Yaesu Rotators use English (Imperial) sized Ball Bearings. Depending on the specific rotator model, the size will be: 5/16" (7.938 mm) for elevation rotor of G-5500 http://ivok.home.xs4all.nl/pa1ivo/G-5500.html 3/8" (9.525 mm) 65 rockwell 65/35(steel/nickel) for azimuth rotor in larger G-1000 rotors w9gb Sent from iPad Air