This die stock is for using with a 1" outside diameter split die. It features screws that allow you to compress the die to take off more material (if the thread is too tight) and alternatively a screw that allows the die to spread open, which allows you to cut unthreaded or damaged bar.
If you have a thread you wish to repair we recommend a Hex Die. These will take off less material and will try to reform the thread. A Circular split die is for making new threads, taking off additional material or when the thread is so badly damaged a die nut will not fit.