NGK A6.

A =18mm 6 = middle heat range.

Maccess had plenty, mainly because Rory kept buying boxes for the Morris-Commercials and the computer kept re-ordering. . . . . . . . . They've sussed it now unfortunately.

I had the pleasure of meeting the NGK rep in there one day and asked why so many old plugs were now replaced with a single type.

Better materials - generally if you mix air and petrol at 14:1 it goes bang with the same properties and heat no matter if its in your lawn mower or Ferrari. All the complex numbers just define the particular mechanics of suppression, thread reach, seating and hex etc; as a glance at the nomenclature chart shows.
http://www.spark-plugs.co.uk/pages/t...ymbol_code.htm

A6's (7's would do) have done thousands of miles in the M-C's and a few hours in my pre-war Allen Scythe too, Daves F15 has had BM6ES from the word go; no B6ES on the shelf so used lawn mower plugs. . . . .M = compact insulator.

R.