Spark Plug Identification For Outboard Motors

 

 

   Spark plugs are the heart of having any motor run well (if at all).  Outboards are even more so important if you have a Dead In The Water situation at a critical time.  And understanding their numbering system can be confusing to the uninitiated (even to some of us who thought we knew).

 

   Any old plug may fire and run your motor, but maybe not really well, EXCEPT if you put a longer reach plug in a head that requires a shorter reach.  This will allow the piston to, on its upstroke hit and collapse the gap, creating a dead motor or at best one that does not run well.

 

   Normally spark plugs are relatively cheap, therefore it may be best to purchase and carry spares of the proper version for your application.  It's rather hard to justify the loss of a week's vacation at a remote Canadian lake because your spark plugs fouled, leaving you sitting on the shore wishing, and for only the cost of a few dollars.

    Spark plugs are made in different size threads and different lengths of threads.   For Champion, they have designations, of for instance a J-6J would be a 14mm thread and a 3/8" reach (thread length, NOT including the electrode) and a medium cold heat range.  The newer plugs could be a J-6C, meaning the electrode was copper coated, for prolonged life.   If the number had a prefix R, like RJ-6C, that would indicate it was a resistor plug, designed to eliminate electrical interference from the plug, mainly used on boats that are equipped it a lot of electronics, (sonar, radio etc.)

    A spark plug is said to be "hot" or "cold" by HOW WELL IT DISPERSES HEAT, NOT HOW HOT THE SPARK IS, as generally thought, by keeping more heat IN, OR out by cooling the tip of the spark plug.  A spark plug is said to be "hot" if it can conduct more heat out of the spark plug tip and lower the tip's temperature.  Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug.

   The degree to which a spark plug disperses the heat it receives is called its "heat range".  Spark plugs with a high degree of heat dispersal are called high heat range (cold type) and those with a low degree of heat dispersal are called low heat range (hot type).

   Colder plugs are ideal for small water cooled engines, where the temperature inside the cylinder is low, as compared to other applications like an air cooled motor that produces higher operating temperatures.  The water cooled need to retain much of the heat to keep the internal temperature high, which allows the plug to self clean and prevent fouling.

  

   It is important that to remember that a spark plug only functions completely when its center electrode temperature is between these temperatures of about 500°C and 950°C.

 

   The heat that the electrode section of the spark plug receives due to combustion is dispersed through the path in the illustration below,  Provided by Densco

   This is largely determined by the temperature of the gas inside the combustion chamber and the spark plug design.

 

    Low heat range plugs have long insulator leg sections and the surface area affected by the flame and the gas pocket capacity are large.  Also, since the heat release path from the insulator leg section to the housing is long, heat dispersal is low and the temperature of the center electrode rises easily.

    On the other hand, high heat range plugs have short insulator legs and the surface area affected by the flame and the gas pocket capacity are small.  Also since the heat release path from the insulator leg section to the housing is short, heat dispersal is high and the temperature of the center electrode does not rise easily.

         As mentioned above, the thread length does not include the electrode

LISTED BELOW
ARE GENERALLY USED CHAMPION OUTBOARD MOTOR SPARK PLUGS

Thread Size Reach	Heat Range	Outboard tyoe	Thread Size	Heat Range	Thread Size
14mm 3/8" Reach	HOT COLD	H-10J J-12J J-11J J-8J J-7J J-6J J-4J J-2J	7/16" 14mm 1/2" Reach	HOT COLD	L-9J L-7J L-4J
14mm 3/8" 14mm 1/2" 14mm 1/2" 14mm 3/4"	Surface Gap	J-19V L-20V L-19V N-19V	14mm 5/8" Reach	HOT COLD	D-16J K-15J D-9J

 

 

 

                        To check the recommended spark plug for your OMC outboard, CLICK HERE.

The above illustrations came from Champion Spark Plug's website

 

 

Shown below is the Champion spark plug code system.  This can help you identify what you have, and suitable replacement spark plugs 

 

Shown below is a cross reference to other spark plug brands for heat ranges

                                                                                     CLICK HERE to view of NGK coding 

   Another thing to be aware of is that the non American made plugs "Reach" length will be in millimeters and can be slightly different than the American made versions.  This may become a problem IF the foreign plug is long enough to protrude inward to where the top of the piston will hit the electrode, smashing it closed.

 

 

Most all articles here are Copyrighted from 2013 to 2023 by LeeRoy Wisner

 

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Originated 07-07-2013, Last updated 06-10-2023
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