NGK B7ES Spark Plug to fit all Norton road going single engines, i.e. Model 30/40, OHV ES2/Mod 18/Mod19 and SV 16H/Big4. It also fits Vincent 500cc and 1000cc models (Series B/C/D).
I use NGK spark plugs on all my bikes now, I have found them to be the best of all modern plugs for quality and reliability. The B7ES is equivelant to a Champion N4 and is an alternative to the slightly softer B6ES (item number 0575). You may find your engine runs slighlty better on this grade of plug if you often run at high speeds or your engine runs hot, as this is a slightly harder (colder) plug.
I tend to keep at least one of each in my tool roll on road bikes, as spares - we sell the softer B6ES grade as well - Item no 0575