ACDelco 520-187 Advantage Gas Charged Rear Shock Absorber Super-finished chromed piston rod inhibits corrosion and provides a more consistent wear surface Piston seal adjusts as it wears to maintain a tight seal between the piston and pressure tube. This minimizes leak paths for a longer, more consistent, product life. O-ring provides constant pressure to the inner tube and makes the valve less susceptible to dirt, wear, and fatigue leading to longer service life Drawn over mandrel (DOM) pressure cylinder for a smoother surface and less resistance, resulting in increased service life