I don’t know who can amend or add to the particular thread…but in the Engine & Transmission FAQ section for NA/NB’s, there is a sub heading of “Spark Plug Diameter” .
Somebody asked if 10mm or 14mm, the reply states that the ‘B’ means 14mm. But I’ve just done mine on my '96 Mk1 1.6 and they are a 10mm but the plugs are BKR6E-11. Meaning the ‘B’ can’t mean 14mm surely…? Maybe different sizes between the 1.6 and 1.8?
Can the thread be amended to show the differences at all for future reference to others?
The way I understand it is there are three main diameters of thread used on spark plugs, 10mm, 12mm and 14mm.
They used to have roughly standardised sizes of socket that was used on each;
10mm thread used a 16mm (or 5/8") socket
12mm thread used a 18mm socket
14mm thread used a 21mm (or 13/16") socket.
I think as engines got smaller and more compact things got a muddled up and more complex.
There is one other thread size I’ve seen on spark plugs, 18mm. It was used on vintage cars and a Ford engine in the 1970’s or 1980’s (think it was one of the Pinto engines in Cortina’s or Capri’s.).
Originally when spark plug threads were standardised as Bannedbiker says a 10mm threaded spark plug was 16mm across the flats of the hexagon and 14mm threaded plug 21mm across the flats.
However modern plugs have improved seat and washer design and so a 14mm threaded plug now is 16mm across the flats instead of 21mm previously. Hence you now use a 10mm plug socket for a 14mm threaded spark plug.
In general most cars use 14mm threaded spark plugs it is just that in older cars these have 21mm a/f hexagons and modern ones 16mm a/f.
rhino666 - from a quick google some manufacturers, Draper and Laser, still refer to the 16mm a/f spark plug socket as a 10mm plug socket whereas others like Sealey refer to it as a 16mm plug socket.
The only way to be certain is to use the NGK guide, see link below, look at page 5 and interpret the plug code to find the socket a/f size required. Looking at this guide there are more plug a/f sizes than I first thought, just to add even more confusion!