parts or assemble parts from various suppliers, it affects balance. No two parts are manufactured exactly the same and there will always be some variation in weight. Stock OEM piston weights can vary quite a bit, and even aftermarket performance pistons may not be perfectly matched in a set (though most high quality performance parts are weight-matched). Even so, balance should always be checked and corrected as needed to match the needs of the application. On a stock engine, close enough may be good enough, but on a performance engine there’s much less margin for error. Changing bore sizes or piston types will affect balance. Changing piston heights, rod lengths or the type of rods (steel, powder metal or forged aluminum) will affect balance. Replacing a stock crank with an aftermarket performance crank or a crank with a different stoke will affect balance. You can’t just throw the parts together and hope the engine will be in balance (which is what a lot of DIY engine builders do!). You have to weigh and balance the individual parts and make sure they work together.

Internal Balance & External Balance

When the counterweights alone can be made to balance the crankshaft, the crank is said to be "internally balanced". If the counterweights are too light by themselves to balance the crankshaft and more weight is needed, an "external balance" can be used. This involves a harmonic dampener or flywheel that has a weight on it in the same position as the counterweight that effectively "adds" to the weight of the counterweight on the crankshaft.  For "external balance" the harmonic dampener (front) or flywheel/flexplate (rear) play a part in the balancing of the assembly, they must be installed on the crankshaft when it is balanced. This is unlike an internal balance configuration where the harmonic dampener or flywheel do not contribute to the balance of the crankshaft and are not required to be installed but can be included when the crankshaft is balanced. Both methods are used from the manufacturers.

An example of some factory internally balanced engines are Chevy 305 and 350 (2 piece rear seal only!), Chevy 396/427, GM LS-series, and Ford "modular" 4.6/5.4. Some examples of factory externally balanced engines are Chevy 400 and 454, Ford 302 and 351W. Some engines are a combination of both being internally balanced in the front and externally balanced in the rear. The most common example of this is the Chevy 350 (1 piece rear seal) including LT1. Regardless of how an engine is balanced from the factory any balancing method is acceptable as long as the required harmonic dampener and/or flywheel is available.

"Is my crank balanced?"

Since different rods and different pistons are different weights, it is impossible to make a crankshaft that is balanced "right out of the box" for any rod and piston combination. All crankshafts must be balanced to your specific rod and piston combination. When a crankshaft is listed as "internal balance" or "external balance" this is stating how this crank is intended to be balanced. It can be balanced otherwise, but it is much more difficult to do so.

Bobweight

When a crankshaft is balanced, the actual rods and pistons cannot be used in the balancing machine, so they must be simulated. This simulated weight is called the "bobweight". Once the bobweight is calculated, weights are bolted onto the rod journals to simulate the weight of the rods and pistons during the balancing process. Due to the configuration of a "V" type engine, just adding all the weights together does not work.

The weight of the reciprocating parts (the piston, ring set, wrist pin and small end of the rod) must be added together to calculate the amount of bobweight needed to balance the crank. Bobweights are attached to each of the crank’s rod journals to simulate the reciprocating mass of the pistons and rods. On a V8 engine, the bobweight will usually be 100 percent of the weight of the rotating components (the big end of the rod, the rod bolts and bearings) plus 50 percent of the reciprocating weight (pistons, rings, wrist pin and small end of the rod). On many racing engines, “overbalancing” the crank by using 55 percent to as much as 70 percent of the reciprocating weight can help smooth out high rpm vibrations. On straight four and six cylinder engines no bobweights are required. With V6 engines, a different fraction of the reciprocating weight is needed because of the angularity of the crank (typically 39.4 percent of the reciprocating weight on a 90° V6).

For example, let's say we are balancing a 460 to 557 BB Ford with the following component weights:
Piston 584g
Pin 150.3g 
Locks 4.7g
Rings 56.7g
Rod big end 533.5g
Rod small end 205.8g
Rod bearings 45.6g

The rod weight is separated into "big end" and "small end". This is necessary because the small end has a reciprocating (back and forth) motion and the "big end" has a rotating motion. This split weight is figured on a special scale fixture that supports one end of the rod while weighing the other end.

