The inner valve spring K 27505 pairs with the outer spring to form a concentric (dual) spring assembly that improves stability and reduces resonance across the valve train of the ZV40/48. The inner spring provides fine control of valve closure at lower deflections while the dual arrangement raises the natural frequency and resists harmonics that can cause chatter or instability. Typically manufactured from high-grade, shot-peened spring steel and precisely ground, K 27505 helps maintain tight valve control under varied loads — replacing both inner and outer springs as a matched set is good practice to preserve timing accuracy and prevent premature failures.