Now we come to the point where Balancing must be attended to before final truing and Assembly. Here Inspection and Machining blend into Tuning. 

So far in this Workshop we have approached rebuild operations from the viewpoint of an enthusiast with limited resources but moderate resourcefulness. A Stewart-Warner dynamic balancer workstation is certainly beyond this scope. My personal experience operating such reaffirms my convictions that with proper understanding of the fundamentals, an equally satisfactory result can be obtained after the original fashion that preceded strobes and the like. 
We must return to the absolute basics then: (Here I very well demand to be corrected!) 
  
You want the round-and-round moving counterweight balanced by only a fraction of the real up-and-down moving weight. 
  
Or:The magic number or factor is the quotient of (what mass will counter-balance the flywheels) divided by (the real mass of the parts flying up and down). 
  
We can weigh directly the complete piston assemblies, and sort of directly weigh the rod tops for a total reciprocating mass. Next we must determine what weight counter-balances the flymass opposite of the pin. Divide this by our total and we know our static factor. (One then adjusts this factor by lightening pistons, drilling the wheels, etc.) 
  
Therefore I chose to build an elevated knife-edge device so that the final true'd assembly could be hung with no need to simulate rotating mass with a bobweight. 

My knife-edges are 'homemade': I had a pair of 16' planer blades matched on a magnetic chuck surface grinder. I then cut four pedestals out of some convenient scrap which happened to be 11/2' hex stock. I milled slots on the ends to support the blades 14' off of a massive 2' plate. The reason for the tall edges is that I often wish to check the balance factor on flywheels that have already been trued. I hang them 'military' style. 
I am certain that rod stock instead of edges would work as well. With a large enough throw, I imagine that even common centers on a lathe could reveal a factor to within a tenth of a percentage point, given proper attention to detail. 

After previously weighing the rods for reciprocating mass as per our earlier posts, one merely adds weight to the rods until they achieve a balance with the counterweight portion of the wheels. This added weight plus the weighed reciprocal mass of the rods can then be divided by the real total reciprocating mass (Rod tops plus piston assemblies) to give a percentage factor. 
There are techniques and disciplines involved of course, such as washing all lube out of the rod rollers (dynamic procedures call for adding an arbitrary weight for oil, perhaps as much as 2 gm), and carefully spacing the rods on the crankpin to swing as freely as possible. Determining that exact point of being in 'balance' is best done by holding the wheels at exact level (crank center through main center through '6 o'clock' on the wheel) and observing the direction of movement when released. My reproducibility using spring shims as weights on leaded wristpins is give or take about 5 grams. If a total counter-balancing mass is very roughly 1000 grams (guessing), then that's about 1/2 of 1% accuracy. 
  
My primitive understanding of the theory leaves me with many questions of my own, so I wish to leave this for other Listers to illuminate. 
  

From: Guy <guyiii@home.com> 
...I must say, you've lost me,..probably we're "looking" at the elephant from different directions... 

I look at it as: the flywheel is over balanced - the counter weight balances 100% of rotating mass, plus lesser % of "reciprocating" mass...."reciprocating" and "rotating" masses are useful concepts but "purely" definitional - they are neither 100% reciprocating nor 100% rotating ....(does the bottom 49% of rod move up & down as well as rotate?; does the upper 49% "rotate" as well as move up & down?) 

From: "Cotten" <Liberty@npoint.net> 
Guy!It's like I said: (Here I very well demand to be corrected!)  
That's the way my tinker-toy mind comprehends the static method as robbed from the military manual. 
I know Mat can straighten me out! 
It's ALL pretty purely definitional: We can only 'model' our understanding of the physics involved with the teeter-totter analogy of balance. I know it all goes far beyond my math comprehension. 

Dynamic methods account for many variables that static technique ignores, however the result is often the same. (Instrumental methods can be very precise, but they require trained skills that will always leave room for operator error: There's lot's of milwaukie wheels out there with holes 
drilled at both 3 and 9 o'clock, thus cancelling each other, and robbing torque mass to boot) 
The 'sweet spot' around the low to mid 60% factor (Chiefs) was discovered by the tedious trial and error experiments of the pioneers of the design, and apparently dynamic techniques have only verified the phenomenon. 

Again I must preach my belief that 'Balancing" is for tuning, and there really aren't wrong factors, just poor choices for a given specialized application. 
Annoying vibrations are most often a result of an assembly concern, as outlined by Moen in part 3 of the Theory section. 

