These M3 Rear arms are an upgrade for the non-M cars. The stock non-M arms are made from sheet metal spot welded together at the ends. The arm tends to flex and bend under even moderate cornering loads. Factor in gripper tires and more torque and this arm is inadequate for stability and consistent handling. The M3 arms are one piece of aluminum offering greater strength and with a sealed monoball inner bushing that allows the arm to rotate (instead of twisting).