Spark Plug Inspection and Maintenance – Removal and Reconditioning

in Engine Ignition and Electrical Systems

Spark Plug Removal

Spark plugs should be removed for inspection and servicing at the intervals recommended by the manufacturer. Since the rate of gap erosion varies with different operating conditions, engine models, and type of spark plug, engine malfunction traceable to faulty spark plugs may occur before the regular servicing interval is reached. Normally, in such cases, only the faulty plugs are replaced.

Since spark plugs can be easily damaged, careful handling of the used and replacement plugs during installation and removal of spark plugs from an engine cannot be overemphasized. To prevent damage, spark plugs should always be handled individually and new and reconditioned plugs should be stored in separate cartons. A common method of storage is illustrated in Figure 4-51. This is a drilled tray, which prevents the plugs from bumping against one another and damaging the fragile insulators and threads. If a plug is dropped on the floor or other hard surface, it should not be installed in an engine, since the shock of impact usually causes small, invisible cracks in the insulators. A dropped spark plug should be discarded.

Figure 4-51. Spark plug tray.

Figure 4-51. Spark plug tray.

Before a spark plug can be removed, the ignition harness lead must be disconnected. Using the special spark plug coupling elbow wrench, loosen and remove the spark plug to elbow coupling nut from the spark plug. Take care to pull the lead straight out and in line with the centerline of the plug barrel. If a side load is applied, damage to the barrel insulator and the ceramic lead terminal may result. [Figure 4-52] If the lead cannot be removed easily in this manner, the neoprene collar may be stuck to the shielding barrel. Break loose the neoprene collar by twisting the collar as though it were a nut being unscrewed from a bolt.

Figure 4-52. Improper lead removal technique.

Figure 4-52. Improper lead removal technique.

After the lead has been disconnected, select the proper size deep socket for spark plug removal. Apply steady pressure with one hand on the hinge handle, holding the socket in alignment with the other hand. Failure to hold the socket in correct alignment causes the socket to tilt to one side and damage the spark plug. [Figure 4-53]

Figure 4-53. Proper spark plug removal technique.

Figure 4-53. Proper spark plug removal technique.

In the course of engine operation, carbon and other products of combustion are deposited across the spark plug and cylinder, and some carbon may even penetrate the lower threads of the shell. As a result, a high torque is generally required to break the spark plug loose. This factor imposes a shearing load on the shell section of the plug. After removing the plugs, they should be placed in a spark plug tray. [Figure 4-51]

Spark Plug Reconditioning Service

A visual inspection should be the first step in servicing spark plugs. The threads on the shielding barrel and on the shell that screws into the cylinder should be inspected for damaged or nicked threads. Inspect the lead shielding barrel for corrosion, nicks, and cracks. The firing end should be checked for insulator cracks, chips, and excessive electrode wear. The shell hex or wrench hex should be checked to see if it is rounded off or mutilated. If the spark plug passes the visual check, then it should be degreased using petroleum solvent. Take care to keep solvent out of the shielding barrel. Never soak the plugs in solvent. After drying the firing end of the plugs, remove the lead compound deposits using a vibrator cleaner. [Figure 4-54] The firing end can now be cleaned by using an abrasive blaster. This is usually done using a spark plug cleaner tester. [Figure 4-55]

Figure 4-54. Spark plug vibrator cleaner.

Figure 4-54. Spark plug vibrator cleaner.

As the firing end is subjected to the abrasive blast, the plug should be rotated so all the area of the firing end is cleaned. After the abrasive blast, the firing end gets a thorough air blast to remove the abrasive material. The shielding barrel insulators may be cleaned with a cotton cloth or felt swab saturated with solvent, wood alcohol, or other approved cleaner. The firing end should be inspected using a light and a magnifying glass. If the plug passes the firing end visual and cleaning checks, then the spark gap should be set using a round thickness gauge. The spark plug should be tested by using a tester as shown in Figure 4-55, which passes a high-voltage through the spark plug and fire the gap. As this test takes place, the firing end of the plug is subjected to air pressure to simulate the pressure in the engine’s cylinder. If the firing pattern is good, the plug should be returned to its holder ready for installation in the engine.

Figure 4-55. Spark plug cleaner tester.

Figure 4-55. Spark plug cleaner tester.

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