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
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.
whichever size fits tightly, probably a metric 15mm