There are several things to note about this calculation. The "oil" value used on the left side of the calculation is an approximation of the weight of residual oil "hanging around" on the assembly. The number used here is a matter of industry standard. The second thing to note is the 50% value used for the reciprocating factor. This number deals with the geometry of the engine itself. A 90 degree bank angle "V" engine will use 50% here. A V6 or a narrow or wide bank angle "V" engine will use a different value (again, consult the engine parts manufacturer). Some engine builders will perform what is call "underbalancing" or "overbalancing". They will use slightly different values here such as 48% or 52%. This is done to help compensate for dynamic effects at extremely high or extremely low rpm operation (again, beyond the scope of this discussion). We use 50% because this value is required for almost all common street or racing engines unless otherwise specified.

Balanced Rotating Assemblies

Most rotating assemblies are sold unbalanced so that engine builders can balance it however they wish. Manufacturers/Retailers do offer fully balanced assemblies balanced. But it must be ordered specifically as a balanced assembly. If you don't know if its balanced or not, ask and request the balance card, don't assume it is. Most forged 4340 steel crankshafts are designed for internal balance. An internally balanced kit don't have to include a harmonic dampener or flywheel because they are not required for balancing – allowing you to use whatever brand you like. Externally balanced kits should include a harmonic dampener and flywheel or flexplate as needed. External balance can be done using "house" OEM external dampers and flywheel/flexplate but the results cannot be guaranteed accurate unless using the actual items to be used on the engine. Be aware of the fact that replacing the flywheel/flexplate or harmonic balancer with different parts can upset balance on an externally balanced engine. If a harmonic dampener and flexplate is provided, it can be an O.E. style replacement or SFI approved depending on your needs. If you’re building a high horsepower high RPM engine, internal balance is preferred. Internal balance is better for longevity of parts and fatigue life.

Balance Reciprocating Parts: Understanding balancing

Balance doesn’t matter with a manifold, or engine block... because they are stationary engine components that don’t move. But balance does matter with everything that spins or reciprocates in the engine and drivetrain. Everybody knows what balance is, right? You maintain your own balance by centering your body mass over your feet. If you lean too far forward or backward, or too far to the left or right, you’ll lose your balance and fall unless you grab hold of something or reposition your feet. Moving your center of gravity creates an imbalance that must be offset or corrected to maintain your balance.

It’s the same with engines.

Reciprocating piston engines have a crankshaft that rotates at high speed, and pistons and connecting rods that oscillate up and down with every revolution of the crank. Both generate forces inside the engine that can cause unwanted vibrations and even engine damage if the forces are too great.When an object rotates, it naturally rotates around its own center of gravity. That’s just the way nature works. Every solid object has a natural center of gravity regardless of its size or dimensions, even an odd-shaped object like an exhaust manifold. Toss an old manifold off a cliff and give it a spin as you do so, and the manifold will rotate around its own center of gravity. Balance doesn’t matter with a manifold because it is a stationary engine component that doesn’t move. But balance does matter with everything that spins or reciprocates in the engine and drivetrain. This includes the crankshaft, connecting rods and pistons, as well as the flywheel or flex plate, clutch or torque converter, the harmonic balancer and drive pulleys, the cooling fan, the turbocharger impeller shaft, the driveshaft, brake rotors and drums, the wheels and tires, and even the camshaft(s). The forces generated by an imbalance in any of these parts depend on two things: the magnitude of the imbalance and the speed of the object. The larger and heavier the object, and the faster it spins, the greater the force generated by any imbalance that exists. For a rotating crankshaft, the force at the main bearings is proportional to the speed of the engine squared. Also, the further the imbalance is located from the center of gravity, the greater its effect on the part as it rotates. With crankshafts, large heavy counterweights are used to offset the forces generated by the reciprocating weight of the pistons and rods. The crank must not only maintain its own balance as it spins around inside the block, it must also offset the forces generated by the mass of the pistons and rods as they pump up and down.

So what does this actually mean in terms of the forces generated inside an engine? An imbalance of only 1/4 oz. (7 grams) located four inches out from the center of the crank on a counterweight will generate a force of about 7 lbs. at 2,000 rpm. At low rpm, you would hardly feel a thing. But at 8,000 rpm, that same force would grow to 114 lbs. with every revolution of the crank. If this same engine had one ounce (28 grams) of imbalance, the forces generated would be multiplied by a factor of four, generating 456 lbs. of unwanted gyrations at 8,000 rpm! That’s enough vibration to rattle your teeth and pound the heck out of the main bearings. It’s also wasted motion that goes into shaking the block instead of spinning the crankshaft. Consequently, imbalance hurts horsepower as well as smoothness and engine longevity.

