Is the quoted BR6/7 ES a typo?
A BR6ES is a resistive spark plug with a non-projecting nose. The correct non-restive plug for that engine is BP6ES, and if you must have resistive plugs (I have a grave disliking of them) then one fits BPR6ES which is the resistive version of the projecting nose.
I'd need to check with my books, but I can decode all the letters on older NGK plug types.
