They balance by "average" weights of parts. Good for a 5,500rpm street build. In a typical "race" balance all pistons/rods are corrected to match the lightest then the crank is balanced to that number. My "kit" balanced crank was 42grams off on front & 12grams off on rear when my shop corrected the piston/rod weights and spun the deal. They had to weld up a new crank because the other balancer drilled out too much and had to be filled in before we could fix the balance. And this was all about 2-3 months ago.
