Spark Plugs

Pity the poor spark plug. It lives in a hole, gets hit with a high voltage shock thousands of times every minute, and has to endure searing heat from explosions. The spark plug has the tough but important job keeping the power producing combustion inside your engine alive and well, for without spark there is no flame. Because the spark plug has to stand up to high voltage and extreme temperatures it is important they are replaced at regular intervals.


The spark plug has two primary functions. The first and most obvious is to produce a hot spark inside the combustion chamber to light aflame the fuel and air mixture when the piston is at the very top of its power stroke. The resulting explosion pushes the piston back down the cylinder and creates the power your car needs to go. If a spark plug fails entirely you will lose fire in that cylinder altogether and suffer a "dead hole."

The Heat Beater
The second and less obvious role of a spark plug is to remove heat from the combustion chamber by acting as a heat exchanger. A spark plug produces no heat on its own, but rather transfers combustion heat to the metal of the hole it's screwed into, and ultimately to the coolant surrounding that metal. Spark plugs are rated from cold to hot on the speed at which they can transfer heat away from the combustion chamber.


It is important to run a spark plug of the correct heat range, for the right rate of heat transfer is required to keep the spark plug electrodes clean and firing. At the proper temperature, combustion byproducts are burned away and the spark plug cleans itself. Too cold a range and the spark plug may become fouled with carbon or combustion deposits and misfire or cease to create a spark. Too hot a range and the tip of the spark plug can actually melt away and the spark plug will fail.

Mind the Gap
After you have the correct spark plugs for your vehicle, the next thing to check is the gap between the two electrodes at the working end of the plug. While most plugs come pre-gapped for a specific engine or engine series, it's a safe bet to consult your vehicle service manual for the correct gap and use a feeler gauge to check gaps before installation. If the gap is too small the spark may not burn hot enough, too large and it may not be able to jump the gap and make the spark.