SBT-1 CS 3/8" Stud Mounted Ball Transfer Unit 1" Main Ball 3/8" Threaded Stem Proudly made in the USA SBT-1 CS 3/8" Stud Mounted Ball Transfer Unit, Main Ball is 1" Inch, 1" Main ball rotates on (80) 1/8" support balls, SBT-1 CS 3/8" stud mounted ball transfer is heavy duty, stud mounted bearing and capable of supporting as many as 75 pounds (per unit).