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What Are Spark Plugs?

A row of installed spark plugs.

Cars have changed a lot in recent years, but gas ignition hasn’t, thanks to the spark plug.

But what are spark plugs? In simple terms, a spark plug creates a spark within the engine’s cylinder head, burning fuel to start the car. Your gasoline-powered vehicle cannot run without them. But there is much more to spark plugs than meets the eye. Read on to learn more about their history and construction as well as replacement considerations.

A History of Spark Plugs

Albert Champion and Robert Bosch have received credit for inventing the first spark plugs in 1899 and 1901, respectively. But decades before them, Jean J. Lenoir had patented what some called a “sparking plug” just two years after he’d built the first commercially successful internal combustion engine in 1858. However, Champion and Bosch offered considerable refinement to Lenoir’s design, and both names are ascribed to modern plugs today.

Spark Plug Construction

A new spark plug ready for installation.What are spark plugs? They are small, simple-looking parts composed primarily of a shell, an insulator and a central conductor. Plugs are specified by size — specifically by thread or nut — sealing type and spark gap.

Most spark plugs in modern cars have 14-mm threads; earlier vehicles had used 18-mm or 7/8-inch (approximately 22-mm) ones. Sizes of 10 mm and 12 mm are common in lawnmowers and certain imported motorcycles.

Thread length varies as well, depending on the manufacturer and engine type. The sealing type or seat is where the plug mates to the head. It can be flat or tapered depending on the manufacturer and engine type. The amount of torque applied ensures a secure fit commensurate with the design.

The spark gap dimension varies according to the age of the vehicle and its energy output (your owner’s manual spells out the specific plug needed for your engine). Typically, there is one plug for each cylinder, but there are exceptions. For example, the V8 Hemi engines used in various Chrysler, Dodge, Ram and Jeep products have 16 plugs, two per cylinder.

Going Bad

Today’s spark plugs work for much longer than those of previous years, with some rated to last up to 100,000 miles. The days no longer exist when cars require an annual tuneup to replace spark plugs and adjust points and timing.

But spark plugs do still wear out, and if you’re finding it difficult to start your vehicle, it could be that your plugs are bad. Other troubling signs include a rough idle, engine misfire or surge, weak acceleration or a spike in fuel consumption. The “check engine” light may come on and offer a telling code as well.

Visual clues may also reveal a bad spark plug. If you remove the plug from the cylinder and notice that the end has burned or sandy-looking deposits have accumulated on the outer electrode, then it’s time to replace it. In a worst-case scenario, a wet accumulation of oil on the electrode may indicate not only that the spark plug is bad but also that there’s significant engine trouble.

Time for a Change

Reference the maintenance schedule in your owner’s manual to follow the correct change interval for spark plugs.

Check out all the spark plug products available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on spark plugs, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Matthew C. Keegan View All

Matt Keegan has maintained his love for cars ever since his father taught him kicking tires can be one way to uncover a problem with a vehicle’s suspension system. He since moved on to learn a few things about coefficient of drag, G-forces, toe-heel shifting, and how to work the crazy infotainment system in some random weekly driver. Matt is a member of the Washington Automotive Press Association and is a contributor to various print and online media sources.

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