Spark plugs, a guide

Spark plugs are a very important part of your engine, they have to ignite the fuel-air mixture at the correct position in the four-stroke cycle, there are some myths and legends associated with Alfa Romeo spark plugs and it is definitely not the case that if you use 'normal' spark plugs in your engine you will blow holes in your pistons, there are huge amounts of clearance between the spark plug and the piston however if you use a plug that is not of the correct heat range you could cause pre-detonation (pinking) and this could cause damage to the piston and to help to avoid this all spark plugs come with a code to tell you something about their properties so on a typical NGK spark plug, say a BKR6EKPA which is the larger plug on twin spark and early JTS engines, the number breaks down as follows

BK is a 14mm plug with a 1.25 thread pitch and the nut is 16mm

R signifies that it is a resistor plug

6 is the heat range, a lower number is a hotter plug

E Thread reach of 19mm

K means it has two ground electrodes

P Platinum centre electrode

A design series designation

Spark plugs are designed with many features in mind so some car manufacturers like Alfa Romeo may use different plugs in different applications and so on a twin spark there is one large plug, the BKPR6KPA and one small plug the PMR7A which is used because of space restrictions in the combustion chamber.

The heat range of a spark plug is very important as if a plug is too cold it will foul and the car won't start and if it is too hot it can cause damage to the spark plug or to the piston as the heat retained in the spark plug is hot enough to detonate the charge without needing a spark. A spark plug in normal operating conditions should run at a temperature of between 450 and 870 degrees centigrade. The temperature of the spark plug is controlled by increasing the amount of the central ceramic core of the spark plug that is in contact with its outer threaded case which in turn transfers the heat to the water jacket of the engine. Interestingly on a Golden Lodge spark plug and certain types of plugs from other manufacturers use the spark itself to clean the ceramic around the electrode, by putting the earth electrodes low down around the threaded case means that the spark has to travel over the surface of the ceramic and this helps to keep the ceramic clean. Modern cars also tend to use long-life spark plugs, the BKR6EKPA will last until 60,000 miles this is achieved by using a Platinum central electrode which does not easily erode. Every time a spark plug fires it takes a very small amount of material with it and this is how a spark plug generally wears out by using platinum the rate of erosion is reduced considerably.

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