That is obviously a mistake if it actually said 14mm. socket rather than 14mm. plug.
Spark plug socket size }:-/
- Thread starter ^woody^
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It actually says "14mm socket" on the card and on the barcode label. Just by looking at it I could tell that it was too big, so I said "Nah mate, it's smaller than that" so he then gave me one labelled as 12mm and said it must be this as they only have the two sizes, and I could see that that one was bigger than 12mm. Then it all kicked off. 
Then it all kicked off.
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As it says :-
https://www.gsparkplug.com/identifying-spark-plugs
A bit deeper :-
https://www.sat.dundee.ac.uk/psc/spanner_jaw.html#Jaw_Size_Table
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https://www.gsparkplug.com/identifying-spark-plugs
A bit deeper :-
https://www.sat.dundee.ac.uk/psc/spanner_jaw.html#Jaw_Size_Table
-0-
What we all know, and knew, as 14mm spark plugs, for example, Champion N5 or N9Y and AC 42XLS, all had hexagons which were 13/16" AF (across the flats). The 14mm referred to the size of the thread. With the wider use of aluminium for cylinder heads and the use of twin overhead camshafts, the amount of metal which had to be removed to accommodate a 13/16" plug socket was too great, so the hexagon was reduced in size to 5/8" AF. This now appears to have become the "standard" for 14mm plugs. To those for whom the inch is an unknown quantity, 13/16" is a fag-paper under 21mm, and 5/8 is just under 16mm.
