A Zerk fitting, commonly called a grease fitting, is a nipple-shaped fitting with a hole in the top that is sealed by an internal check ball with a spring behind it. Zerk fittings are usually found in automotive U-joints and steering components such as tie-rod ends and ball joints. They are also found in some driveshafts. A grease gun has a snap ring that locks over the tip of the Zerk fitting; as grease is fed through the gun under pressure it pushes the check ball down against its spring, allowing grease to flow into the part being greased. When the grease gun is removed the spring pushes the check ball back against the hole, sealing it.
Spray the area around the old Zerk fitting with parts cleaner and wipe clean with a shop rag.
Use a ratchet and socket to remove the old fitting. Some parts may have a plug in them where you want to install a Zerk fitting. Use a ratchet and socket to remove the plug.
Thread the new fitting into the hole by hand, then tighten with a ratchet and socket. Some new parts, like U-joints, come with a Zerk fitting but it is not installed. Just screw it into the hole on the side of the U-joint.
