I am not sure the name of the tool but it is made by Dayton and I believe McMaster Carr sell a similar model. It has a set of smooth spacers attached to a group of serrated teeth. The smooth spacers ride in the good threads and the serrated teeth clean and reform damaged threads. I recently used it on a 1 inch bolt that had the end threads damaged. I do not have a die that large and it would have been hard to get one on the damaged threads anyway. This tool slides over the damaged part to the undamaged threads. Tighten the tool onto the undamaged threads and then turn in a direction to run the serrated part over the damaged threads. Very useful but the disadvantage is that you need enough room to swing the tool around.