The Alloy USA JK HD ball joints were designed with a low profile (grease) zerk fitting, a feature not offered on factory replacement parts. Inner insert is replaced by a high strength steel socket. (Replaces factory plastic nylon insert). This steel insert increases Ball Joint strength. This is a must for lifted vehicles running tires larger than stock. Ball and socket use an exclusive threaded preload system that prolongs Joint life. Ball and shaft are constructed from SAE 4150 Chromoly steel. Will not work with RCV shafts.