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Installation & Removal of Spark Plugs

Doing the job right can help prevent performance problems, cylinder head damage and hours of hassle


Anyone who has had to remove a cylinder head to retrieve a broken spark plug knows how important it is to use care in spark plug installation and removal. In this article, we’ll cover some of the most common plug installation issues and how to avoid some common pitfalls.

Over-tightening spark plugs or improperly cleaning the threads in the cylinder head before installation of the NEW spark plug is a very common issue that can cause thread damage. Overlooking these basics can lead to breakage issues during the next replacement.

Another issue is thread galling in aluminum cylinder heads. Galling or binding between the plug and head can destroy the thread and possibly result in plug breakage. To avoid this, it is important to let the engine cool down before trying to remove a spark plug.

Installation recommendations vary by spark plug design; however always be sure there is no carbon or other contamination in the cylinder head threads prior to installation. There are special taps available designed to “clean” the threads and remove residual carbon if necessary. Also, be sure the threads on the spark plug are undamaged. Never consider installing a spark plug – even one that’s brand new – that has been dropped. Generally spark plugs come pre-gapped from the factory, however it is critical to always verify the spark plug gap meets the manufacturer’s recommendation for the application you are working on before installing a spark plug into an engine.

Tapered-seat spark plugs should be installed by hand until finger-tight. Then use an additional 1/16th turn applied with a spark plug wrench to ensure the spark plug has been tightened to the manufacturer’s recommendation for a 14mm thread. Alternatively, using a torque wrench, tighten the spark plug to approximately 7 to 15 lb. ft. for 14mm threads and 15 to 20 lb. ft. for 18mm threads to ensure a gastight seal. Overtightening a spark plug will stretch the plug body and potentially result in breakage upon removal.

For gasket-type plugs, install the plug until finger-tight. Then if the plug is being installed with a new gasket, use a spark plug wrench to apply a 3/8 to 5/8 turn – depending on head material. (See chart below.) When reinstalling a used spark plug with a used gasket, only ¼ turn is necessary, regardless of head material.

The Tougher Job: Plug Removal

It’s not unusual to have difficulty removing a used spark plug. Be sure to allow the engine to cool before attempting the job, and always be sure to clean away all debris around the plug prior to removal or use compressed air to prevent entry into the engine. If the plug appears to be overly tight and/or there is corrosion surrounding its base, it is acceptable to apply a quantity of penetrating oil (rust breaker). After allowing the penetrant to do its job (most require at least 5 minutes), apply steady pressure with a spark plug wrench until the spark plug loosens. Do not yank on the wrench, as this could cause breakage.

For especially difficult to remove plugs, try loosening ¼ turn, then retightening. Repeat this process while reapplying penetrating oil until the plug is removed.

It’s always a good idea to use a thread chaser tool to remove carbon and other contaminants from the threads in the head before installing a new spark plug.

Changing Your Vehicle’s Spark Plugs

Labor Time: 30 minutes to 4.0 hours

Things you will need:

  1. One set of NEW spark plugs for your application
  2. Safety glasses/Goggles
  3. Gloves
  4. Spark plug socket and ratchet (size and length of socket will depend on your application)
  5. Spark plug boot remover or spark plug boot removal pliers
  6. Shop air and blow gun
  7. Penetrating oil (see MSDS for proper handling/ disposal and safety procedures) See sales associate for more details
  8. Spark plug thread chaser
  9. Di-electric grease (this may come in the replacement wire or spark plug boot set) See MSDS for proper handling/ disposal and safety procedures. See sales associate for more details

