Spark Plug Replacement Do’s and Don’ts – UnderhoodService
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Spark Plug Diagnostics

Electronic

Spark Plug Replacement Do’s and Don’ts

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Tooling has changed from the old 13/16-inch spark plug socket to a multiplicity of 5/8-inch long, short, intermediate and swivel, thin-wall sockets needed to ­replace spark plugs on many modern engines. Similarly, tightening spark plugs with the old flex-head, long-handled, 3/8-inch drive spark plug wrench is asking for trouble on engines that don’t have long-reach spark plug threads in the cylinder head.

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Spark Plugs

Photo 1: Modern engines require modern tools for spark plug replacements. The in.-lb torque wrench is vital to achieve correct seating of the spark plug against the cylinder head.

Consequently, the compact in.-lb torque wrench is a necessary tool for correctly torquing spark plugs into modern aluminum cylinder heads. See Photo 1.

Modern spark plugs are generally torqued between eight and 17 ft.-lbs. To illustrate the degree of these values, eight ft.-lbs can easily be achieved using a short quarter-inch drive ratchet. Seventeen ft.-lbs can be easily achieved using a short 3/8-inch drive ratchet.

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If the spark plug is overtorqued into an aluminum head, the aluminum threads begin to “roll over,” causing a high degree of friction when the spark plugs are removed. In worst-case scenarios, the threads actually separate or strip from the cylinder head casting and may require removal of the cylinder head to repair. In other cases, overtorquing a spark plug can distort and break the seal between the metal shell and porcelain insulator, which

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Photo 2: Overtightening a new spark plug can cause combustion gas leakage past the ceramic seal on the spark plug.

causes combustion gases to leak into the spark plug boot and blow it off the spark plug. If a spark plug boot keeps blowing off, suspect a seal leak caused by an overtorqued spark plug. See Photo 2.

If the spark plug is undertorqued, combustion gases begin to leak past the tapered seat or metal gasket. This leakage causes carbon to form around the threads that, in turn, causes the spark plug to seize to the cylinder head. A properly torqued spark plug will show a bright register mark on the tapered seat or the gasket will show compression. See Photo 3.

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All of the above are good reasons to take a little extra time to properly torque a new spark plug.

Spark Plug Removal & Installation

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Photo 3: The loss of contact between spark plug and cylinder head seats through insufficient torque can cause the spark plug to overheat and cause preignition. A dull tapered seat or insufficiently compressed gasket indicates insufficient torque.

Although not critical on all engines, always let an engine with an aluminum cylinder head cool down to help expedite safely removing the spark plugs. Next, it’s extremely important to use compressed air to blow dirt and other debris away from the spark plug. If dirt remains after the spark plug has been removed, insert a long air nozzle into the cylinder itself to blow the remaining dirt out of the spark plug threads.

Many manufacturers recommend soaking the spark plug threads with suitable penetrating oil before removal. In some cases, it’s advisable to back the spark plug out about a 1/4 turn to allow the penetrant to soak into the threads. If the spark plug still feels seized into the cylinder head after the penetrant has soaked, continue to remove the spark plug from the head by adding penetrant and alternately tightening and loosening the plug to lubricate the spark plug threads.

Spark Plugs cylinder heads

Photo 4: Expanding collet and conventional spark plug thread chasers are handy for cleaning or restoring the spark plug threads in the cylinder head.

If the spark plug threads appear damaged or caked with carbon, use one of many spark plug thread chasers now on the market to clean the threads. See Photo 4.

If the threads are stripped, a number of aftermarket tool companies offer repair kits that allow the threads to be replaced in chassis.
Remember that secondary ignition voltage has increased from about 20,000-25,000 volts on contact point ignitions to about 60,000 volts on modern distributorless or coil-on plug ignition systems. Spark plug installation in a modern engine requires care and cleanliness to contain these high-firing voltages. A greasy fingerprint on the exterior insulator can, for example, rapidly develop into a misfire-causing carbon track or flash-over. If a spark plug shows evidence of flash-over, the spark plug boot or wire contains a companion trace and should also be replaced to prevent a comeback.

Spark Plug insulator

Photo 5: Using a spare spark plug boot or length of fuel hose to handle the spark plug during preparation and installation can prevent contaminating the spark plug insulator with greasy fingerprints.

To avoid leaving fingerprints on spark plugs, remove the spark plug from its box by grasping the metal shell. Next, insert the spark plug into a recently cleaned rubber hose, spark plug boot or spark plug socket to allow for checking the air gap and other service operations. See Photo 5.

If it appears advisable to apply some anti-seize compound to the spark plug threads, prevent contaminating the center electrode by using a small tinning brush to dab a small amount of compound to the ground electrode side of the threads. Use anti-seize compound sparingly because a misfire can develop if anti-seize spatters onto the spark plug’s center insulator.

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Next, use an in.-lb torque wrench to tighten the spark plug to specifications. Remember that multiplying a ft.-lb torque specification by 12 will make the conversion to in.-lbs on the torque wrench. If you’re using anti-seize compound on the spark plug threads, use the lowest specified torque value. Lastly, use a small, round brush to apply a coating of dielectric grease to the inside of the spark plug boot to seal the boot against moisture and expedite future removal.

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