It is 22mm, but a 7/8" hex will work.......
Many spark plug sockets have a 7/8" hex on the top of the socket, so you can just invert an extension into the plug socket and use a ratchet or breaker bar on the other end of the extension. This will give you a 7/8 hex on the end of the extension and you most likely
already have these in your tool box................