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.
most 50cc 4 stroke takes 5/8
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.
21mm socket or wrench
no,just make sure you get the right one that fits it correctly.