image of spark blocks on engine block. car repair concept

These little parts pack a big punch in your vehicle. For gasoline-fueled cars and trucks, spark plugs provide the catalyst for igniting the air and fuel mixture in the combustion chamber.

Anatomy of a Spark Plug

Spark plugs cost about $10 each, but they contain components that provide invaluable contributions to the function of your vehicle.

Copper Electrode

The spark plug contains a copper electrode in the center.

Nickel Alloy or Precious Metals

The electrode is surrounded by nickel alloy. More expensive spark plugs are composed of precious metals, such as platinum or iridium, which surround the copper electrode instead of nickel. It’s believed that premium spark plugs can improve gas mileage but, in reality, any new set of plugs can generate the same results.


The center electrode is surrounded by porcelain, which is a conductor of heat. When performing spark-plug-related auto repair, be mindful to avoid hitting the porcelain with your tools. 


The connector/terminal nut is the top of the spark plug. Below the terminal are the ribs of the plug. The ribs may be made of ceramic, and are delicate; you’ll want to avoid touching the ribs with any of your tools to prevent damage.

Head Hex

Below the terminal is the hex head is where your socket wrench makes contact with the plug when you install or remove it from your engine.


At the base of the spark plug ribs is the gasket, also known as a crush washer or a seat. 


The threads are the bottom of the spark plug, and where it screws into place in your engine.

Ground Electrode

At the end of the threads is the ground electrode, which is the metal arm of the spark plug that may or may not need adjusting. It can be made of stainless steel or titanium.

Gapping Spark Plugs

The ground electrode is where the electronic arc occurs. Pre-gapped plugs are common, but they aren’t always exactly the right size for your engine. You can measure the gap on your plugs using a simple tool. The spark plug gapper is a metal disc that has a beveled edge. The edge ramps to different thicknesses around the edge so you can measure the gap of your spark plug. Labeled like a ruler with .020 to .100 inch measurements, as the beveled edge changes in thickness, you can slide your plug around to determine how it measures.

When measuring, you’ll gently slide the arm of the spark plug (on the threaded end) around the edge of the gapper until you feel resistance. Even pre-gapped plugs should be checked to ensure they’re accurate for your engine’s needs. If you need to adjust the gap of your spark plug, you can use the gapper tool to manipulate the size of the gap by gently pressing the arm of the plug against the gapper to make it smaller. If you need to widen the gap, you can hook the arm of your plug on an inner ring in the gapper tool to give leverage for pulling it open.

Other tools for measuring spark plug gaps:

  • OEMOOLS Blade Master
  • CTA Tools Spark Plug Gap Gauge
  • Performance Tool Spark Plug Gap Gauge
  • A & E Hand Tools E-Z Grip Spark Plug Gap Gauge
  • Performance Tool Wide Gap Wire Spark Plug and Gauge
  • Stanley Proto Gauges Spark Plug Gap Set

These tools can range in price from $3 to $11, so it’s an inexpensive way to ensure your spark plugs perform well.

Types of Spark Plugs

Cold spark plugs have less insulation, allowing more heat to be transferred from the combustion chamber to the rest of the engine. Cold plugs are used in high horsepower engines. In contrast, hot spark plugs are better insulated and are more common in standard engines. 

Diesel engines use glow plugs because they have a different ignition system than gasoline-powered engines. Glow plugs have a heating coil in their centers instead of a center electrode.

The number of cylinders in an engine determines the number of spark plugs. For example, four-cylinder engines have four spark plugs, while six-cylinder engines have six spark plugs., and so on.

What Does a Spark Plug Do?

Spark plugs work in conjunction with the ignition coil to create an arc of electricity. 

The job of a spark plug is to ignite the air/fuel mixture that’s drawn into the engine’s combustion chamber. Spark plugs also transfer heat away from the combustion chamber to the car’s cooling system. This helps prevent the engine from overheating.

Engine pistons move up and down in their cylinders. When they move down, they draw in the air/fuel mixture. When the pistons move up, the mixture is compressed toward the spark plugs at the top. 

Replacing Spark Plugs

The recommended schedule for replacing spark plugs is every 30,000 miles. However, if you notice your gas mileage is changing for the worse, it could be time to swap out the old plugs for new ones. If you’re not sure, you can always remove one plug at a time and check for signs of wear.

Tools for Changing Spark Plugs

  • Spark Plug Socket
  • Gap Gauge

Each spark plug has a wire attached to it. Auto repair experts recommend that when you change them out, focus on just one plug at a time. You want to keep track of the wiring because it determines the order in which the plugs fire, which is important.

Follow one individual wire to a spark plug and unscrew it from the cylinder. Replace it with a new plug and tighten it gently to avoid misaligning the threads. Once the plug is tightened, reconnect your spark plug wire before moving on to the next one.

If you have questions about spark plugs, give the experts at Master Muffler a call.


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