Engine Balancing

Engine Balancing

Postby jp75vette » November 14th, 2010, 11:12 pm

Does anyone have tips on engine balancing?

I bought an externally balanced 383 that ran fine in a 67 Chevelle with an automatic transmission. I had the new flywheel and clutch supposedly balanced to the flex plate that was in the Chevelle. I installed the 383 in my 1975 Corvette with a 4 speed BW ST-10 4 speed. I have vibrations at mid teens and 3300. From what Iâ€™ve read this is likely clutch/transmission related. I believe the main bearings are wearing as oil pressure seems to be dropping a bit.

My plan is to pull engine, take it apart and have all components balanced. The guy I have located will balance the piston/rod, crank, balancer and all the rotating elements. What tolerance should the balancing be done to?

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Re: Engine Balancing

Postby grumpyvette » November 15th, 2010, 8:16 am

precision balance jobs generally try to get the engine to be balance to within a 1/2--1 gram tolerance, naturally the more time it takes, and the more precise the work, the more it tends to cost
the only way your going to get a precise balance job is to have all the components weights and balance known and all the components matched in individual weight, first balanced individually, as each connecting rod big and small end weight needs to match, and each piston needs to match, then have all the bob weights on the crank match the weight the piston & rods weight as a complete assembly, the crank with its flywheel or flex- plate and damper need to be balanced as a rotating assembly .
I generally use and suggest use of scat 4340 forged cranks or occasionally the 9000 cast steel scat cranks for daily driver applications, SCAT, usually does a good job of carefully checking crank dimensions but its your job to check clearances , anytime you build an engine its the builders responsibility to verify clearances, and youll almost always find a rotating assembly needs to be balanced, obviously the crank manufacturer can,t dictate what rods, pistons,piston pins, rings, damper or flywheel are used so the crank counter weights won,t be perfectly balanced
its generally best to buy a complete matched rotating assembly, that includes rods,crank,pistons,rings,bearing damper and flywheel or flex plate. from a single source and ideally with INTERNAL vs EXTERNAL BALANCED COMPONENTS
If your crank counter weights are the correct size and weight, and located on the crank in the correct location,they can be drilled to remove weight, if the counter weights are not large enough, mallory metal is added to crank counter weights because they are not large and heavy enough to compensate for the weight of the pistons and connecting rods, if the crank was designed to allow the counter weights to clear a piston skirt on a 5.7" connecting rod, t
they are significantly smaller, than a crank designed for a 6" connecting rod and its very unlikely to have counter weights large enough to counter balance a 6" connecting rod assembly.
only the balance shop can determine whats required after inspecting the components youve selected, in some cases youll be far better off to buy a new crank or a complete rotating assembly rather than to try to patch together random selected components, but like I stated discus this with the shop balancing your engine.
some times you can select lighter piston pins, or pistons but keep in mind random parts are un-likey to balance as a matched set, and cap screw connecting rods with 7/16" rod bolts can be 200% stronger and yet have more clearance than stock connecting rods
good question, so let me show you a easy explanation
with an internally balanced crank each counter weight compensates for the loads directly adjacent from it, on an externally balanced crank the flywheel and dampers at the ends of the crank
so the total imbalance is averaged and a weight is placed to compensate for that average, if both cranks were placed on roller bearings and spun both would rotate fairly freely
now for a simple explanation
grab a single strand of uncooked spaghetti, and draw a line along one edge with a black marker
an internally balanced engine has stress loads compensated adjacent to the stress location, so try rolling the strand of spaghetti back and forth from two closely spaced locations between your fingers, notice the marked edge stays strait,now an externally balanced engine is balanced from the ends of the crank, so grab both ends of the strand between your fingers in the same direction at the same time,then roll with one end but resist or add drag with the other set of fingers, notice the line on the edge starts to spiral back and forth as you change direction on the loads induced, thats exactly whats going on over the length of the crank on an externally balanced crank. its flexing a good deal more and loads induced stress the whole length.


there can easily be 600- 700 psi of pressure exerted on a piston in a high compression performance engine as the engines reaches max cylinder pressure, theres over 12.5 square inches of piston surface area in a 383 engine so thats easily over 8500 pounds of pressure being exerted on each crank journal and bearing several times a second, this causes a crankshaft to flex , the piston and connecting rod also weigh a significant amount a can induce significant inertial loads as they change directions at higher rpms, loads can easily exceed 10,000 lbs in combined stress on a crank journal 50 plus times a second at 7000rpms
http://rlengines.com/Web_Pages/Cranksha ... ncing.html

keep in mind if your building a common 383 sbc as an example the crankshaft designed for 5.7" rods will have slightly smaller counter weights than a similar crank designed for the slightly heavier 6" rods so try hard to get the lighter weight pistons and specify the lighter than standard tool steel tapered piston pins that cost a bit more because the extra cost of the pins is usually more than off set with the slightly reduced cost incurred during having the assembly balanced, because Malory metal slugs are rather expensive.
counter weights designed for 6" rods will not generally clear piston skirts on a combo using 5.7" connecting rods and a crank with counter weights designed to clear 5.7" rods won,t have enough weight to balance 6" connecting rods without mallory metal added to the counter weights.
this is one good reason to buy a MATCHED rotating assembly as a package deal from a well known and trusted supplier like SCAT,CROWER,LUNATI and specify internal or external balance and the correct rod length, piston bore size and steel alloy ETC.

