1. Heat range.
A heat range refers to how much heat a spark plug is capable of
removing from the combustion chamber. Selecting a spark plug with
the proper heat range will ensure that the tip will maintain a
temperature high enough to prevent fouling, yet be cool enough to
prevent pre-ignition. If you modify a motor a lot it is best to start with a
colder plug. If it ain't fouling its OK. On motocross bikes its unlikely
you would modify a motor enough to need the plug heat range

2. Resistor?
Always use a plug with a resistor. I will not affect power and will give
the general public one less reason to want to ban bikes.  Spark plugs
use resistors to suppress the emitted electrical "noise" from ignition
systems. We must do this to comply with government regulations on
the amount of electrical "noise" allowed by ignition systems. Radio
frequency interference (RFI) can also be suppressed by using resistor
(or inductor) wires or plug caps. We use these things to avoid
interference with sensor signals and with your listening radio. Sensor
interference can occur if a sensor, or its wire is un-insulated and is
routed close to the plug or the plug wires.  The resistor get rid of the
initial voltage spike and its harmonics that cause the radio frequencies
to be produced. Since we rely on the trailing edge of this voltage
spike for ignition, a resistor has a small effect on your power.

3. Size of the electrode.
This is what all the number at the end of a plug are for. Basically if
you need to use a narrower electrode then much more expensive
metals (well they justify the cost using this reason) must be used as
normal material would erode in no time.  A narrower electrode is not
used to stop fouling and possibly could even foul easier as it only
needs one little blob to foul it.

Basically the smaller the electrode (has less Capacitance for those
into electrical stuff) then less voltage it needs to fire. The bigger the
electrode the more current it can flow. (good in cars with heaps of
power available, huge sparks)  So in most bike bikes when the
ignition power (current) is limited the ignition are generally set up for a
small diameter electrode.

If dollars are short the standard "S" plugs will not do you engine any
harm. It does however place extra stress on you electrical system as it
will be required to produce a higher voltage than normal. I have also
seen dyno chart with very small increase in horsepower with the "G",
Whether in "real life" you can tell any difference is unlikely. Like all
thing a New "S" is much better than a rat shit "G" .

Below is a table of how to decode a NGK plug.

The "CM" is not listed but it would be a "EG" electrode. So they are
basically a short "EG" plug because room is tight in a KTM 250.   The
shorted body will not cause any hassles.  If thread length is the same
(very very important) and you can fit in an EG then that's ok as long
as the plug cap isn't hitting anything.

Here the table of values for NGK spark plug names.
The breakdown is like this: [B] [CPR] [6] [E] [S] - [11]
Six fields.  Some, e.g. the second field, are optional.  Some fields may
have multiple letters.

Start with the code written on the plug. It will have something like
BR8EQ, B8EG or B10EGV.  This code can be broken down into
seperate codes that tell us exactly what the plug is designed for, like
B R 8 E Q
Field One:  Thread Diameter
A = 18mm
B = 14mm
C = 10mm
D = 12mm

Field Two:  ConstructionC = 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 supressor

Field Three:  Heat Range

Field Four:  Thread Reach
E = 19mm
F = tapered seat
H = 12.7mm (1.5")
L = 11.2mm (7/16")
* NOTE *
If this field is blank, an 18mm plug has 12mm reach,
and a 14mm plug has a 9.5mm (3/8") reach.
Field FIve
A, B = special design (Details unknown)
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 engines
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 = tungston electrode
X = booster gap
Y = v-groove centre electrode

Field Six:  Wide Gap
8 = .032"
9 = 0.36"
10 = .040"
11 = .044"
13 = .050"
14 = .055"
15 = .060"
20 = .080"
Everything you need to know about spark plugs!
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