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by Jon Mikelonis

Introduction
Do you want us to balance it? Well of course I do. Do you know what that
means? Not really. With regard to engine building, the term "balance" can
cause an enthusiast to become conflicted or anxious. On one hand you're
confident that almost nothing in life that is "imbalanced" is desirable
and on the other hand you don't know what's involved in engine balancing, whether it's absolutely necessary for your application, how long it will take, or how much it's going to cost.

 
If you are having an engine built, the balancing stage is the customer's first test of preparedness. In this shot, the rotating assembly for FordMuscle's 400 build is being balanced. Before any block work commenced, we were already at the shop with a pressure plate.
   


Engine balancing refers to the process of equalizing the weights of the engine's reciprocating and rotating components, also known as the rotating assembly. The rotating assembly is made up of the pistons, pins, rod bearings, connecting rods, crankshaft, balancer, flywheel or flexplate, and the pressure plate. While that may be apparent to you, if you do decide to have your engine balanced during a buildup, then making sure you have already acquired your pressure plate and balancer even before you've had any block work done, may not be so obvious.

So does your rotating assembly need to balanced? The same question can be asked in a better way. Does your motor NOT need to be balanced? Really, the only circumstance that one might consider NOT having their rotating assembly balanced is if they were doing a stock rebuild with factory or factory-equivalent components. However, even in the case where you are having a flat top piston replaced with a flat top piston, variations in manufacturing tolerances between different makers of the same piston cause many machinist's to balance every engine they build. With regard to performance engine building, most high performance pistons are designed to be lighter than factory pistons in order 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 it is verified.

Stock rebuild or performance rebuild, it only takes 1 to 2 hours to
balance most V8's. With that said, not having your rotating assembly balanced during a buildup is equivalent to not having your front end aligned after you spent $1000 on tires.

Let's Get Started
Assuming that you believe that the maximum reduction of the internal
loads and vibrations that stress the metal in your engine is a desirable
attribute, then let's get started showing you what's involved in engine balancing. The information and pictorials on the following pages were documented at Superior Machine in Sparks, NV. The work was performed on FordMuscle's ongoing Ford 400 buildup Keep in mind that this is a stock displacement engine that is externally balanced.

Calculating Bob Weight (Reciprocating Weight + Rotating Weight)
The process of engine balancing begins by determining the crankshaft "bob weight". A common misconception surrounding rotating assembly balancing is that there is some kind of miraculous jig that enables the entire rotating assembly (rods and pistons included) to be bolted together and spun on a machine. Clearly this doesn't exist except in the form of an engine block of course. So without that magical apparatus, the combination of the rotating assembly's reciprocating and rotating weight must be mimicked in the form of weights that can be attached to the crank's rod journals. These are known as bob weights. For a V8 like our Ford 400, four bob weights must be assembled that represent both the reciprocating and rotating weight of the piston and rod assemblies. If you are asking yourself why there aren't eight bob weights required for a V8, just remember that there are two rod and piston assemblies per journal. Each bob weight accounts for two rod and piston assemblies. Therefore, only one bob weight is clamped to each journal.

Reciprocating Weight
Reciprocating weight refers to the "up and down" forces each piston and rod assembly exerts on the crank. Reciprocating weight is made up of the piston, piston pin, locks (for full floating pins), piston rings, and the "small end" of the connecting rod. The following chart shows the values of each and total for the Ford 400 rotating assembly used in this article.

Reciprocating Weight- Ford 400 Build
(all values in grams)
Piston
500
Pin +
140
Locks +
0 (press fit)
Rings +
56.5
Rod Small End +
216
Reciprocating Weight =
912.5 grams

The following sequence shows how the calculation of reciprocating weight was actually performed at the shop.


Each piston is weighed to determine the lightest one. Pistons in the set that exceed the weight of the lightest piston in the set will
have material removed from the underside with a ball mill. Pistons are usually balanced within +/- .05 grams of one another.
 
Before any material is removed however, a pin is weighed and the scale is tared. continued next page
     
(Calculating Bob Weight Continued, On the Balancing Machine, Balancing the Pressure Plate)
     
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In This Article...
Engine balancing can evoke misconceptions of complicated equipment, untouchable machine shop skills, and high costs for the enthusiast. FoirdMuscle takes you step-by-step through the rotating assembly balancing process. We'll demonstrate that while attention to detail is important, the procedure is something every FordMuscle'r can understand.

Related Articles:
Roddin' Real - OEM Connecting Rod Preparation, Reconditioning, and Balancing
About the TCI® Rattler Harmonic Balancer
The lead image of this article shows that we chose TCI's Rattler to serve duty as our 400's harmonic balancer. Can't find it? It's the round yellow and black thing with the snake on it! Anyway,
The Rattler® is an unconventional balancer with a specialized design derived from the "bifilar mounted pendulum absorbers" used in Lycoming aircraft engines like the one shown below. While TCI® applied the design to internal combustion automotive engines, the duty of the pendulum absorbers are the same in each application. Both work to absorb crankshaft torsional vibration


The Rattler's design warrants further explanation. But first, some basic background on the requirements of a harmonic balancer.

A harmonic balancer's job is to control crankshaft torsional vibration. Torsional vibration is inherent in a combsution engine where changes in cylinder pressure exert unsteady forces on the crankshaft.

Harmonic balancers are either energy dampers or energy absorbers. Elastomer type balancers are classified as energy dampers since they work to dissipate energy. Unlike regular balancers, The Rattler® provides torsional control by producing counteracting forces to the forces that actually cause torsional vibration.



As the exploded view shows, inside the Rattler® are steel rollers (centrifugal pendulums) that fit loosely into a specific number of holes. By using an exact, patented, mathematical relationship, the rollers will roll forward during compression strokes and roll backward during the power stroke to keep the engine speed variations and torsional vibration to a minimum. The rollers store and release energy back into the crankshaft rather than converting the mechanical energy into heat energy as dampers do.

Find out more about
The Rattler®:

TCI
www.tciauto.com


The Hines Crankshaft Balancing Machine in Action. During the process of balancing our 400's rotating assembly, the crankshaft, harmonic balancer, flywheel, and bob weights are spun to 500 RPMs numerous times between rounds of incrementally removing material from the crankshaft's counterweights. Lighter than stock piston and rod assemblies always require balancing the rotating assembly.

**Pay close attention and you'll hear the TCI Rattler's pendulum absorbers "Rattle" as the Hines machine winds down.


 


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