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How does a rethreader work? 

Shop for Rethreaders

screwing a screw A rethreader is rotated into a hole in the same way a screw or bolt is.
hard steel rethreader shaft The shaft of the rethreader is made from an extremely hard steel, which can alter the threads in the hole.
sharp edge on flute As the thread passes a damaged area, the sharp edge between the flute and thread initiates the repair by pushing the thread back into its correct position.
rethreading an electric back box The rethreader also dislodges debris from the hole by getting into the gaps to ensure they are clear, while the flutes collect waste so it does not become stuck again.
rethreader flutes and swarf Occasionally, a small amount of damaged thread can be shaved off by the rethreader. These small off-cuttings are called ‘swarf’ and are prevented from jamming by the flutes, where they collect and are subsequently removed with each turn of the tool.