Spark Plug

A model engine spark plug can vary in size, but 1/4” x 32 is common as it fits the same thread and rough dimensions of a model engine glow plug.

1/4 inch spark plug

I took this photo to give you an idea for just how small this 1/4” x 32 sparkplug is.

A spark plug is made from a number of parts:

  • Main threaded body
    • allows the plug to be fixed into the cylinder head
    • acts as the main earth path
    • supports the insulator
  • Insulator
    • made from a ceramic
    • provides electrical insulation between centre electrode and main body that is earthed
    • helps conduct heat away from the plug towards the cylinder head
  • Middle electrode
    • conducts electrical energy from connector at the top to the electrode at the end
    • spark jumps between end and electrode that is connected to the main body

Voltage at the spark plug can range from 40,000 to 100,000 volts.

Fouling – A deposit on the electrodes of a spark-plug of essentially non-conducting material that may, but will not necessarily, prevent the plug from operating. These deposits normally get burnt off when the plug reaches 400°C to 450°C.


The distance between the tips of the electrodes.

The rule of thumb for the spark plug gap on older engines is coil ignition 0.025 thou and magneto ignition 0.018 – 0.020 thou. (thou => thousandths of an inch)

Too small a gap will mean that the spark duration will be very quick and the spark will be thin and weak. This may lead to:

  • Too small to ignite fuel
  • Bad starting
  • High exhaust emission levels.
  • Increase in fuel consumption.

Benefit of a small gap is the plug always fires on each cycle.

Too large a gap will mean that the ignition system may not be able to generate a spark. This may lead to:

  • Bad starting
  • Miss-firing at high speeds

The benefit of wide-gap is the spark is strong for a clean burn.

Gap Growth – The spark plug gap will gradually increase with wear. The normal gap growth in a 4-stroke engine is 0.01 to 0.02mm/1000km. For 2-stroke engines the gap growth is around 0.02 to 0.04mm/1000km.

Heat Range

At the optimum firing region the tip of the spark plug where the spark is formed will run between 450°C and 800°C. Below 450°C the plug is likely to be in the fouling zone. Above 800°C and the plug may cause pre-ignition.

Plugs are made to operate at different temperatures depending upon the thickness and length of the porcelain insulator as measured from the sealing ring down to the tip.

The heat rating of a spark plug is related to the amount of heat it can dissipate. A hot plug will have a longer insulator nose at the spark end and so will conduct less heat away to the head and so run hotter. A cold plug will have a shorter insulator nose and so a shorter path to the main body and the cylinder head, resulting in more heat being conducted away and the plug running colder.

  • If a plug is too cold, it will foul.
  • If a plug is too hot, it will cause preignition.

Heat Range Reserve – The distance to the start of pre-ignition under further increasing thermal loading of the spark plug. This reserve is expressed in degrees crankshaft, the amount by which the factory-set ignition timing can be further advanced without preignition occurring.

Spark Breakaway – The end of spark duration.

Surface Gap Spark Plug – A type of spark plug in which the spark is fired across the insulator surface between the centre electrode and the shell. There is no side electrode.

Whiskering – A deposit of conductive material that tends to form a bridge between the spark-plug electrodes or to a ground, thus shorting out the plug.

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