Repair Topics > Spark Plugs and Wires > How to Replace Spark Plugs: A DIY Guide

How to Replace Spark Plugs: A DIY Guide

Learn how to replace spark plugs yourself and save money.

Replacing spark plugs is critical for keeping your vehicle’s engine running smoothly and efficiently. Over time, spark plugs wear out, leading to hard starting, especially in colder weather. Worn spark plugs cause poor engine performance, reduced fuel efficiency, and potential engine misfires. Fortunately, replacing spark plugs is a relatively straightforward process that can be done with basic tools and a little know-how. This guide will walk you through the steps to replace spark plugs yourself and save money while ensuring the job is done properly.

The tools you need to replace spark plugs

• The right spark plug socket— There used to be just two sizes of spark plug sockets. Not anymore. Some spark plugs in late-model engines are longer, requiring a deeper socket. In addition, some spark plugs are located so deep in the spark plug tube, that the socket needs extra grip to pull it out. Some sockets use a rubber boot, others use snap clips, and still others use a magnetic ring. Talk to the clerks at the auto parts store for a recommendation for the best spark plug socket for your particular engine.
• Ratchet and Extension— For reaching and turning the spark plugs.
• Torque Wrench— You’ll need a 3/8″ drive torque wrench rated in inch-lbs. To ensure the spark plugs are tightened to the manufacturer’s specifications.
• Gap Gauge— Use a wire gauge gapping tool, never a ramped “coin style” tool. Those can bread off the brittle iridium tips.
• Dielectric tune up grease— To apply to the spark plug boots for better electrical connection and easier future removal.
• Anti-Seize Compound— This is rarely needed for late-model engines but is required on some non-plated spark plugs and when re-installing a previously used spark plug.

Tips for removing and installing spark plugs on late-model engines

1) Use compressed air to blow away dirt or grit surrounding the plug before removing it

If you don’t remove it, it’ll fall into the cylinder after you remove the spark plug.

2) Spark plug manufacturers no longer recommend applying anti-seize to spark plug threads

Most new spark plugs have a factory-applied nickel coating on the threads. So, they no longer require an anti-seize application. If in doubt, consult the spark plug manufacturer’s website to see if they want you to apply anti-seize. If you apply anti-seize, you MUST reduce torque by at least 10%.

NOTE: The nickel coating is considered a one-time use. If you remove the spark plug, you MUST apply anti-seize when reinstalling it.

3) Always use a torque wrench to tighten a spark plug

Spark plug torque has been reduced substantially over the years due to the use of aluminum heads. This means you MUST use a torque wrench to seat spark plugs properly.

• Over-torquing a spark plug can lead to misfires— The porcelain portion of the spark plug is sealed to the metal shell with an adhesive. If you over-torque the shell, you’ll distort it and break the adhesive seal, resulting in a leak and misfires.
• Over-torquing can rip the threads right out of an aluminum cylinder head— In addition to distorting the metal shell, over-torquing causes incredible stress on the soft aluminum threads. During the compression stroke, the high pressure can cause the weakened threads to let go, spitting the spark plug out of the hole and taking the threads with it.
• Under-torquing causes misfires and thread damage— Under-torquing causes cylinder leaks and misfires. Worse yet, low torque can cause the spark plug to blow right out of the cylinder head, taking the threads with it.

The average spark plug torque is around 12-15 ft/lbs. That’s not a lot of torque! It’s way below the lowest set point on all 1/2″ drive torque wrenches.

4) Always apply a light film of dielectric grease to the inside of the spark plug boot

Dielectric grease does two things: It prevents the high voltage from

This image shows where to apply dielectric grease on the coil boot

Wipe a thin layer of dielectric grease inside the spark plug or coil boot

finding a path to ground down the side of the spark plug and prevent the rubber boot from welding itself to the porcelain.

5) Don’t over-torque the coil-on-plug retaining bolts

I see this all the time. DIYers have installed the spark plugs and placed the coils back in place. Then they crank down on the small 10mm coil bolts and break the bolt head off. The average torque specification for C.O.P. bolts is just 6 ft/lbs! If you crank on it, you’re applying about 45 ft/lbs. No wonder the head breaks off.

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Avoid these common spark plug installation mistakes

• Not removing dirt and sand from around the spark plug before removing it— Blow out the dirt around the spark plug before removing it. If you don’t, all that cuff will fall into the cylinder after you’ve removed the plug.

• Using an impact driver or wrench to remove spark plugs— Some spark plugs are seized in the threads. You can’t tell that if you’re using an impact. Using that much power will remove the plugs, but it may also rip the threads out of the cylinder head or, worse yet, distort the spark plug’s metal shell and break off the porcelain, leaving you with a broken spark plug still in the hole.

• Stripping the cylinder head threads— Always start a spark plug with your finger to ensure the threads have properly engaged.

Using a coin gap tool— A coin gapping tool twists the side electrode as you try to increase the gap. The twisting motion can not break off the iridium tip on the center electrode but also bends the side electrode so it is no longer oriented squarely with the center electrode.