I’ve not had a problem thread for about eighteen years. To encounter three on the same motor fitted by a well-known Sydney wrecker, suggests I might be the “discoverer” - not the creator.
These, sadly aren’t sold in M10x1 for secondary spark plugs:
https://www.ebay.com.au/itm/361302625145
So I’ve resorted to this:
Worst case I have to get the mobile thread guy onto it.