A

18 mm

B

14 mm

C

10 mm

D

12 mm

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

2

Hot

10

Cold

E

19 mm

F

tapered seat

H

12.7 mm (1.5")

L

11.2 mm (7/16")

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

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

8

.032"

9

.036"

10

.040"

11

.044"

13

.050"

14

.055"

15

.060"

20

.080"