ACDelco 540-5018 Specialty Premium Monotube Rear Shock Absorber Single tube and multi-stage sealing design minimizes chamber temperature and debris entry for longer lasting performance Larger 1-13/16 inch (46mm) bore size allows the shock to react quickly to bumps and other road imperfections Greater heat dissipation, improved cooling, and reduction in fade, compared to conventional twin tube shocks Large internal working area helps reduce stress and fatigue for component durability Monotube technology and rod guide bearing system offer outstanding response for controlled handling