watch video



http://www.mime.eng.utoledo.edu/faculty ... 1-0258.pdf





http://www.adperformance.com/index.php? ... x&cPath=71


http://www.maintenanceresources.com/ref ... alance.htm

http://www.circletrack.com/enginetech/c ... rminology/



http://www.lunatipower.com/ProductGroup ... 186&cid=18

and several other companys supply balanced kits

What is Balancing?
Balancing is the action of matching the weights of the reciprocating parts of the engine. These parts include, but are not limited to:
Pistons and Piston Pins
Piston Rings
Rod Bearings
Connecting Rods (large and small ends, need to be weight matched)
Damper (harmonic balancer)
Flywheel/Flex Plate
Pressure Plate/Clutch (frequently over looked)
Also, an "Estimated" Weight of Oil is part of the calculations

once you get the assembly balanced ask for the SPECS so any future replacement parts can be easily matched
example if your piston weights 589 grams you need to know that.
once balanced your clutch pressure plate should have a obvious index mark that matches the identical mark on the flywheel so the two components are always assembled together the same way as a unit. SOME SHOPS stamp a BL some shops JUST drill or punch a small DOT, on both the pressure plate and flywheel ,so be aware and look for and match components indexed correctly, and MENTION that you WANT the pressure plate balanced with the flywheel, theres a good chance that if you don,t mention and insist on getting it done that its ignored and this can be a source of vibration if not done(one more reason to get a good SFI rated blow proof pressure plate and BILLET fly wheel, and use a LAKEWOOD blow proof bell housing)

read thru these links



you obviously need a correctly installed bell housing that correctly centers the transmission input shaft with the crank center-line or that can cause problems




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Re: Engine Balancing

Postby grumpyvette » March 8th, 2011, 10:53 am

Crank Journal: Installation Procedures For Placing Heavy Metal Into Crankshaft Counterweights

external balance weights

external balance weights
By Duane Boes


Weâ€™re all familiar with the old saying, "We learn from our mistakes." Recently, I was able to learn from someone elseâ€™s misfortune. I make plenty of mistakes of my own, so taking a lesson without the expense was a welcome change.

Installation procedures for placing heavy metal into crankshaft counterweights is a simple procedure to explain and implement. If things are done right, there is little to fear from a shaft with metal placed in the counterweights.

However, when a heavy metal slug does begin to move, things can get ugly fast. Consequently, proper installation steps to prevent a slugâ€™s movement are important to know and understand.

Counterweights are subjected to fairly simple forces. Their vibrations, on the other hand, are quite complex. These vibrations can create strange reactions in a slug placed into a counterweight. The photo below is an example of something unpredictable happening.

Notice how the slug has rotated within the counterweight. The pen in the photo points out a portion of the slugâ€™s O.D. that has been exposed through the balance hole. This can only happen if the heavy metal begins to turn.

You would expect a slug to slide one way or the other should it begin to lose its press fit. But to see it rotate is completely unexpected. Interestingly, while this piece of heavy metal rotated roughly .200", it shifted very little. For me to try and explain these vibrations would be a case of trying to explain more than I really understand.

Diameters are "Job #1" when it comes to installing heavy metal. The best insurance against a costly problem starts with the press or interference fit between the heavy metal and the hole itâ€™s placed in. Slugs of 1" diameter require between .002" to .004" interference. To maximize the holding power of this interference fit, both the O.D. of the slug and the I.D. of the hole need to be as round as possible.

Even with every effort possible being made, the slug and the hole will not match perfectly. To overcome this variable I recommend using Loctite 640. I have tested the advantage of using this product several times; the results have been consistent.

Without the use of such a bonding agent a 1" diameter slug installed with a .004" press fit will require nearly seven tons of force to be placed upon it before shifting occurs. With the bonding material, the amount of force needed to shift the slug jumps to roughly 15 tons. Once the slug has been broken free, the force required for continued movement drops to seven tons.

In consideration of the oily environment and operating temperatures, Loctite Corp. recommends its 640 compound. The product can be used very sparingly and still be effective. Anyone who is not comfortable with their press fit, and is staking the O.D. of the slug, should seriously consider using some type of bonding agent.

The problem shown in the photo to the left is the direct result of poorly planned placement and drilling. A slugâ€™s placement is nearly as important as its fit. Figure 1 below is a drawing of recommended minimum section thickness for both the outer and separating walls of a counterweight that is expected to securely hold a piece of metal.

