Sparking Plugs - curing heat range problems

The humble sparking plug is one of those essential parts of our Austin Sevens that is rarely regarded with the importance it deserves.  The more studious of us throw a new set in every season or two, but how many of us get to know these four little firebombs properly?

There are just three criteria to consider when you saunter down to your local auto factors.

1.  Thread size. Obvious really, as Sevens use a mix of 18mm and 14mm threads, and a hybrid plug that has the body of an 18mm but the tread form of a 14mm so that they look “correct” under their brass terminals.  It is hard to get this wrong, as they simply will not fit. 

2.  Reach, the technical word for length.  Obviously the plugs must be long enough to get down into the cylinder, so that they can access the pockets of fuel-air mixture.  Too long is obviously a problem, as the electrode will come in to contact with the top of the piston, or get dangerously near to it, causing a local hot spot that is detrimental to the crown of the piston itself.  Too short is also an issue, as the flame front will not start at the right place and the engine will lack power.  It has much the same effect as being retarded, as the flame occurs much too late and the piston is on its way down, running away from the power, or “flame front”

3.  Heat range. For any given type of plug, they can be available in different heat ranges, often just referred to as Hot plugs or Cold plugs.  

For a plug to work properly, the tip must be between two vital temperatures.  If the tip is below about 500 deg C, it will not be hot enough to burn off the carbon deposits, and the plug will soot up.  Electricity will leak across the insulator, and the plug will stop sparking.  Above 950 deg C, the tip will be so hot that the fuel air will ignite without a spark.  This gives pre-ignition and power will be lost.  

Plug Heat Range 1

1: High heat range = COOL          2: Medium            3: Low heat range = HOT  

Perversely, a high heat range plug (1) is designed to work in a high temperature (highly tuned) engine, and conducts heat away faster resulting in a COOLER tip.  It does this by having more of the insulator in contact with the body, giving a short path for the heat to get away to the cylinder head.  Conversely, a low heat range plug (3) is for engines that are in a low state of tune, and therefore are designed NOT to conduct heat away, (by having a long path from the tip to the body) and consequently the tip is HOTTER.

Have a look at   which is an excellent website by the DENSO Corporation covering:  Spark and Ignition,  Spark Plug Construction,  Heat Range, How to Choose a Plug,  Service Life,  Spark Plug Installation,  Inspecting and Exchanging Plug / Service Life and Troubleshooting.

So, in order to stop our Sevens from sooting up, especially when driving about the country lanes at moderate speed,  it is necessary to have an adequately LOW heat range plug.  If the tip temperature falls below 500C, then all that carbon begins to build up leading to misfires and power loss as the spark weakens.  That’s why our members who blast down the A30 at full power to get to runs tend to have fewer plug fouling problems than the more sedate or cautious drivers.  


This article, written by Geoff Hardman,  originally appeared in CA7C Seven Focus in April 2015 pp16-18.


See also:

Sparking Plugs