Mac Tools 14mm Spark Plug Reverse Rethreader - $40 (san anselmo)

Mac Tools 14mm Spark Plug Reverse Rethreader SPKTHR14 Metric This is a hard to find tool.
I used it once or twice and it worked well.
Clever design slips into the cylinder, then you snug it up and begin re-threading from the inside out so it follows the existing threads.
Spark plug rethreader features an expanding collet design where the rethreader can be inserted then expanded to the exact size before winding out to repair the spark plug thread