An adequate amount of material must be provided between the slugâ€™s O.D. and the circumference of the counterweight. You can visualize that in the case of a section that is too thin; the counterweightâ€™s material will stretch either during the installation or while in operation due to the centrifugal force. In either event the press fit is reduced, opening the door for unwanted movement.

The same type of deformation can occur at the separating wall. In situations where two slugs are adjacent, the wall is holding one-half of each slug. In this system, the separating wall must withstand a load equal to the total weight of one slug at maximum rpm.

Drilling locations
Drilling locations are the next point of concern. The root cause of the problem in the photo stems from a poorly planned drilling into the counterweight. The wall thickness dimensions detailed in Figure 1 may seem a little extreme. Unfortunately, itâ€™s not uncommon to find yourself in a situation where the weight to be removed will fall directly on a series of slugs. In these instances the extra material will be good insurance.

Figure 2 illustrates the effect that drilling has on the counterweight material responsible for holding the heavy metal in place. Attention has to be given to these drill locations. The object being to remove weight without significantly weakening the interference fit between the slug and its surrounding counterweight material. This can be accomplished by limiting drilling to the locations detailed in Figure 2.

In summarizing heavy metal installation, there are four important points to remember. First, make sure your press fit is adequate and that the hole and slug are round as possible. Second, use some type of bonding fluid to fill voids, ensuring a solid footing. Third, make sure the metal is placed into the counterweight in a manner that will maintain a good press fit. Fourth, and last, be careful not to drill out the counterweight material responsible for holding the slug.

A number of engine rebuilders work on modified big block engines. Most of these engines are balanced externally from the factory. In OEM applications external balancing presents few problems. This situation, however, changes as rpm and compression ratios are increased.

In an externally balanced system number 2 and 4 main bearing loads are significantly increased. Externally balanced crankshafts that have broken as a result of a fracture starting at the main bearing side of the overlap are common. Most of these broken cranks also exhibit signs of main bearing wear particularly on numbers 2 and 4. This is the result of higher loads placed on those mains.

The simplest cure for this on higher horsepower engines is an internal balance. In cases where this is not an option, the crankshaft should be heat-treated. By placing a hard wear layer on the journal, bearing life is significantly increased.

The photo below is a close-up of a broken big block Chevy crankshaft. The fracture on this shaft began at the main bearing and progressed toward the rod journal. The photo also shows heat discoloration on the main bearing. The discoloration is uniform around the journal circumference.

This type of heat is consistent with a bearing that is running in distress. Heat generated on a broken shaft is not uniform around the journal; the heat from this type of failure is localized in the areas of interference as the shaft is wedged into the block.

On this crank, the primary failure was the loss of an acceptable bearing surface due to wear. Clearances opened, oil left the journal at a rapid rate taking with it the film barrier, resulting in the generated heat.

A customerâ€™s perception of the problem will usually begin and end with the failed component. For the rebuilder, however, being able to explain why the component failed will always require a much deeper understanding.

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Re: Engine Balancing

Postby mtrhead » March 8th, 2011, 8:52 pm

I was just going to post why you don't hear about clutch/pressure plate being included in the balance conversation. I see it should be!

Also, new ATI's they state in the instructions DO NOT include in the balance because they take a set.

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Re: Engine Balancing

Postby grumpyvette » April 27th, 2011, 2:49 pm

Understanding Crankshaft Balancing
generally the small and large ends of each rod is weighted and the heavier rods ends are belt sanded on the ends until all connecting rods in a set weight the same on both ends, after the rod caps are numbered to prevent parts beings swapped between rods, the piston pads under the pin boss are milled to get the pistons to the same weight and the cranks counter weights are milled, drilled or have weights welded into drilled holes to add weight[/b]
the areas with green Xs are milled to balance piston weight
if the engines EXTERNALLY BALANCED theres off center weights added to the balancer and flex plate or fly wheel to compensate for the lack of extended counter weights on the crank shaft

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 (but not stonger) so the weight of the counterweight will increase as a result.

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.

Since the harmonic dampener (front) or flywheel (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 when the crankshaft if balanced. Both methods are used from the manufacturer.

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. 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. Regradless 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.

Eagle crankshafts, for example, are listed with a "target bobweight". This is an approximation (+/-2%) of the bobweight the crankshaft is roughly "out of the box". Because of the tolerance (+/-2%) the crankshaft cannot be considered balanced. For instance, for a crankshaft listed as having a 1800 target bobweight.

The actual range of bobweights one of those cranks might have is from 1764 (1800-2%) to 1836 (1800 +2%). It might even be at the high end of that range on one end and the low end of that range on the other! This is not usually a problem because Eagle crankshafts are designed to have a target bobweight higher than most typical rod and piston combinations. Therefore, in most cases you will only need to remove material to balance the crankshaft instead of adding material.

The main benefit of the target bobweight is to help the machine shop know what to expect before balancing so that a more accurate price estimate can be made. Eagle will balance a new crankshaft at the time of purchase. You will need to provide the bobweight you want it balanced to, which must be below the target bobweight listed for the crankshaft.


