8221 Shaft Couplings by Shaft Couplings Shaft couplings permit positive thread-to-thread adaptation from machine to core bit. Male Female 5/8"-11 3/4"-16 3/4"-16 5/8"-11 1-1/4"-7 5/8"-11 5/8"-11 1-1/4"-7 . Posted in Concrete & Masonry Tools, Core Drill Equipment