ACDelco 530-251 Professional Premium Gas Charged Rear Shock Absorber A hardened, chromed piston rod with micro cracks provides an ultra-smooth, self-lubricating, corrosion- and scratch-resistant surface, helping increase seal life Drawn over mandrel (DOM) pressure cylinder for high quality sealing properties and reduced internal friction to assist with longer product life Gas charged for reduced fluid aeration, which can result in better response, less foaming, and more consistent control than non-gas charged shocks Full perimeter, multi-lip, piston rod seal is permanently lubricated and helps keep dirt and contaminates from reaching the interior of the rod O-ring hydraulic piston seal resulting in minimized piston blow-by and consistent control