The Garage - Spark Plugs
Reading Spark Plugs!

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Last Updated: 12/3/2013
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With a new or unknown Spark Plug / Engine combination, play it safe!
Always start at least 1-2 heat ranges on the cold size of the mean heat-range for the series of plug you are running. At worst, you may experience some plug fouling. On the other hand, a plug that’s too hot can cause detonation and damage the engine.

Determining the optimum heat range is a trial-and-error process. You run the car, then “read” the plugs by closely inspecting and analyzing the condition of the plug tip and insulator. Once you find the correct heat range that prevents fouling without contributing to pre-ignition or detonation, changing to a hotter or colder plug won’t alter engine performance.

Set up the engine for optimum air/fuel ratio and timing first, then fine-tune plug heat range. Reading plugs on the street is not the same as in racing. On the street, as mileage piled up, a properly burning plug traditionally had a clearly visible brown or grayish-tan color.

The below was from a free hand out issued by NGK Spark Plugs around 1977


Oil Fouled
Oil Fouled
Carbon Fouled
Too Cold
Too Cold
Cold or Rich - OK
Cold or Rich - OK
Cold or Rich - OK
Good
Good
Good
Good
Real Good
The Best
Best
Best
Best
Good
Good
Good
Kinda Hot - OK
Hot or Lean - OK
Hot or Lean - OK
Hot or Lean - OK
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range


NOTE: Today’s pump gas may use additives that cause a discoloration of the plug core nose; they could be pink, purple, or blue.
Do not consider this coloration as an indication of heat range when reading spark plugs.




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