My best guess is that it's all about the leverage thing: 
I suspect that at low factors, the pistons have an advantage over the flymass, but want to pull themselves apart. I would guess that at an overly high factor, accelleration/de-accelleration would be prolonged, but I have never encountered a motor built to the upper extreme. 

Another reason why I static balance is that it gives you the freedom to alter the reciprocating mass as well to drill the wheels (which is a one-way trip, unless you are prepared to "plug 'n stuff". The pistons are shorter lived than the crank, we assume, so why not carve on them?) 

From: "Moen" <moen@get2net.dk> 
Could you run this bit past me again in slow-motion? 
>Determining that exact point of being in 'balance' is best done by 
>>holding the wheels at exact level (crank center through main 
>>center through '6 o'clock' on the wheel) and observing the direction of 
>>movement when released. 

From: "Cotten" <Liberty@npoint.net> 
BALANCING revisited: 

When one looks at a flywheel, the portion opposite the crankpin is disproportionately heavier. (On a Z wheel there is an obvious 'shelf' in the forging.) Let the crankpin denote 12 o'clock by rotating it to its highest position. Therefore 6 o'clock is 180 degrees down, bisecting the 
countermass. We should mark it accurately with a line for reference when placed on the 'edges'. 

This extra weight see-saws against the rod weight when the axis of the mainshafts is held level. (Building the fixture to do this properly is important, and we should investigate that soon.) 

'Balancing' is adjusting the weights on either side of this 'scale' to bring it to equilibrium with a hair less than 2/3rds of the real weight of "what goes up-and-down". It seems a bit arbitrary that the top half of the rods, plus the obvious piston assemblies, are used to calculate this, but it works for static methods. 

We do this to tune the motor. The 64% isn't carved in granite, but it is where the gives-and-takes of the pistons' leverage over the flymass is at an optimum for harmonic vibrations and the like. A lower factor seems to make them jump like rabbits, but edgy at sustained highway speeds. 

So we lay the mainshafts of the complete rod/flywheel assembly across a pair of suspended, perfectly level, stable, precision surfaces (such as 'knife-edges' which are like metal rulers stood up on their lengthwise edge, or precision round stock) that will allow the rods to hang free. The crankpin will automatically roll straight up to 12 o'clock. We want to slip enough weight into the wristpin bores so that it will roll back down to level (3 or 9 o'clock, parallel with the horizon). You can mentally draw a line through the crankpin center through the mainshaft center through our countermass reference mark. This line should perfecly parallel the 'edge' or horizon. 
Determining when we have reached this equilibrium is best observed by holding the wheels from rolling positioned at this parallel geometry. If it rolls slightly upward when you let go, it is light; downward indicates too much has been added. 
By weighing what you added, you can figure what your factor will be. 
Then you must correct it. (When drilling in the counterweight portion, it is possible to skew the wheels from the force of the drillpress. I use a bolt and nut cut down to place between the wheels as an expandable support.)  

From: Patty Duffy <MICHIGANDER@Worldnet.att.net> 
I have a set of origional Ind 80 wheels and piston's which I had to replace, I had to go to .005 oversize pistons and when replaced them, I had to add some weight to the top of the wheel's. The picture of the balance stand in the origional overhaul manual can be made by the home mechanic.  

Anyway you are correct as to the methods used by the factory, the magic 64% did work for the 74 and 80 motors. Some might say 65% but if the wheels are set-up on the stand with piston complete like the picture, you will find that the rod top total's and complete piston come out to about 64% factor. The flywheel drillings on different wheels from 40's through the 50's have some simple explanations, Indian had to change piston weights more from the lack of scarce war time aluminum stock (reused aluminum or reclaimed and thus thicker piston wall for example) but the factor remained much the same. 
  
From: "Marc Gunderson" <indian@beachaccess.com.au> 
I true up flywheels in the lathe using a dead center in the chuck which I machine first and a live center in the tail stock. The pressure on the flywheel assembly is most important when doing this. Only a skilled operator can feel when it is right. 
My balancing jig is 2  bits of heavy duty angle iron machined with a pointy edge set up on my lathe bed using an engineers level. And balanced like the book says. Works great. 
I too like Cotten and Duff have not got great amounts of money to spend on top notch equipment, so improvising is the only way to go. 

 
Drilling for balance; Note support between wheels.