The factory balance of crankshafts can vary a great deal depending on the application and the OEM tolerances. For a low rpm stock engine, plus or minus 8 to 10 grams or more may be close enough for the average Joe. For a street performance engine, those numbers should come down to plus or minus 3 grams or less. And for a high revving NASCAR engine that spends most of its time at 8,500 to 9,500 rpm, plus or minus 1 gram or less is the rule. How much is 1 gram? Not very much. A dollar bill weighs one gram. A penny, by comparison, weighs about 2-1/2 grams. An ordinary sheet of office paper tips the scale at a whopping 5 grams, which is more than the amount of imbalance that’s generally desired in a street engine.

The longer the stroke on the crankshaft, the more important balance becomes because of the distance factor. A longer stroke moves metal further from the axis of rotation and magnifies its effect on balance. In recent years, some racers are even scrutinizing camshaft balance. Cam balance is usually not much of a factor because the cam only turns at half the speed of the crankshaft, and the lobes do not protrude very far from the shaft itself. But in a high revving NASCAR engine, cam imbalance can cost the engine as much as 20 horsepower because of the valvetrain harmonics it creates.

SPIN BALANCING

To spin balance the crank, the crank is mounted on the balancing machine support stanchions. The crankshaft should is checked for straightness because a bent crank will wobble as it rotates – which may fool the balancer into thinking the wobble is due to imbalance. Straightness is checked with a dial indicator at the center journal.

The balancer will then spin the crank to about 500 rpm and determine the magnitude and location of any imbalance it detects. Our state of art PC based balancer can detect imbalance as small as .01 grams, which is far more than what’s actually needed. The balancer detects imbalance by measuring the displacement of the support stanchion sensors while the crank is spinning. Readings are taken in 1 degree increments on our machine (6 degrees on older balancers), and compared to the position of the crank as it rotates. Imbalance changes the crank’s center of gravity, causing it to wobble off center as it rotates, and the greater the imbalance, the more it wobbles and shakes.

Now comes the magic part. The balancer software looks at the sensor inputs and calculates the amount of the imbalance and its location. The weight is then displayed in ounce-inches or gram-centimeters, and its estimated location is shown in degrees from a reference position.  The unbalance amount and position are imported into a special computer program called "Heavy Metal Analysis" (HMA). This program allows our technician to plot the position and amount of material that will be required to correct the crankshaft. The program lets tech's create multiple scenarios based on rotation and radius position, weight amounts and sizes of Mallory – all of which can be simulated without having to cut the first chip.

Corrections can then be made by drilling holes or machining down the outside diameter of the counterweights to remove metal. Or, if more weight is needed, you can weld metal to the counterweights, or drill holes (or use existing holes) to add plugs of heavy metal to the counterweights (Mallory metal is 1.5 times as heavy as lead, and is often needed on stroker cranks and ultra-light racing cranks).

Removing weight requires locating the drill bit precisely and drilling to an exact depth. The software on our PC based balancer can calculate the size and exact depth of the hole(s) to be drilled, as well as the best place to locate heavy metal if weight needs to be added. On older machines, corrections typically require much more skill and guesswork. You drill out what you think is the right amount of metal to remove, then cross your fingers and spin the crank again to check the balance. Then you drill or plug some more, spin again, and repeat as many times as needed until you finally get it right.

Making corrections also involves separating one end of the crank from the other because imbalance at one end will affect the other. On older balancers, corrections made at one end of the crank often upset the balance at the other end, requiring repeated spins and corrections until both ends are in balance. Our new balancers software takes this into account and splits the forces apart so corrections that are made on one end won’t upset balance on the other end. It’s like cutting the crank in half and balancing each half separately while also taking into account the balance on the other end. This is called “dynamic plane separation” and is a time-saving feature you want if you’re shopping for a new balancer. The main advantage of dynamic plane separation is that it does a superior job of isolating vibrations so corrections can be made more quickly and accurately. It reduces the back and forth corrections and repeat spins that can eat up valuable shop time and cost the customer money.