Tips and Warnings

  • Evaluate the service procedure recommended by the manufacturer and be certain all of the necessary parts and tools are available. Many engine platforms may require removal of the upper intake manifold to gain access to the spark plugs. (Special tools and additional gaskets maybe required).
  • Allow the engine to cool down to a room/shop temperature. (most applications). Attempting to remove the spark plugs from a HOT engine may cause the spark plug threads to seize, causing damage to the cylinder head threads.
  • Remove spark plug boots by first twisting the boots back and forth until they move freely on the spark plug and then pull off while holding the spark plug boot, not the spark plug wire (if equipped). Label the wires to ensure that they are reinstalled in proper order.
  • Aggressively pulling on the spark plug boot can lead to spark plug wire terminal pull out which will require replacement of the spark plug wire.
  • Clean the spark plug recess area before removing the spark plug. Failure to do so could result in engine damage because of dirt or foreign material entering the cylinder head. Also contaminated threads may prevent the proper seating of the new spark plug.
  • If the spark plug is difficult to unthread, STOP, and use penetrating lubricant and allow it to soak for at least 5 minutes. Then tighten the spark plug back up and try removing it again. It may be necessary to follow this procedure several times until the spark plug is removed.
  • Use ONLY hand tools when removing or installing the spark plugs or damage can occur to the cylinder head or spark plug.
  • Use only spark plugs specified for use in the vehicle. Do NOT use spark plugs that are either hotter or colder than those specified for the vehicle. Installing spark plugs of another type or heat range can severely damage the engine.
  • Always check the gap of all new spark plugs before installing them into the engine. Installing spark plugs with the wrong gap can cause poor performance and may even damage the engine.
  • Be sure that the spark plug threads smoothly into the cylinder head and the spark plug is fully seated upon install. Use a thread chaser, if necessary, to clean threads in the cylinder heads. Cross threading or failure to fully seat the plug can cause overheating of the plug, exhaust blow-by, or thread damage.
  • Always tighten the spark plug to the manufacturer’s recommendations.

Follow These 12 Easy Steps

  1. Purchase a service manual for your vehicle. It contains essential information for getting the job done safely and correctly the first time.
  2. If you don’t already have them, obtain the tools necessary for spark plug replacement.
Tools necessary to replace the spark plugs:
  1. Spark plug socket – This will have special padding inside to protect the spark plug’s porcelain insulator
  2. Torque wrench – It is important that the spark plugs are tightened to the recommended torque
  3. Ratchet – Standard hand tool to fit the spark plug socket
  4. Extension – Depending on model and accessories, you may need special extensions and universal joints to reach one or more of the plugs
  5. Spark plug gap gauge – This gauge for checking the gap comes in a variety of styles. Make sure the gap for your engine is included
  1. Determine if there are any special procedures required to access the spark plugs. Sometimes other components will need to be removed or repositioned. Your service manual will give the specifics for your vehicle.
  2. The spark plugs are located near the top of the engine. On some vehicles, you’ll have to remove an ignition coil for access to each spark plug. Other vehicles will have large wires attached to the spark plugs. Grasp the wire by the boot, rotate it and pull it off.
On vehicles with ignition coils over each plug, disconnect the electrical connector (A) and remove the bolt (B).
  1. If compressed air is available, use it to blow any dirt or foreign material away from the spark plug hole. This prevents debris from falling into the cylinder as the spark plug is removed.
  2. Unscrew each spark plug, using the special spark-plug socket, a ratchet and extension. Replace only one spark plug at a time.
  3. Check each of the plugs as you remove them - they will indicate the general running condition of the engine. In an engine running normally, the spark plug tip should be brown or grayish tan in color. If it is any other color, or if there are deposits on the tip, there is a problem with the way the engine is running. Your service manual should have a chart that will help you determine the running condition of the engine based on the appearance of the spark plug tip.
  4. The gap between the spark plug electrodes is a critical dimension that must be correct. Most spark plug gaps are set at the factory, but sometimes gaps can be wrong, even on new spark plugs. In most cases, you’ll want to check the gaps of the new plugs, assuring they are to the specification listed in your service manual. In some cases, particularly with iridium spark plugs, the manufacturer recommends against checking and adjusting the spark plug gaps. Always follow the recommendations of the spark plug manufacturer.
Unless the manufacturer recommends against it, you’ll want to check the gap on each spark plug.
  1. Before installing the new plugs, apply a thin coat of anti-seize compound to the threads.
Apply a thin coat of anti-seize compound to the spark plug threads, being careful not to get any near the lower threads.
  1. Thread one of the new plugs into the spark plug hole until you can no longer turn it with your fingers, then use a torque wrench to tighten it to the specification listed in your service manual. It is a good idea to slip a short length of rubber hose over the end of the plug to use as a tool to thread it into place. The hose will grip the plug well enough to turn it, but will start to slip if the plug begins to cross-thread in the hole – this will prevent damaged threads.
A length of snug-fitting rubber hose will save time and prevent damaged threads when installing spark plugs.
  1. Attach the ignition coil or spark plug wire to the new spark plug, assuring it is fully seated on the plug.
  2. Repeat the procedure for the remaining spark plugs.