Spark plug thread repair kit
It is all too common that a spark plug thread becomes damaged where some monkey has rammed a spark plug in rather than checking the thread is correctly started before starting to wind it in with any tools.
Traditionally a damaged thread would have to be fixed by taking the head off and taking to a machine shop. The other option had been a helicoil that would work once but next time around would likely rip out and render the head scrap.
This kit uses solid inserts and are much stronger than the old style wire helicoil method. With alloy cylinder heads these can be fitted without even removing the head.