Ariel Owners Motorcycle Club

Hi all,
I have a 28 557cc SV, what is the correct 18mm 1/2" spark plug for it I have a selection but all different makes also a Champion which has the wrong thread size a slightly to small which I think is a counterfit one which has gone in the bin.
I have been onto the green company for one but there is so many different types, even looking through my old books they all seem to recommend different types. Regards Clive.

I use NGK AB6.

Thanks Tony will get a NGK and try it also trying to get the carb right but thats another story the joys of a nearly 100 year old bike. thanks again Clive.