Technical NGK Plug References

I was searching for plugs for my older Offy heads and came across this tidbit of info that I figured could be shared here. Its how to read the NGK spark plug numbering system. Explains it quite well. Maybe this will help others searching for weird plugs etc or something to help cross reference.


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Here is the table of values for NGK spark plug names.

The breakdown is like this: [CPR] [6] [E] - [11]
Six fields. Some, e.g. the second field, are optional.
Some fields may have multiple letters.

Field one: Thread diameter.
A = 18mm B = 14mm C = 10mm D = 12mm

Field two: Construction.
C = hex size 5/8" K = hex size 5/8 with projected tip (ISO)
M = compact type P = projected insulator type
R = resistor SD = surface discharge for rotary engines
U = semi-surface discharge Z = inductive suppressor

Field three: Heat Range.
2 = hot, up to 10 = cold. There's no 1, I guess.

Field four: Thread reach.
E = 19mm F = tapered seat
H = 12.7mm (1.5") L = 11.2mm (7/16")

If this field is blank, an 18mm diameter plug has 12mm reach,
and a 14mm plug has a 9.5mm (3/8") reach.

Field Five: Firing end construction.
A, B = special design (no details given)
C = special ground electrode
G = racing use
GV = racing use V type
H = half thread
K = 2 ground electrodes for certain Toyotas
L = half heat range
LM = compact lawn mower type
M = 2 ground electrodes for Mazda rotary engine
N = special ground electrode
P = platinum tip (premium)
Q = 4 ground electrodes
R = delta ground electrode for BMW
S = standard 2.6mm centre electrode
T = 3 ground electrodes
V = fine-wire centre electrode, gold palladium
VX = platinum tip (high performance)
W = tungsten electrode
X = booster gap
Y = v-groove centre electrode

Field Six: (after the dash) Wide gap.
8 = .032" 9 = .036" 10 = .040"
11 = .044" there is no 12 13 = .050"
14 = .055" 15 = .060" 20 = .080"

Resistor plugs are used for two reasons --

1. They cut down electrostatic interference.

2. They provide a sharper "edge" to the voltage spike, making for
a stronger, shorter spark. On high RPM motors, this is important.

The projected insulator simply describes the shape of the plug head.
A projected insulator sticks out a little further into the combustion
chamber.

 

Cool info. Thanks!

 

Would not use anything else. Been using them in my motorcycles forever. I put them in everything.