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Mind the Gap: How to Replace Spark Plugs

Dirty Spark Plugs

While every part of your vehicle’s engine is important, spark plugs are often underestimated. They are critical components that can significantly impact performance and efficiency. Critical functions, such as power output, fuel economy and even engine temperature are affected by the condition of your plugs, so knowing how to replace spark plugs is essential to any DIY car owner.

Know Your PlugsSpark plug by Razor512 on Flickr

Starting off, it’s good to know what makes one plug different from the next. The central electrode is normally made up of copper, nickel or chromium, but it can also be made of rare metals such as platinum or iridium, which are more durable and produce a more powerful spark. You should check your vehicle’s manufacturer specs, found in your owner’s manual, before upgrading to rare-metal plugs.

Removing Old Plugs

First, remove your old plugs: Pull off the spark plug wire by grasping the wire’s boot — never pull by the wire itself. If your vehicle uses coil-on-plug wires, be extra careful not to damage the coil at the top of the plug wire as well. Next, use a brush and some compressed air to remove any debris that could fall into the engine after you remove the plug. Using a plug socket and extension on your ratchet, loosen the plug. Once it is loose, disconnect the ratchet and finish the job by hand. Stuff the plug hole with a clean shop rag to keep debris out.


Before you throw that old plug away, inspect it for signs of a deeper problem. A spark plug can tell you a lot about how the engine is running: Black soot on a plug could mean that the engine’s fuel mixture is too rich, while black moisture is a telltale sign of an oil leak. Blisters or burnt spots on the insulator and electrode are indicators that a fuel mixture is too lean, the engine is overheating, the gap was too wide or that you have a timing issue.

Set The Gap

Every engine has a unique specification for their spark plugs’ gaps. The gap is the space between the electrodes that the spark jumps across to ignite the gas inside the cylinder. Before installing your new plugs, check your manual for the correct gap and set it using a feeler gauge. Use the gauge’s hook to open the gap slightly if necessary; if it’s too wide already, gently press the electrode down on a non-marring surface until it meets specifications.

Installing New Plugs

Before inserting the new spark plugs, apply a  small amount of anti-seize to the first two threads, making sure not to get any on the electrodes. Next, thread it by hand with the socket and extension; be careful not to cross-thread the plug. Once its hand-tightened, use your ratchet to finish the job. Tighten the plugs to the manufacturer’s recommended torque specs, reinstall your plug wires in the correct order, and you’re all set.

Knowing how to replace spark plugs is simple and can save you money when doing a tune-up. Fresh plugs help your engine start and run more smoothly, and can even improve your fuel economy.

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

Image courtesy of Flickr.

Erich Reichert View All

Erich Reichert has been an editor and on-air personality in the radio control car hobby for 12 years. A certified car nut since birth, he has written for internationally published titles such as RC Car Action, RC Driver and Xtreme RC Cars, as well as Stuff Magazine, Road and Track and Super Street. He's covered everything from product reviews and tech articles to high-profile lifestyle pieces and celebrity interviews. Erich found his passion for writing after a successful career as an art director, working with brands such as Pepsico, NASCAR, MTV, Nintendo, WWE, Cannondale Bicycles and HBO. He's also a father, an avid hockey fan and an FIA race license holder who enjoys hiking, playing drums and movies.

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