Spark plug diameter for helicoil insert

Hi guys
Not posted for a while, been busy in the garage, anyone know the correct size helicoil insert I need for my Chaly spark plug? iv'e just managed to strip it (never stripped a plug hole for 25 years )

10mm metric fine ?
Check spark plug manufactures website maybe to confirm.

...or go with an oversized spark plug. (drill & cut new threads) - spark plug size 'D', 12mm instead of 10mm.

Thanks guys, wasnt sure of the pitch thats all
Cheers
Phil.