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.

There are also some dynamic considerations to be made when balancing the crankshaft. Explaining those is beyond the scope of this discussion. If you want to study those topics further, contact a crankshaft balancing machine manufacturer and they can go into greater detail.

To calculate the bobweight of a particular assembly, the following formula and balance card is used:

For example, let's say we are balancing a Chevy 383 with the following component weights:

Piston 416g
Pin 118g
Locks 2g
Rings 35g
Rod big end 458g
Rod smal end 186g
Bearings 46g

The rod weight is seperated into "big end" and "small end". This is necessary because the small end has a reciprocationg (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 preference. There is no solid "rule of thumb" for this. Eagle uses 5g for small block assemblies and 15g for big block assemblies. Since it is impossible to accurately represent this value, it is just an estimate. The actual amount of oil can change constantly and can even be different from cylinder to cylinder! We have found through experience that the numbers we use estimate this property well.

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 balancer manufacturer). Some engine builders will perform what is call "underbalancing" or "overbalancing". They will use slightly differnet 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). Eagle uses 50% because this value is required for almost all common street or racing engines.

Balanced Rotating Assemblies

Most Eagle rotating assemblies are sold unbalanced so that engine builders can balance it however they wish. Eagle (and other manufacturers) do offer fully balanced assemblies balanced. But it must be ordered specifically as a balanced assembly. Part numbers for balanced assemblies will begin with the letter B. For instance, if you want assembly part number 12006 balanced and in +.030" bore size, you would order assembly number B12006-030.

All Eagle forged 4340 steel crankshafts are designed for internal balance. An internally balanced kit will not include a harmonic dampener or flywheel because they are not required for balancing â€“ use whatever brand you like. Externally balanced kits will include a harmonic dampener and/or flexplate as needed. If a harmonic dampener and flexplate is provided, it will be an O.E. style replacement, not SFI approved. If youâ€™re building a high horsepower engine, internal balance is preferred. Internal balance is better for longevity of parts and fatigue life.

If your assembling a EXTERNALLY BALANCED 383 or 400 based engine,you'll need externally balanced flex-plate and damper components, obviously its best to have all components used balanced as a unit if possible


damper install instructions

there are 153 and 168 tooth flex plates (12.8" and 14.1")and flywheels
youll need to get one that matches and is also externally balanced

â€“ Technical Tip courtesy of Eagle Products (http://www.eaglerod.com)

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Re: Engine Balancing

Postby Indycars » April 27th, 2011, 3:56 pm

For those people that want to calculate the BOB WEIGHT, there is a calculator in the " spread sheets and engine related forms"
section, along with many other calculators.

Here is a link to the spread sheet section:

Here is a link directly to the calculator:

This is what the main page looks like.

Too much is just enough!!!


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Re: Engine Balancing

Postby grumpyvette » December 13th, 2012, 1:02 pm

Racing Rotating Assemblies: Cranks, Rods and Pistons

http://www.enginebuildermag.com/Article ... stons.aspx

The crankshaft, pistons and connecting rods inside an engine convert thermal energy in the cylinders into rotational energy that produces usable horsepower and torque at the flywheel.

By Larry Carley

Larry Carley

Building a performance engine requires assembling the optimum mix of rotating components that are compatible with the block and heads, properly matched with each other, and balanced to precise tolerances.

The easiest way to get the right combination of parts is to buy a complete rotating assembly from a supplier who offers such kits. Most suppliers offer a wide range of rotating assemblies for street, strip or circle track applications. A complete kit takes the guesswork out of matching the rod lengths and piston configurations with a stroker crank, or matching piston and rod weights with the counterweights on a crank (particularly lightweight cranks). For an extra fee, many suppliers will balance their rotating assemblies for you, which they say reduces the risk of balancing errors that sometimes occur when cranks are incorrectly balanced or over-drilled to correct a heavy spot.

Though crankshaft, connecting rod and piston kits are often marketed directly to racers who want to assemble their own engines in their garage, kits are certainly an option for professional engine builders who are working within time and/or budget restraints, or who lack their own balancing equipment. Buying a complete rotating assembly (which may also include bearings and piston rings) versus sourcing the crank, rods, pistons, rings and bearings from different suppliers reduces the risk of mismatched parts that can cause assembly problems, durability and balance problems. It’s one-stop shopping – and only one supplier to deal with if there are any problems.

For example, it makes no sense to spend big bucks on a lightweight crankshaft, then mate it with a set of relatively heavy connecting rods and pistons. The advantages of using a lightweight crank (faster throttle response and more rapid rpm changes) would be reduced because of the heavy pistons and rods. A lightweight crank must be used with lighter pistons and rods to take full advantage of the reduced rotating mass of the crank.

