Tech. Tip:How to"read" a spark plug?

Reading spark plugs can be a valuable tuning aid. By examining the Insulator firing nose color, an experienced engine tuner can determine much about the engine's overall operating condition.

In general, a light tan/gray color tells you that the spark plug is at the optimum temperature and that the engine is in good operating condition.

Normal Condition

 

Dark coloring, such as heavy black wet or dry deposits usually indicate a fouling problem. Heavy, dry deposits can indicate an overly rich condition, too cold a heat range spark plug, possible vacuum leak, low compression, overly retarded timing, or too large a plug gap.

Dry/Wet Fouling

Common causes for this problem are:



If the deposits are wet, it can be an indication of a breached head gasket, oil control from rings or valvetrain problems, or an extremely rich condition, depending on what liquid is present at the firing tip.

One must also look for signs of detonation, such as silver specs, black specs, or melting or breakage at the firing tip.

Overheating

 

Common causes for this are:

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and to prevent possible engine damage.

Here's a chart on how to read NGK sparkplug codes..

 

NGK SparkPlug Codes:

NGK code:  [B]  [P]  [6]  [E]  [S]

Field one:  Thread Diameter

A

18 mm

B

14 mm

C

10 mm

D

12 mm

 

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

10

Cold

 

Field four:  Thread reach

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.

 

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:  Wide gap

8

.032"

9

.036"

10

.040"

11

.044"

13

.050"

14

.055"

15

.060"

20

.080"