Yes -my mate was having nightmares with his MGB - he had fitted new NGK plugs - but didn't tell me that! We struggled to change everything in the dizzy, and tried another dizzy I had with me, had the carbs off - stripped, checked back on again - still wouldn't start. In desperation I put in a set of old Champs I had in my bag - and it started right away! That's when he said he had just fitted these new NGK plugs. I threw them right down the bottom of the field at the back of his house - to make sure he didn't try to use them again.
Why? Surely with all these 'problems' with NGK one of you should contact the manuacturer?
I have NGK equivalent of the Champion L10 in the sidevalve and have had no problems, in fact I thought they looked a nicely made plug.
NGK are OK in the side valve provided you use the short nose plugs.
Incidently I have a quantity of Autolite 425 plugs for sale for a very reasonable price. They also work well in side valves.
Lucky you! I merely report what happened on the MGB. It's run fine ever since - still on my old set of Champs. I use N6Ys in my own Trav - harder than the N9Y - more suited to the modified engine.