Synergy Jeep TJ/LJ, XJ, YJ, ZJ (Dana 30/44) HD Ball Joint Full Set (4120) Ball joint stud made from 4140 steel with an induction heat treated bearing surface for smooth operation and maximum life Fully greasable metal on metal design eliminates the stock style nylon liner for improved performance and durability Includes (1) upper and (1) lower 12 Month Unlimited-Warranty Ball joint boot is made from tear/wear resistant silicone polyurethane