The “N” series plugs are 3/4 reach, 13/16 hex plugs. I pretty much never use an “N” plug any more. The ”C” is the same plug without that clunky 13/16 hex and required bigger plug socket.
It appears that Champion and NGK are eliminating plugs in the “N” series very quickly. There is no real reason to use them except maybe a 100 point restoration or similar.
I can’t remember the NGK nomenclature for the 3/4 reach 13/16 hex plugs or Autolites. I have looked at the NGK catalog and Autolite’s web page and they are eliminating that plug series too.
