Take a close look at a thread chaser set- - - -the sharp edges at the top of the "tap" and the bottom of the "die" aren't there so the tool doesn't cut at all. It just pushes the thread back into shape if it isn't severely damaged. A good-quality tap and die set will be sharp, and it will be made of chrome-vanadium tool steel, not "carbon steel". Carbon steel tooling is for short-term use, and isn't expected to last for repeatedly cutting threads.
I make my own thread chasers by grinding a few thousandths off the crest of the thread of a tap that's started to get a little dull, and painting the shank of the tap red to alert me to the fact it's been modified. The same thing can be done to a die with a small cylindrical grinding wheel in a Dremel tool. These modified tools seem to work a lot better than commercially-made chasers.
Jerry