Mismatched parts can also create balance problems if the mass of the counterweights on a crank don’t closely match the reciprocating mass of the pistons and rods. A lightweight crank has smaller counterweights because it is designed for lighter pistons and rods. If you try to use pistons and rods that are too heavy for the crank, balancing the crank will require adding slugs of expensive Mallory (heavy metal) to offset the added mass. That adds weight back to the crank and undermines the advantages of buying a lightweight crankshaft to reduce weight.

It’s a Balancing Act
A precision balance job is absolutely essential for any high revving performance engine, but it’s also recommended for street performance engines, too. Balancing reduces loads and vibrations that stress metal and can eventually lead to component failure. What’s more, a smoother running engine is a more powerful engine. Less energy is wasted by the crank as it thrashes around in its bearings, which translates into a more usable power at the flywheel.

If a rotating assembly is put together without balancing all of the individual components, it’s impossible to say whether or not the assembly will be within acceptable tolerances for balance. The counterweights on stock cranks are sized for stock connecting rods and pistons. Replacing the stock rods and/or pistons with aftermarket performance parts will likely upset the balance because the new parts will usually have a different weight (usually lighter, but not always).

A difference of a few grams may not seem like much, but it all adds up. A few grams here, a few grams there, and pretty soon you’ve created an imbalance that can produce noticeable vibrations and harmonics at various engine speeds. Imbalance usually gets worse the higher the engine revs due to centripetal forces that multiply exponentially with rpm. Double the rpm and you quadruple the force of the imbalance.

Many crankshaft suppliers publish “target bobweight” specifications for their cranks. This allows engine builders to choose rods and pistons that more closely match the design bobweight, or to estimate how much effort it will take to balance the crank if the bobweight of the rods and pistons vary significantly from the target bobweight.

The counterweights on the crankshaft are supposed to offset the reciprocating and rotating mass of the pistons, rings, wrist pins, rods and bearings. The mass of each counterweight should equal 50 percent of the reciprocating weight (the piston, wrist pin, rings and small end of the connecting rod), and 100 percent of the rotating weight of the big end of the rod and rod bearings (which you have to multiply times two on V6 and V8 engines because each throw on the crank is connected to two rods and pistons).

Though many suppliers publish factory weights for pistons and rods, the only way to know for sure how much these parts actually weigh is to weigh them on an accurate gram scale. The same for the bearings and rings. A special support must be used when weighing the small and big ends of the rod to determine the weight of each end.

Once the reciprocating and rotating weights have been measured on a scale and calculated, a bobweight that equals 50 percent of the reciprocating weight and 100 percent of the rotating weight can be assembled and mounted on the crankshaft journal before the crank is spun on a balancing machine. The balancer will then detect any imbalance and show you where weight needs to be removed or added to achieve proper balance.

Most of the piston suppliers we spoke with said their off-the-shelf performance pistons and custom pistons sets are within plus or minus one or two grams of each other, though some piston sets can vary up to 3 or 4 grams or more. One gram is the approximate weight of a dollar bill, and it takes 28 grams to equal one ounce.

The basic idea behind matching pistons is to weigh each piston, note all their weights, then match the entire set to the weight of the lightest piston. Some engine builders say they weigh and match pistons to within 0.5 gram or less when balancing an engine. Others say plus or minus a gram is close enough, so there’s usually no need to check or match piston weights provided you are sourcing your pistons from a quality manufacturer.

All the piston manufacturers we spoke with cautioned against trying to lighten performance pistons significantly either for balancing purposes or to further reduce weight because doing so may weaken the piston or create stress risers that could cause a piston to fail. A lightweight piston has already had most of the “unnecessary” metal removed to minimize its weight. Drilling or machining away additional metal in the pin boss area or under the crown could weaken the piston to the point where it might crack, collapse or pull apart under high load or speed.

If you have to remove weight from a piston, the safest areas for doing so are usually behind the oil ring or along the edges of the pin boss towers. Avoid drilling or cutting near the pin boss radius, or under the piston crown. If you don’t know where to remove metal, contact the piston supplier for their advice.

An alternative method of matching piston weights is to also weigh the small ends of all the connecting rods, then mix and match the rods and pistons to equalize the total reciprocating weight of each piston and rod assembly as much as possible (combining lighter rods with heavier pistons, and vice versa). This may eliminate the need to drill or grind altogether.

Rod weights tend to vary more than piston weights, as much as 4 to 5 grams in many instances, though some rod manufacturers say their rods are within plus or minus one gram. Rod weights on the large and small ends are matched by grinding away metal until the weights are equalized to within 1 gram or less. Rods should always be ground in a direction perpendicular to the crankshaft and wrist pin, never parallel as this can leave scratches that may concentrate stress causing hairline cracks to form.

If you’re building an engine with a stroker crank, the rod length will obviously be different than stock rods to accommodate the longer stroke. This will also change the rotating and reciprocating weights of the assembly. Longer rods are heavier, but not as much as you might think because the counterweight only has to offset 50 percent of the reciprocating mass. What’s more, a longer rod moves the pin up higher in the piston, which usually means the piston can be lighter (which helps offset the added weight of a longer rod). For higher output applications, shorter rods may be the way to go. A taller piston is heavier, but it can also have thicker and stronger ring lands.

