R   Resistor 3k ohm/ 10k ohm

K   Resistor 1k-2k ohm

T   Resistor 7k-15k ohm

Q   Low resistance, inductive suppressor

X   Combined resistive/inductive suppression

U   Internal auxiliary air gap (booster gap)

If the very first character listed in the plug type number is not one of the characters listed above, the plug is a non-resistor type (doesn't contain a resistor)

THREAD x REACH x SEAT x HEX

A   12mm x 1.25mm, .750", gasket/joint, 5/8" or 18mm

C   14mm x 1.25mm, .750", gasket/joint, 5/8"

CJ   14mm x 1.25mm, .375", gasket/joint, 3/4"

DJ   14mm x 1.25mm, .307", taper, 5/8"

E   14mm x 1.25mm, .984", taper, 5/8"

EA 12mm x 1.25mm, 1.043", gasket/joint, 14mm

EC 14mm x 1.25mm, 1.043", gasket/joint, 5/8"

ER 12mm x 1.25mm, 1.043", gasket/joint, 5/8"

ES 14mm x 1.25mm, .984", taper, 5/8"

ET 12mm x 1.25mm, 1.043", taper, 14mm

F   18mm x 1.5mm, .429", taper, 13/16"

G   10mm x 1.0mm, .750", gasket/joint, 5/8"

J   14mm x 1.25mm, .375", gasket/joint, 13/16"

L   14mm x 1.25mm, .500", gasket/joint, 13/16"

N   14mm x 1.25mm, .750", gasket/joint, 13/16"

P   12mm x 1.25mm, .500", gasket/joint, 11/16"

S   14mm x 1.25mm, .689", taper, 5/8"

V   14mm x 1.25mm, .441", taper, 5/8"

Y   10mm x 1.0mm, .375", gasket/joint, 5/8"

Z   10mm x 1.0mm, .500", gasket/joint, 5/8"

The tabulated values generally hold true for the more common plug types. There may be minor variations from the tabulated values for the less common plug types.

1 to 20  Automotive & small engine

20 to 49  Historical aviation

50 to 65   High performance racing plugs

65 to 100  Industrial & small engine

Please note, within a given plug family the plugs run hotter as the number gets higher. Generally, the Racing and Industrial plug families run colder than the automotive plug family even though they have higher numbers.

CENTER ELECTRODE MATERIAL

W  Iridium

P  Platinum

G  Gold palladium

CORE NOSE PROJECTION

RETRACTED  No Projection

H  .020"-.030"

Y  .060"-.090"

M  .100"-.140"

L  .180"-.220"

E  .240"-.300"

GROUND ELECGTRODE MATERIAL

C  Copper

CC  Double copper

P  Platinum

G Gold palladium

2  .020"

3  .030"

4  .040"

5  .050"

Z  Skirted plug

LM Lawn mower plug

V  Surface gap

B  Fine wire rivet design

On a skirted shell, the threads on the metal shell stop before they reach the bottom. Non-skirted plugs have threads all the way to the bottom of the shell.

*Some plugs have special design features. If the first character after the heat range (number) is not one of the center electrode references, check the Special Features matrix.