[JSeery]

In little words (and over simplified) you weight all the parts (pistons, rings, rods, piston pins, etc.) individually and get them to match each other. So, you would weigh all the pistons, find the lightest one and then get all the other pistons to match that weight. And so on with all the individual parts. Then you add up the weight for one cylinder (again, piston, pin, rings, rods, etc.) and use a math formula to come up with what is called a bob-weight. This weight is placed on each crank throw to the represent the weight of the parts that would be there in the engine. The crank is placed in a balancing machine and spun at different rpm. The machine tells you where the crank is out of balance and weight is added or removed to get it into balance. Then you can add things like a flywheel and balance it as part of the assembly, and so on. The only part that is different are the rods, they have to not only match weight as a whole, but for each end. So, all the small ends have to match and all the large ends have to match. That is a simplified version of it.

If you are interested in detail you might want to read some of the links A bones posted.