With longer strokes, it may be necessary to reduce the outside diameter of the counterweights so the pistons will clear the crank at bottom dead center. Smaller counterweights mean lighter pistons and rods are necessary to achieve proper balance (or you have to add heavy metal to the counterweights).

Internal and External Balancing Facts
On “internally balanced” engines, the counterweights are balanced to the pistons and rods. On “externally balanced” engines, additional weights on the flywheel and/or harmonic damper assist the crankshaft in maintaining balance. An engine may have to be externally balanced if the counterweights are too thin or too small to achieve internal balance by themselves (as is the case with small block Ford engines and Chevy LS engines).

The main advantage of internal balance is that once the rotating assembly is balanced, it will stay in balance. You can change the flywheel, clutch and/or harmonic damper without affecting the engine’s internal balance. These external parts should also be balanced separately to make sure they don’t cause any vibrations.

On an externally balanced engine, the flywheel and damper must be mounted on the crank prior to balancing. Once balance is achieved, the index position of the flywheel has to be marked so it can be reassembled in the correct position to maintain proper balance. If the flywheel is later removed for resurfacing and is replaced without indexing it back in its original position, balance is lost. The same holds true if the flywheel is replaced with a different one. The whole engine will have to be rebalanced with the new flywheel.

Racing cranks also require a good harmonic balancer. The balancer helps dampen torsional vibrations that may cause a crankshaft to fail or the nose to crack. One crankshaft manufacturer recommends using the lightest and smallest diameter dampener, and balancing the crankshaft with the damper bolted in place. Not balancing the crank with the dampener in place is like trying to balance a wheel with a flat tire.

For racing applications, the same crank manufacturer also recommends using a dampener with an elastomer ring rather than a fluid filled dampener or one with moving parts. They say engine speed changes too quickly in a racing engine for dampeners that are “self-balancing.” A conventional elastomer dampener, in their opinion, eliminates any chance of cold start vibrations and reduces the risk of nose failure on the crankshaft.

Balancing Tips
One crank manufacturer we interviewed said some shops don’t know how to balance cranks correctly, don’t check the accuracy of their equipment often enough, and don’t use the best procedures for correcting imbalances. A precision balancing shop, we were told, should use an engine lathe rather than a drill press to remove metal when balancing cranks.

When a crankshaft is spun in a balancer, sensors detect wobble that reveal the amount and approximate index location of any imbalance. The machine then shows the user where metal either needs to be added or removed to achieve proper balance. It usually takes several spins to narrow down and correct the crank to the point where it is within the desired range of balance (plus or minus a few grams or less).

If the crank has a heavy spot, metal is removed from the counterweight by drilling or machining. Drilling is quick and easy, and can usually be done while the crank is still mounted on the balancer provided the balancer is set up with a drill press. But if a lot of metal needs to be removed, the crank can end up looking like swiss cheese. Holes create turbulence and may also create stress risers that could lead to cracking and failure down the road.

If more than two holes per counterweight are required to correct an imbalance, the counterweights should be machined in an engine lathe. Machining the counterweights to trim weight requires extra labor, but is a cleaner, safer approach to balancing that also helps to reduce windage inside the crankcase and the risk of fatigue failure.

If an engine is externally balanced, and is heavy on one end, one crank manufacturer says it’s better to take the weight off the flywheel or damper than the crank. In cases where a crank needs extra weight added (as may be the case with some stroker cranks), counterweights have to be drilled so slugs of heavy metal can be inserted in the holes.

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Re: Engine Balancing

Postby grumpyvette » December 13th, 2012, 1:07 pm

Balancing Equipment: a weighty matter
http://www.enginebuildermag.com/Article ... atter.aspx

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

By Larry Carley

Larry Carley

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.

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. It’s not unusual to find imported stroker cranks with as much as 80 to 90 grams of imbalance! These cranks obviously need to be reworked if they’re going into a high revving performance engine.

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.

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 mail order DIY engine builders do!). You have to weigh and balance the individual parts and make sure they work harmoniously together.

Balancing requires several pieces of equipment: a highly accurate digital scale for weighing pistons, wrist pins and connecting rods, and a spin balancer for rotating parts such as the crankshaft, flywheel or flex plate, torque converter or clutch, harmonic balancer and crank pulley. You’ll also need a drill stand and/or milling machine to make corrections.

The first step in balancing a crankshaft is to weigh all the reciprocating parts starting with the pistons, wrist pins and ring sets. The idea here is to determine the weight of each part, find the lightest pistons, then match the weights of all the remaining pistons to the lightest one by grinding or machining metal off the balancing pads, skirts and/or pin bosses of the heavier pistons. The goal is to match weights to within one gram or less.

