Spark plugs use SAE and not Metric. The one that fits it is the one that fits it.
18MM
5/8"
54mm
11/16" (18mm) deep well socket
18 mm
The CR9EH-9 spark plug needs a 16mm or 5/8" socket.
I believe that is 18 mm.
Get a spark plug socket and wrench that fits the plug body and remove it by turning counter-clockwise.
The BPR6ES spark plug typically requires a 5/8-inch (16mm) socket for installation and removal. This size is standard for many automotive spark plugs. Ensure you have a spark plug socket with a rubber insert or magnet to help secure the plug during installation.
most 50cc 4 stroke takes 5/8
A Champion 8815 spark plug requires a 5/8-inch (16mm) socket for installation and removal. This size is standard for many spark plugs, making it easy to find in most tool sets. Always ensure the socket is compatible with your ratchet for optimal use.
It's called a sparkplug because... It creates a spark to ignite the fuel in the pistons, and as with any plug - it fits into a socket.