Hello again just a quickly . I've run my thunderbolt on champion nc3 plugs for about 100 miles and the wright hand side seems to be breaking down what plug would be suitable. Thanks
N4 was listed up to 1968 then N3/NC3.
But NC3 should be OK. Its just a slightly colder plug.
If the electrode or insulation is breaking down then its getting much too hot and there is a problem which needs sorting - over advanced timing or too weak mixture being possibilities.
My bikes have never got on wel with Champion plugs. Try NGK B7ES.
The book states N4 plugs for the Thunderbolt, with resistive HT lead.
Regards TTJohn