The next step is to weigh the big ends and small ends of the connecting rods. The small end is reciprocating weight while the big end is rotating weight. Each end must be weighed separately by supporting one end on the scale while the other end is supported by a stand. The small ends of the rods can then be equalized by grinding metal off the balance pads at the top of the rods. The weights for the large end of the rods can be equalized by grinding metal off the balance pads on the rod cap. As before, the goal is to equalize all weights to one gram or less.

NOTE: Removing metal from the big end of the rod will change the overall weight of the rod slightly, so it may be necessary to go back and recheck the small end weights and do some additional grinding to keep the small end weights equal.

Next comes some math. 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).

On some balancers, it is also possible to use “simulated” bobweights via the balancer software if you’re balancing a run of identical crankshafts. Once you’ve determined the required bobweight for one journal, you can simply enter that value for each succeeding crank without having to actually bolt weights on the crank. This saves time and improves consistency.

To spin balance the crank, the crank is mounted on the balancing machine support stanchions. The crankshaft should be 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 can be 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. Many balancers today can detect imbalance as small as .01 grams, which is far more than what’s actually needed to achieve a balance of plus or minus one gram or less. The balancer detects imbalance by measuring the displacement of the support stanchion sensors while the crank is spinning. Readings are taken in 1 to 3 degree increments (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.

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 many newer balancers 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. This can sometimes leave the crank looking like a piece of Swiss cheese, which is not good because too many holes may weaken the crank or create windage and drag problems at high rpm.

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. On most newer balancers, the 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.

Multi-plane balancing is also possible on some machines 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).

If a crank is externally balanced, the flywheel and harmonic balancer must be bolted to the crank for a final balance check, and corrections made as needed by adding or removing weight on the flywheel or harmonic balancer to even out the forces. Externally balanced cranks typically have smaller counterweights that are not sufficient by themselves to cancel out all the vibrations of the pistons and rods. An externally balanced crank can sometimes be converted to an internally balanced crank by adding heavy metal to the counterweights. Otherwise, the index position of the flywheel must be maintained with respect to the crank if the flywheel is removed and reinstalled to maintain proper balance.

Whether you’re shopping for your first balancer, adding an additional balancer to expand your business, or replacing an ancient balancer that has outlived its usefulness, there are a variety of machines from which to choose. Balancers have improved a great deal in recent years thanks to better software, better hardware and more user friendly controls. Many balancers now have full color displays and Windows-based software that make them easier to use, even novice users. Graphic displays that show you exactly where the corrections are needed reduce errors and save time.

Balancer manufacturers typically sell two kinds of equipment: balancers for use with existing milling machines, and stand-alone units that include a milling stand or drill press as standard equipment or an option. Balancers come with basic digital displays or PC-based full color displays and various software packages. Bobweights may be included or available at extra cost. Obviously, the more features you want, the more you’ll pay.

Randy Neal of CWT has been making waves in the industry with his “three plane” balancer. He says dynamic two-plane balancers can sometimes be fooled by certain kinds of crankshaft imbalance. His “third plane analysis” software and hardware does a better job of isolating and identifying force vectors to reduce the time it takes to balance a crank by a third or more. Additional software includes unlimited parts and history data base, “PDQ” (Precision Drill Qualifying) to help the user determine the best locations for drilling correction holes (to minimize the number of holes that have to be drilled), and “HMV” (Heavy Metal Vector) to plot the best locations for adding metal if more weight is needed.

CWT sells three balancers: the Multi-Bal 1000 for Mill mounted applications and the stand-alone Multi-Bal 2000, are both standard two-plane balancers with polar and vector displays at a base price of $9,995 and $12,500. The Multi-Bal 5000, which is CWT’s three-plane balancer starts at $15,900. The Multi-Bal 5000 comes with touch screen controls, heavy duty work cabinet and a 4,100-lb resin-filled steel base for added rigidity, which eliminates any residual vibrations that could affect balancing accuracy. Neal says the balancer’s sensors can detect motions as small as .25 microns (.00000975?) and inspects imbalance position 720 times per revolution, which adds to the machine’s high degree of accuracy. Options include; heavy duty gear head mill/drill, digital drill depth monitoring, heavy metal drill stand and “moment-matched bobweights,” universal flywheel arbor and direct communication weight scales system.

Gary Hildreth says his company does not make their own balancers, but they do sell Turner balancers. “Our niche is balancer repair and service. We have belts for every balancer ever built and can service any brand of balancer.” The company also sells bobweights and other balancing accessories.

Hildreth says that within the next 10 to 12 months, he will be introducing a new electronics package to upgrade older Stewart Warner balancers. He estimates there’s still 4,000 of these older balancers out there, and most could benefit from an electronics upgrade. No word yet on the selling price of the upgrade package.

Hines sells three basic models of balancers: the “Eliminator,” a $12,495 hard bearing balancer with a black and white LCD plasma display, the $14,900 “Dominator,” which is a PC-based hard bearing balancer, and the $15,900 “Liberator,” their top-of-the-line PC-based balancer.

