There are three basic things that describe a plug.  ( well there lots
but I'll cover three)

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 heatrange changed.

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 horse power with the "G", Whether in
"reallife" 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 a EG then thats 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 this.

B R 8 E Q

Field One:  Thread Diameter
    A = 18mm
    B = 14mm
    C = 10mm
    D = 12mm

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 supressor

Field Three:  Heat Range
    2 = HOTTEST
    11 = COLDEST

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:  Firing End Construction
    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"


Mark Van Echteld