18 mm
Spark plugs use SAE and not Metric. The one that fits it is the one that fits it.
18MM
5/8"
Get a spark plug socket and wrench that fits the plug body and remove it by turning counter-clockwise.
11/16" (18mm) deep well socket
The CR9EH-9 spark plug needs a 16mm or 5/8" socket.
most 50cc 4 stroke takes 5/8
18 mm spark plug socket
You will need either a spark plug wrench or socket that fits the plug. The socket will need to be not only the right size, it will need to be deep enough to allow the socket to engage the nut built into the plug. Be careful not to force the plug if it is seized as breaking it off leads to major projects. Most plugs unscrew easily.
5/8 inch spark plug socket.5/8 inch spark plug socket.
You need an 18 mm spark plug socket.
In many automobile engine configurations, a spark plug socket is the only way you will ever get at the plug. But even in situations whee the hex on the plug is easily accessible (lawnmower, for example) a spark plug socket is still the best way to remove it without damage to the plug.