NGK Sparkplug Codes

Following is a brief list of various codes found in the NGK spark plugs. This is actually a copy of a document that used to be on the Motorcycle Tuning Resources page. Those pages are apparently down so I decided to post the codes here on the TL pages. As I have gotten a surprising number of additions and requests to update these pages via e-mail I have now collected all additions and posted them here.

It is important to note that NGK racing plugs follow a completely different numbering system, if the plug code STARTS with R the table below is not applicable.

Basically the code for

Example NGK codes: [B] [P] [6] [E] [S] [D] [8] [E] [S] [-] [L]

Field one: Thread Diameter and socket size

Code	Thread Diameter	Socket Size
A	18 mm	1" (25.4 mm)
B	14 mm	13/16" (20.6 mm)
C	10 mm	16 mm
D	12 mm	18 mm
E	8 mm	13 mm
G	PF 1/2"	15/16" (23.8 mm)
AB	18 mm	13/16" (20.6 mm)
BC	14 mm	16 mm
DC	12 mm	16 mm
	mm	mm
	mm	mm
	mm	mm
	mm	mm
R	Racing plug, this table is not applicable!

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	Hottest
10	Coldest

Field four: Thread reach
E	19 mm
F	tapered seat
H	12.7 mm (1.5")
L	11.2 mm (7/16")
	If this is blank, an 18 mm diameter plug has 12 mm reach; and a 14 mm plug has 9.5 mm (3/8") reach.

Field five: Firing and construction
A,B	Special design (no details given)
C	special ground electrode
G	Racing use
GV	Racing V-type
H	Half thread
K	2 ground electrodes
L	Half heat range
LM	Compact lawn mower type
M	2 ground electrode for Mazda rotary engine
N	Special ground electrode
P	Platinum tip (premium)
Q	4 ground electrodes
R	Delta ground electrode for BMW
S	Standard 2.6 mm electrode
T	3 ground electrodes
V	Fine-wire electrode, gold-palladium
VX	Platinum tip (high performance)
W	Tungsten electrode
X	Booster gap
Y	V-groove center electrode

Field six(after the dash): Wide gap
8	.032"
9	.036"
10	.040"
11	.044"
13	.050"
14	.055"
15	.060"
20	.080"
L	Half heat range
N	Special ground electrode (note: appears to be limited to BMWs)

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Last modification February 16, 2003

NGK Sparkplug Codes