Chuck insert reducers fit into the back of the chuck and onto your lathe headstock and onto your lathe headstock spindle. Which one you need will depend on the spindle size of your lathe. There is a wide range of inserts to suit most common lathes.
The inserts are threaded both inside and out. The inside thread comes in a variety of sizes common to lathe manufacturers. The outside thread is a unique inhouse thread made to fit into the Teknatool chucks.