Tamiya 50590 4mm Ball Connector.
These ball connectors join linkages to sockets and are used in a variety of R/C cars.
They are of metal construction with 2mm threads and a 4mm nut built-in for easy threading. They are 5mm tall not including the threaded area.
Install in desired location and tighten with 4mm wrench.
Each pack contains 5 ball connectors.