Applies to string insulator units of the cap and pin and long rod types and their associated metal fittings for overhead lines. Includes six standard sizes designated by the nominal pin diameters which form the basis of the standard. Defines each standard size by the dimensions of the pin ball, of the socket and the hook-on 'Go' gauge. States dimensions of twin-balled pins for coupling of two sockets. Tabulates clearance and locking conditions. Appendices give information on extreme positions of the pin ball in the socket as well as on gauges for checking dimensions.