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Spark plugs for my Onan CCKB

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No electronics on a 1975 Onan that would be bothered using non resistor spark plugs and my nearest neighbour is over a mile away. my Simplicity 9020 has always been easy to start even in the coldest of Manitoba winter days.


And my Allis 720 a beast to start even on a good summer day, its now the only tractor left for snow duty. This afternoon wanted to use the loader fro some yard work, it's a nasty experience running a snow blower into something big covered with snow and frozen to the ground.


Its not that cold although we do have a cover of frost in the mornings, well there was no way that Allis was going to start and i'll never resort to starting fluid.


Pulled the spark plugs from the 920 engine, installed them and the Allis fired right up, I'm happy and the old lady is off my case about the yard.


What i noticed on the plugs is the the one from the 9020 is that the electrode protrudes from the end much further from the main plug body than those pulled from Allis.


After reading the article below have decided to go with non resistor spark plugs might not be a good idea fro those of you having neighbours in close proximity.


At the moment the spark jumps the gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs). 


Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level. Most resistor spark plugs use a monolithic resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode.


Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark. It is for this reason that most racing plugs are non-resistor types.


However, in most automotive applications, a resistor plug is required for proper vehicle operation. Use of non-resistor plugs in vehicles that call for a resistor type can result in rough idling, high-rpm misfire, and abnormal combustion.




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What is a Spark Plugs Heat Range?


The term heat range refers to the speed with which a plug can transfer heat from the combustion chamber to the engine head. Whether the plug is to be installed in a boat, lawnmower or race car, it has been found the optimum combustion chamber temperature for gasoline engines is between 500°C–850°C. Within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits that cause fouling.

Heat Disspation Illustration

The spark plug design determines its ability to remove heat from the combustion chamber. The primary method used to do this is by altering the internal length of the core nose. In addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges.

   *When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, keeping the firing tip cooler.

   *A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.

An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as adding a turbo or supercharger, increasing compression, timing changes, use of alternate fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature, necessitating a colder plug. 


A good rule of thumb is, one heat range colder for every 75–100hp added.


In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.




The heat range numbering system used by spark plug manufacturers is not universal. 

For example, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite.

Heat Range Comparison Chart

Some manufacturers numbering systems are opposite the other - for Champion, Autolite and Bosch, the higher the number, the hotter the plug. For NGK, Denso and Pulstar, the higher the number, the colder the plug.

It is not recommended that you make spark plug changes at the same time as another engine modification, such as injection, carburation or timing changes. Performing too many modifications or tune-ups at once will lead to misleading and inaccurate conclusions if any issues occur (an exception would be when the alternate plugs came as part of a single pre-calibrated upgrade kit).

When making spark plug heat range changes, it is better to err on the side of too cold a plug. Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage.



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The Allis had NKG B-6L installed. on shutting down would always give a muffler backfire, now to see if the plugs installed now do the same. If the problem has been cured will post the plug number.


The 9020 plugs have a protruded electrode.


Heat Range Conversion Chart


Heat Range Cross Reference Chart

HOTTER 2   9 18, 19 10
  4 1 14 14, 16 9
  5 1 16 11, 12 8
  6 1 20 9, 10 6, 7
  7 1 22 7, 8 5
  8 2 24 6, 61, 63 4
  9 2 27 4, 59 3
  9.5 2 29 57  
  10 2 31 55 2
  10.5   32 53  
  11   34    
  11.5   35    
COLDER 12   37    


Where is Autolite?

Autolite's numbering system does not have a standardized heat range identifier.

The last digit in an Autolite part number represents the heat range within a plug family.  

For example, take the 254x plug series.  Autolite has 2544, 2545 and 2546 available.  2544 is the coldest plug in that plug family, and 2546 is the hotest. 

However, according to an Autolite reference chart, Autolite 86 has the same heat range as 2244 and Autolite 303 is a hotter plug than 2974.

Therefore, it is impossible to chart an Autolite heat range hierarchy, let alone a standardized comparison to other spark plug brands.

Edited by bcgold
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