NGK B6ES 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 B6ES is equivelant to a Champion N5 and is the 'normal' roadgoing fitment for Norton singles and Vincent models. The other common fitment for these engines was the B7ES (Champion equivelant N4) which is a slightly harder plug and may be more suitable if you prefer harder/hotter running. I tend to keep at least one of each in my tool roll on road bikes, as spares - we sell the B7ES grade as well - Item no 0576