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, but 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. 

 

   For a water cooled outboard motor that is otherwise well tuned, and in good shape BUT still fouls spark plugs, it would be advantageous to go to a COLDER range of spark plug, to burn off the fouling before it starts.   This would especially be so if the motor was used for a lot of trolling.   Air cooled motors which run at a HIGHER temperature require a hotter plug.

 

    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

                                      You will note that NGK's numbering system is reversed from Champion

   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.

 

BELOW WAS COPIED OFF A OMC  9.9/15  MESSAGE BOARD

 

David Stanton   ll-star contributor
The B7HS is for the 1974-1976 models with points, and no electronic ignition, gapped at .030. The 1977 and up models with electronic ignition should have BZ7HS-10. The -10 means it is made to have a wider gap, and should be gapped at .040.

I run NGKs in all six of my 15 HPs and they all run fine. I have worked on an early 9.9 for a neighbor and it wouldn't idle good no matter what I did with the carb, timing, etc. using Champion plugs, even Champion plugs that worked well in another motor. NGK B7HS cured the idle problem. Several experts at the antique outboard club website have also found that NGKs magically fix idle in some of these motors, but nobody can explain why.

Now - to answer your question, hotter plugs will probably not help your problem. Heat range of spark plugs has nothing to do with how hot or how fast the motor runs or performs. It's all about the temperature of the electrode tip in the plug to keep it from fouling. Hot enough to burn off oil, etc., but not so hot that it causes pre-ignition (knock/ping), which is like hitting the pistons with a hammer because the gas ignites early (before the spark) from the hot plug tip and not from the spark.

If your plugs aren't fouling bad, you have nothing to gain by going to a hotter plug.

Note that if plugs are fouling bad, that could also mean a carb problem making it run too rich.  Is your motor new enough to have the plastic-topped carb?  And is that top cracked?

Also - be sure the mechanical spark advance with the throttle is properly sync'd to the linkage that moves the carb throttle plate - very important. Another thing to check - is your motor running on one cylinder up to 15 MPH, and then the other kicks in?  You could have problems with ignition on one cylinder, or bad reeds, or crankcase pressure leak on one cylinder's half of the crankcase.

David Stanton  All-star contributor
Forgot to add that just about all 2-cycle outboards have a speed a bit above high idle where they run rough. That is normal because you are at a throttle point where the carb is transitioning from the low speed needle to the fixed high speed jet, and it's in-between and confused until you put the throttle high enough so most of the fuel is coming from the high speed jet. This throttle band is usually narrow, but you can easily take any of these motors and find that rough running throttle setting.

CHAMPION  (807M)   J-6JC  (Copper) =  (4122)  BR7HS    (Resister)
                                   J-4JC             =   BR8HS           (colder plug)
NGK    B7HS    (5110)  (non-coated)  - -  BR7HS  (7067)    (Resister - Iridium coated)
           BR7HS  (4122)  (Resister)    - -    BR7HIX  (7067)   ( Iridium  coated)

 

Most all articles here are Copyrighted from 2013 to 2025 by LeeRoy Wisner

 

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Originated 07-07-2013, Last updated 04-28-2025
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