Genuine OEM Mercedes-Benz Caliper Spread Spring, part number A0004212001. This spring is a small but vital component within the brake caliper assembly. Its function is to apply a slight outward force, helping to retract the brake pads away from the rotor when the brakes are not applied. This ensures that the pads do not drag on the rotor, reducing unnecessary wear and improving fuel efficiency. Genuine OEM springs are designed for the correct tension and durability.