We balance crankshafts, rods, and pistons using our HINES MC500 computerized engine balancer.

All pistons and rods are balanced to inside 1/2gram.

Crankshafts are dynamicaly balanced to 1/2 gram or less by drilling, grinding, turning and/or  adding heavy metal.

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 stronger) so the weight of the counterweight will increase as a result.
Forces In Action
To better understand the mechanics of balancing, let’s look at the theory behind it. As everybody knows, a rotating object generates "centripetal force." Centripetal force is an actual force or load generated perpendicular to the direction of rotation. Tie a rope to a brick and twirl it around and you’ll feel the pull of centripetal force generated by the "unbalanced" weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple the force.
The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft with only two ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs.
This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.