The correct spark plug for this motorcycle is: NGK D8EA
Spark plugs use SAE and not Metric. The one that fits it is the one that fits it.
5/8"
what is the spark plug gap for a 1979 Honda odyssey
Get a spark plug socket and wrench that fits the plug body and remove it by turning counter-clockwise.
18 mm
11/16" (18mm) deep well socket
The CR9EH-9 spark plug needs a 16mm or 5/8" socket.
NGK D8EA/DR8EA/DR8EIX or equivalents.
For XR and XL 250 (pre 83 year), use NGK D8EA
most 50cc 4 stroke takes 5/8
18 mm spark plug socket