I am trying to fill in the blanks in the succession of Champion spark plugs as applicable to the XK 120-150, mainly for the std 8-1 comp ratio…but may as well fill in the race application and 9-1 NA10, N12) and 7-1, as well. Roger Payne has documented much of the original factory specs, as well as the original type plug details as to colors (ie black metal ) and marks (ie Made in England etc). My interest is in the evolution of the later superceding Champion plug numbers and the heat ranges (and how the number system changed about 1957-58). . It all starts with the NA8, then the N8B. (the NA8 is one step colder than the N8B.) The factory Champion specs for the 1950s Champion numbers are well documented thru the Service Manuals and the Service Bulletins 95, 95A, 99, 136, 156, 164, 217. It seems clear that he plugs became the N5 and then the N5C (copper): but post N5C is when it becomes foggy to me…as the N7, (maybe an N8?), the N9…(and N9Y, N9YC), and the oft used N12YC. Cross ref charts show the N5 = NGK BP5ES…one step hotter than the usual BP6ES. The N7 (N7YC) = BP7YC…one step colder than the BP6ES, while the N9YC = the BP6ES.: Now it seems the heat number system changes…with the N11YC then = BP5ES…and the popular N12YC now back to the often recommended BP6ES…the mid point heat range recommended most. BTW…in Champion N=14mm 13/16, 3/4 reach…that is what the XKs have. The Y is for a std projected tip, The C for copper. Note than an first letter R is for extra resistance, for modern hi resistance spec cars…not suitable for the XK). Any further info…comments to clear up the Champion numbers, correct the above info, for the XK…appreciated. Nick