Engine balancing is the process of reducing the forces created by moving parts in an engine. The goal is to get each piston assembly within 0.1 grams of each other. This is done by adding weights inside or outside the engine to counteract the forces. Engine balancing is not necessary if you are rebuilding an engine using all factory components. However, if you are using aftermarket components, such as a different crank, rods, or pistons, you should balance the engine.