i have a 1984 yamaha 400cc engine that i am working on and it had some wierd NGK plugs. anyone ever hear of a spark plug using a 18mm nut? weird! i thought they were all 5/8 or 13/16...anyhow i need to go get a 18mm deep well now.

i ended up busting off the ceramic part of the plug to use a standard 18mm socket to get the thing out.

 

What's the NGK number on the plug? I can't offhand think of any that are 18mm...seems like an odd one.