Multi-plane balancing is also possible on our machine to segment the crankshaft electronically into even smaller sections. This can be helpful in situations where one area of a crank has a lot of imbalance (one end or near the middle).

Weighing the Benefits of Engine Balancing

Balancing goes hand-in-hand with performance engine building. Balancing reduces internal loads and vibrations that stress metal and may eventually lead to component failure. But is it worth the time and effort for mild performance applications, everyday passenger car engines or low-buck rebuilds?

From a technical point of view, every engine regardless of the application or its selling price can benefit from balancing. A smoother-running engine is also a more powerful engine. Less energy is wasted by the crank as it thrashes about in its bearings, which translates into a little more usable power at the flywheel. Reducing engine vibration also reduces stress on motor mounts and external accessories, and in big over-the-road trucks, the noise and vibration the driver has to endure mile after mile.

Though all engines are balanced from the factory (some to a better degree than others), the original balance is lost when the pistons, connecting rods or crankshaft are replaced or interchanged with those from other engines. The factory balance job is based on the reciprocating weight of the OE pistons and rods. If any replacements or substitutions are made, there’s no guarantee the new or reconditioned parts will match the weights of the original parts closely enough to retain the original balance. Most aftermarket replacement parts are "balanced" to the average weight of the OEM parts, which may or may not be close enough to maintain a reasonable degree of balance inside the engine. Aftermarket crank kits are even worse and can vary considerably because of variations within engine families.If the cylinders are worn and a block needs to be bored to oversize, the larger replacement pistons may be heavier than the original ones. Some piston manufacturers take such differences into account when engineering replacement pistons and try to match "average" OE weights. But others do not. Most high performance pistons are designed to be lighter than the OE pistons to reduce reciprocating weight for faster acceleration and higher rpm. Consequently, when pistons and rods are replaced there’s no way of knowing if balance is still within acceptable limits unless you check it.If you’re building a stock engine for a passenger car or light truck that will spend most of its life loafing along at low rpm, you might question the value of balancing such an engine. But if you value durability and smooth operation don't pass on balancing.On the other hand, if you’re building a performance motor, a stroker motor or an engine that’s expected to turn a lot of rpms or run a lot of miles, balancing is an absolute must. No engine is going to survive long at high rpms if it’s out of balance. And no engine is going to last in a high mileage application if the crank is bending and flexing because of static or dynamic imbalances.

Why Balancing Has Become Absolutely Critical Today

Most shops that are doing custom engine building today are assembling new parts that have never been in an engine before. The only parts that are reused in many performance engines are the block and maybe the cylinder heads – and often even these parts are replaced with aftermarket castings.Most engines are put together with a new crank (usually a stroker), new connecting rods and new pistons. Depending on where the parts are sourced, the weights of the rods and the weights of the pistons may be fairly even. Even so, it’s always a good idea to check the weights, and to equalize the weights as needed.The weights of the reciprocating parts then have to be balanced to the counterweights on the crankshaft. Aftermarket performance parts (rods and pistons, that is) are almost always lighter than the stock parts they replace. So if the original crank is being reused, the counterweights will have to be drilled to compensate for the reduced mass of the reciprocating parts.

If the engine is being built with a stroker crank, balancing is an absolute must. Some suppliers of stroker cranks publish a “target bobweight” for their cranks so engine builders can more easily estimate how much work it will take to balance the crank with a given combination of parts. Its just an estimate and cannot replace and actual balancing of your parts.It’s not unusual to see brand new stroker cranks that are out of balance by as much as 200 to 300 grams because of the broad rod/piston weights to be accounted for! That’s a lot of extra mass on the counterweights that will have to be removed to balance the crank. Another reason why many stroker cranks are heavy is because they are forged with extra metal in the counterweights so the engine builder doesn’t have to add heavy metal (tungsten plugs) to achieve proper balance. Drilling holes is cheaper and easier than installing heavy metal.If an engine is being built with a lightweight racing crank, on the other hand, there’s less metal to work with when it comes to balancing the crank. On some of these cranks, you may have to use heavy metal or ultralight rods and pistons to bring balance down to where you want it.

*excerpts taken from various web and mfg sources