Thank you for your answer AC Ace Bristol, but my engine is an AC, not a Bristol...
mmouss.I appreciate your engine is AC whereas mine is Bristol, I posted links as your original post referred to plug temperature ratings, so I thought the links
would be of interest and helpful.
According to NGK catalogue NGK's equivalent to Champion L85 / L86/ L86C/ L86CC & L87 is their B6HS not B5HS, Which is a hotter / softer plug .Regards your latest question, reference 14mm & 18mm plugs, Brian is right , 14mm /18mm adaptors were ( are ? ) available so 14 mm plugs can be screwed into 18mm heads, Just be cautious regards over length thread reach into your cylinder head. / Combustion chamber.
B =14mm Thread pitch 1.25mm
6 = heat rating
H = thread pitch 12.7mm
S = Firing end construction ( Std super copper core centre electrode.)
A = 18mm thread pitch 1.50mm
NGK tel number 01442 281000 ( Hemel Hempstead, Hertfordshire. HP2 4SD
Keith.