Centralab (CRL) R2-81 Fastatch Rear Dual Volume Control Type B, Tap 900K, outer shaft diameter (upper) measures 3 1/4 inches round (quarter way from lower top), inner diameter 1/4 inch flat and second section of outer shaft diameter (quarter way from bottom measures 1 inch round ; control base diameter measures 15/16 inches (7/8") by overall shaft length measures 4 1/4 inches long.