John Witt of Hines says the Dominator and Liberator both include a Graphic Depth Encoder which ties the drill quill to the display screen to show the operator where and how deep to drill correction holes. Both machines also offer dynamic plane separation for faster, more accurate corrections – a capability, Witt says, that Hines has had since 1979. The Liberator also includes a 19? flat screen color display, 24? x 36? workstation cabinet so new rod and piston corrections can be done without leaving the machine.

Rich Idtensohn says his company is the world’s largest manufacturer of dynamic balancing equipment. Its CS30 Balancer is an industrial-based, multi-plane, hard bearing balancer that starts at $18,000 and can go as has high as $30,000 depending on options. The CS30 Balancer has two options for instrumentation, the CAB 700 digital display or the CAB 803 PC-based Windows NT touch screen display.

Both instrumentation packages provide an automatic tolerance calculation for each plane, and provide an electronic protractor so the operator can visually pinpoint the exact location to make corrections. The system includes optional drill depth indicator software that shows how much drilling is needed to make a correction. The machine also comes with a heavy-duty drill correction unit that rotates 360°.

“Many manufacturers offer similar features that make balancing easy,” says Idtensohn. “But the real difference is a matter of accuracy, sensitivity and repeatability. The fact of the matter is, if engine builders take a closer look at their results many just aren’t getting what they think they’re getting. A few simple repeatability tests can be very revealing and can make a substantial difference in performance.”

Schenck is also offering a new balancing certification program, the first of its kind says Idtensohn. The three-tiered program offers Level 1 Balancing Operator Certification, Level II Balancing Technician Certification, and Level III Balancing Specialist Certification. The program is designed to provide a standard benchmark for excellence and productivity, as well as individual recognition for technical skill and aptitude. Anyone who has completed their Fundamentals of Balancing, Jet Engine Balancing or Pump & Impeller Balancing seminars are eligible for Level 1 or II certification. Candidates for Level III certification is open to those who can demonstrate in-depth knowledge of balancing theory principles and practices, vibration analysis, rotor dynamics and measuring instrumentation.

Sunnen sells two balancers, a DCB-750 with digital controls that starts at around $17,000 and a DCB-2000 with Windows XP instrumentation touch screen display that starts at around $20,000. Tim Meara of Sunnen says both machines are mechanically the same except for the controls. New features on the DCB-2000 include counterweight cutting software for calculating how much metal to remove from the counterweights if a user prefers to turn down the counterweights rather than drill holes in them. New vectoring software shows where to add heavy metal to cranks and allows the use of existing holes to save extra drilling.

Meara says Sunnen’s balancers are two-plane balancers. The DCB series balancers are extremely precise, measuring unbalance from .01 to 1,000 grams on work pieces weighing up to 500 lbs. (226 kg.). The balancers can handle anything from small engine cranks to diesel cranks. Also included is a heavy-duty drill tower and steel base that is concrete-filled for extra stability for more precise results.

Tim Whitley says his company has a marketing agreement with Pro-Bal to sell their balancers. The PB500 is a basic microprocessor controlled hard bearing balancer that starts at $11,000 bare up to $23,000 complete with the drill press and automotive tooling package while the “Edge” bolts to a vertical milling machine and sells for $7,500.

Placing the “Edge” on a milling machine saves floor space and offers the best drilling machine possible when correcting the imbalance.

“These balancers are capable of force and couple balancing, which some people now call three plane balancing. Separating the forces out allows the user to better understand what’s going on so corrections are easier and faster to make,” said Whitley.

Accuracy is to .01 oz./in. detectable unbalance, and spins take only 20 seconds. The compact design of The Edge takes up less space than other balancers and can be converted to surface heads and blocks, and do seat and guide work.

Turner Technology sells three models of balancers. The Series 1 balancer is for automotive and small engine parts, and can handle cranks up to 7? in stroke and flywheels up to 16-1/2? in diameter over the table. The Series 2 balancer is a heavier, taller machine that can handle cranks of greater stroke with larger flywheels. The Series 3 balancer is taller yet to handle even larger shafts. Series 2 and 3 balancers can have optional roller bearing tops for balancing very heavy shafts.

Turner Technology’s Michael Turner says its balancers offer powerful dynamic plane separation with third plane analysis for fast, accurate balancing, even with a large amount of couple. “Our software allows you to balance any section of a crankshaft without changing the balance elsewhere, so fewer corrections are needed. We also use a closed-loop motor control design for improved accuracy.”

The machines use an interface system that allows the balancers to be connected to any laptop or desktop running Windows 98 through XP. This helps keep the cost of the equipment down, says Turner, and simplifies upgrades. Turner says heavy metal software is included in the standard package along with free lifetime software upgrades. Digital scale readings are automatically transferred into the control software to save time and reduce errors. Balancer packages start around $8,000. You also get one of the best warranties in the industry with an option to extend the warranty at any time, according to Turner.

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