Thanks for the advice!
Just to make sure: is the old Champion 16 the same as the more recent D-16? Champion have had so many changes in their numbers so you get easily confused!
Furthermore I noticed one strange thing. The ferrules that holds the spark plugs seems, on my engine, to be counterbored. Under the 1/2″ thread reach the hole is bigger! So fitting a longer reach plug is no good, after some use you probably not get it out due to carbon buildup!