18MM
5/8"
18 mm
Spark plugs use SAE and not Metric. The one that fits it is the one that fits it.
11/16" (18mm) deep well socket
5/8
The CR9EH-9 spark plug needs a 16mm or 5/8" socket.
8mm square
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.
30 mm socket
most 50cc 4 stroke takes 5/8
Take your socket set and try various sockets until you find one that fits...then look at the size on the side of socket...BINGO!!! now you know the size.