NGK Spark Plug Codes

Field:  1   2     3   4   5     6
       [B] [CPR] [6] [E] [S] - [11]

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

Field two: Construction (optional)
  C = hex size 5/8"
  K = hex size 5/8 with projected tip (ISO)
  M = compact type
  P = projected insulator type (Note 1)
  R = resistor (Note 2)
  SD = surface discharge for rotary engines
  U = semi-surface discharge
  Z = inductive suppressor

Field three: Heat Range.
  2 = hot,
  3
  4
  5
  6
  7
  8
  9 
  10 = cold

Field four: Thread reach (Note 3)
  E = 19mm
  F = tapered seat
  H = 12.7mm (1.5")
  L = 11.2mm (7/16")

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 (optional)
   8 = .032"
   9 = .036"
  10 = .040"
  11 = .044"
  13 = .050"
  14 = .055"
  15 = .060"
  20 = .080"
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1) The projected insulator simply describes the shape of the plug head. A projected insulator sticks out a little further into the combustion chamber.

2) Resistor plugs are used for two reasons:

• To reduce electrostatic interference
• To provide a sharper "edge" to the voltage spike, which produces a stronger, shorter spark. On high RPM motors, this is important.

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