How to Match up the Correct NGK Racing Spark Plugs for Your Engine

How to Match up the Correct NGK Racing Spark Plugs for Your Engine

Important – Heat Ranges Explained

First things first, if you’re going to fit racing plugs, you’ll need to know what heat ranges are and how crucial they can be. 

For most stock or mildly modified engines, the stock heat range your vehicle comes with is sufficient to provide optimal performance and combustion. If you have a highly modified engine however with increased compression, forced induction, or other performance enhancements, you may need a spark plug with a different heat range. Using the incorrect heat range can lead to inferior engine performance, knocking, misfires, and potentially even engine damage.

It’s important to consult with a knowledgeable tuner to determine the correct spark plug heat range for your specific engine setup. Using the appropriate spark plug heat range ensures that your engine runs smoothly and efficiently, maximizing performance and longevity.

For our example and to make things easy, we are sticking to the standard plugs heat range, that is the same as factory fitted. Before you decide to alter the heat range, it is imperative to eliminate as many standard mixture running problems as possible:

  • Carburettors – Check the jets aren’t blocked and are the correct size, check the float height, check the air filter is not clogged, check the carbs are balanced and the mixture screws are set correctly.
  • Fuel Injected – Scan the vehicle for any error codes, look out for any fuel mixture formation codes such as mass air flow meter P0101.

Standard Heat Range – How to Find

The first number in the spark plug’s full part number sequence is the heat designation. This number can range from 2 to 11. 

Examples of part numbers with the heat range highlighted in red:

  • CR6HIX
  • CR7EIX
  • CR8HIX
  • DPR7EIX-9
  • PFR6B


You can decode your own spark plug part number by following our guide – How to Decode your NGK Part Number.

Don’t know your plugs part number? Please use the NGK Part Finder.


What Does the Heat Rating Number Mean?

The heat range indicates how much heat can be dispersed from the spark plug. Too hot and the spark plug electrodes will melt, too cold and the spark plug will not get hot enough to burn off carbon which leads to misfires.

Each NGK spark plug is assigned a heat rating number, with lower numbers representing hotter plugs and higher numbers representing colder plugs.

Image Source: NGK

Heat Range – Checking the Existing Plugs Condition

It’s a well-known fact that you can tell a lot by how well an engine has been running from the appearance of a plug.

Carbon Fouling – If your plugs become carbon-fouled, a lower heat range number is required to make the plug run hotter. If a plug doesn’t run hot enough it cannot burn off any carbon deposits and misfiring can occur.

CRUCIAL FACT – Carbon deposits are conductive! If your plugs are fouled, they will not be sparking correctly. Plug fouling creates an alternative electrical path on the white ceramic insulator; resulting in a misfire. Instead of arcing between the centre and ground (bottom) electrodes, the arc could occur anywhere else along the ceramic insulator to the metal shell. 

Overheating – If the plug appears to have overheated or melted, you will need a colder heat range to compensate. (1) 

Image source: NGK

  • How to Find the Correct NGK Racing Spark Plugs

    #1 Download

    Please download the .pdf from the link below. This official guide made by NGK is essential in matching up the correct racing spark plugs. We also highly recommend reading all the information provided inside. 

Example Buyer Scenario

We will use an example buyer for this guide. They own a 2017 KTM RC390 and want to test an NGK racing plug for a track day. This guide will outline the process of properly cross-referencing the appropriate racing plugs.

Step 1

As mentioned above, it is essential to download the NGK Racing Spark Plug Guide (.PDF) and thoroughly read the information provided. You will also need this guide for matching up the correct plugs later on.


Step 2

Using the NGK Part Finder, we obtain the full specification of the standard spark plugs for our engine. Note that ‘Date – 15->’ in the guide means 2015 onwards. Our example bike is a 2017 KTM RC390, so this is correct.

Click on the view link to see the specifications highlighted in yellow:

Motorcycles-Scooters-NGK-Partfinder (1)

The specifications will pop up as an image. We recommend saving this and printing it out.



Step 3

Write down your own plug specifications in the following format:

Part number = thread diameter x thread reach, hex, terminal type

Example: LKAR8AI-9 =  12mm x 26.5mm, 14mm, solid


Step 4 

Scroll down the PDF (that we previously downloaded) and match the diameter, thread reach, hex and terminal type shown in the PDF.

Our plugs specification that we wrote down earlier is LKAR8AI-9 =  12mm x 26.5mm, 14mm, SOLID

IMPORTANT – Unfortunately not all spark plugs can be replaced with a racing plug by merely cross-matching the thread size and length. Other factors need to be addressed such as if the original plugs are JIS or ISO, and if the firing end is the correct type etc. If unsure, contact a tuner. 

Race-Cat-2019 20 UK-version-ver2-pdf-png-406×248-

Our plug has a solid-type terminal, so we know that R2558E- is a match:

terminal type


Step 5

Once you’ve matched the dimensions, write down the racing plugs part number, our example is R2558E-. The blank space with a circle is where you need to add the heat range of your standard spark plugs. This will be the base for your tuning journey. In our case, our heat range is 8. From the guide we can see that  R2558E- is luckily available in heat range 8, so our full part number needed is R2558E-8 (NGK Stock code #95920)

If you know what heat range you need or would like to try a different one, the guide will display which are available for the said part number. For our plug shown above, the available heat ranges are 8, 9 and 10.


Step 6

Use our website search to find the correct racing plug part number, we are an NGK UK direct distributor so you can have confidence the plugs we supply are 100% genuine.


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