Balance BasicsEngine balancing is an attempt to balance the forces of the reciprocating and rotating internal components of the engine. Balancing an engine involves figuring out the effective mass of the components hung on the rod journal, which amounts to the bob weight. The bob weight includes the pistons, pins, locks, rings, rods, and rod bearings. It is determined by weighing the parts and then using a formula to account for the parts' motion. It seems simple, but often the parts have widely varying weights; for example, some rods may weigh more than others. So, the first step in a balance job is to weight-match the components, particularly the rods. Once weight-matched, a bob weight can be determined. The crank is put on a balancing machine with bob weights of the derived weight attached to the journal, and the crank is dynamically balanced, much like a tire.
The factory balance job is a rough estimate, but it is close enough to provide acceptable smoothness. If the stock components are used in a rebuild, rebalancing isn't necessary. The same is true if replacement parts such as pistons are of similar weight to the originals. However, if aftermarket race parts such as pistons or rods are used which differ greatly from the stock weight, the factory balance job will not even be close. It's not uncommon for an aftermarket piston to be several hundred grams lighter than the originals, making balancing mandatory.