The Do’s and Don'ts of Installing Spark Plugs

The Do’s and Don’ts of Installing Spark Plugs

This article discusses the “conscious” steps you should perform during spark plug replacement.

Spark plug replacement is often considered one of the least complicated tasks at a shop. But, ignorance of what you are dealing with can get you into trouble before, during and after the new plugs are installed.

Blindly removing and replacing components without inspection can cause your shop to fall into a diagnostic black hole trying to find the cause of a misfire that was not there before.


Modern engines tend to have the spark plugs placed as close to the combustion chamber as possible. With twin cams, long runner intakes and other obstacles, access to the spark plugs can be difficult. Take your time. Often you are dealing with brittle plastic clips and covers.

When you are pulling the coil or boot out of the hole in the head, look at how clean it is. If there is any trace of oil or debris, it should be cleaned out, and the leak should be addressed. It could mean a new valve cover gasket or tube seals, but it will ensure the plugs last for another 100,000 miles. 

If the source of the leaks is not addressed, it could mean a slow death of the components in the hole. The ignition components from the shell of the spark plug to the top of the coil are electrostatically charged. This can attract particles of debris and oil to the spark plug, boot and coil. 

Ozone can be produced by errant current inside the hole where the plug and coil are located. Ozone can quickly degrade the insulating materials of the boot and coil. This is not a new phenomenon; it is the reason why some distributor caps have vents.

When installing a spark plug, make sure the top insulator is free from finger prints. The prints could attract debris that could lead to carbon tracking and misfires.

Coils shouldn’t have any visible damage, and the boot should be fully seated. While assembling coils to the engine, disregard the coil if you notice any visible damage like a cracked coil housing, broken anti-rotational tabs or a broken connector. Any damaged part could lead to a premature failure, and a poorly seated boot could lead to engine misfire.

Pulling the Plugs

Removing the plugs while the engine is hot can damage the plugs and the head due to differing rates of thermal expansion. Always wait until the engine is cold. While there is not a set temperature, take your time and use your best judgment. 

All the plugs should have close to the same appearance. Corona stains on the exposed ceramic body are normal. They are caused by the high amount of static electricity attracting particles of oil and dirt to the body of the plug — not blowby gases or thermal distress.

Back in the days of leaded gas, the porcelain insulator on a functioning spark plug would have a tan color. Today, some gas additives have been known to turn fresh insulators blue, pink, or purple. But, stains and deposits from oil coolant still leave the same marks. 

Coolant will leave a white chalky deposit on the electrodes and insulator. But, don’t expect to see the crusty deposits like you may have seen in your shop textbooks. New coolant formulations have lower phosphate levels and different chemistry, so the deposits are not as noticeable. If you have any concerns that the head gasket is leaking coolant, look at the piston with a borescope. In most cases, the piston in the leaking cylinder will be considerably cleaner. This is because the flash steam from the leaking coolant does a great job at breaking up carbon deposits. 

Oil will leave black or gray deposits. Most deposits will leave a flat and not glossy appearance. If the plugs have a glossy sheen, it could be a sign of a fuel problem.

Picking the Right Plug

Even if a spark plug fits in the hole in the head and the correct heat range, it might not be designed for that application. If a copper core plug is used in an engine designed for a fine wire platinum plug, it will require extra voltage to fire the plug and put stress on the ignition coil. But, if a platinum or iridium is used on a copper core application, it could help to reduce the voltage needed to fire the plug and reduce stress on the secondary coil.

It is highly recommended to follow the recommendations in the spark plug supplier’s catalog to find the right plug for the engine. 

For modern fine-wire spark plugs, don’t measure the gap because you could damage the center electrode. However, it’s still a good idea to inspect the new spark plugs for damage or defects. The majority of these plugs come pre-gapped from the factory. On these types of plugs, bending the ground electrode(s) can alter the geometry and thermal performance of the electrodes to the point of causing a misfire.

On older applications, gapping may still be required. Gapping specifications can be found in the service information or emissions tag under the hood. They can also be found in electronic and paper catalogs.

Measuring the gap requires the right tools. A coil-style gap gauge will work for copper core plugs. But for fine-wire or precious metal plugs, a wire-type gauge is recommended.

If it is necessary to widen the gap, do so with a tool that pulls back only on the ground electrode without touching the center electrode or the porcelain. To close the gap on a plug, gently tap the plug electrode first, on a hard surface.


Most spark plugs come from the factory with a coating or plating that is designed to prevent galling of the threads. This shiny silver or gold plating is not to make the plug look expensive. Applying a coating of anti-seize will only lead to incorrect torque readings. Use of anti-seize can increase torque values by 20 percent. 

Always apply the specified torque to the specified mounting thru-bolt that holds down the coil. Loose thru-bolts could lead to failures under vibration. Overtightened thru-bolts could lead to a cracked coil mounting ear and premature thru-bolt failure. The misfire codes could occur when the vehicle is started in the bay, or they could occur a thousand miles later after the bolt and coil work their way loose.

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