I'm surprised this issue is not more common but for those who are sometimes confused about spark plug numbers here's a guide I did many years ago.
I'll start with the big 2 champion and NGK because they are available everywhere.
Champion, eg RN4V
R = Prefix number for plug type (either a resistor or auxilary air gap)
This sample is obviously a resistor
N = Thread diameter and reach, as you already know this would be 14mm plug with a 3/4 inch reach.
4 = heat range, The higher the number the hotter the plug, this would be a cold plug
V = Sufix, special feature, this one is a surface gap type plug
NGK, eg BR7ES
B = Thread size, again no surprises there, this would be a 14mm plug
R = plug type, this would be a resistor type plug
7 = heat range, on this range the lower number indicates the hotter plug, the range is 2-10
E = Thread reach, if you haven't guessed theis one would be 3/4 inch
S = Sufix for special design, the S indicates a standard copper core center electrode
Of the 2 plugs I would use NGK, why, simple, the champion range of plugs leave the electrode over exposed and the NGK strap is much thicker and shorter, in my oen expierience the strap eroded very quickly on my race engine whiel the NGK held up nicely, other than that the Champion are every bit as good as the NGK, but I thought I'd add why I like the NGK's
AC eg, S124F
S = designed usage, this one is from my snowmobile
12 = thread size, this would be an easy 12mm.
4 = heat range, the lower the number the colder the range, range is 0-9
F = suffix, special feature, this one is special reach for foreign applications
Bosch eg,W3CI
W = Thread size, this one is alsoa 14mm plug
3 = Heat range, the lower the number the colder the type. The range is 0-12
C = Thread reach, this one would be 3/4 inch
I = Special design, not sure whats so special about I but it's special.
Hopefully this would shed some light on someone who is sitting there trying to decide what number plug means what, if not, sorry for taking up your time.
Spark plug referance
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