Keeping the Fire Going
Some spark plug maintenance tips
By Joe Escobar
Spark plugs are an integral part of every reciprocating engine. Many of us know enough about them to get by, but there is some important information on spark plugs that can help ensure the continued safe operation of the engines they are installed on. This article will touch on some of that information.
At first glance, spark plugs appear to be simple in nature, but don’t let their appearance fool you. Spark plugs are precision-engineered and constructed to withstand the extreme temperatures and pressures associated with high horsepower aircraft engines, while providing the spark necessary for combustion. They are also expensive parts, and require careful handling and special attention during maintenance to prevent dollars from going out the hangar door.
Hot or cold?
The terms hot and cold are used to describe both engines and spark plugs. Low horsepower engines are often referred to as cold because of their lower operating temperatures. Hot engines are the higher horsepower engines that operate at higher pressures and temperatures.
In relation to spark plugs, the terms hot and cold are used to describe their ability to transfer heat from their firing end to the engine cylinder head. A cold plug has the ability to transfer heat more readily, while a hot plug has a slower rate of heat transfer.
To understand why different plugs are used with different engines, it is necessary to understand the conditions necessary for optimum spark plug performance. The operating temperature of the spark plug’s insulator core nose is a factor affecting the formation of combustion deposits. For peak performance, they need to operate at a temperature range between 1,000 and 1,300 degrees Fahrenheit. If they are operated at lower temperatures, especially at or below 800 degrees Fahrenheit, they are susceptible to carbon deposits. In addition, they are more susceptible to lead deposit formation at lower temperatures because the bromide scavenger contained in tetraethyl lead is non-active at low temperatures. At the higher temperatures associated with peak performance, the bromide scavenger is fully activated, disposing of lead deposits with the combustion gases during the exhaust cycle.
Which spark plug to use?
The engine manufacturer works closely with the spark plug manufacturers to determine the correct spark plug for each engine application. In terms of hot or cold plugs, hot plugs are generally used in low horsepower (cold) engines to keep the insulator core nose at the higher temperatures necessary for peak performance. Cold plugs are used in the higher horsepower (hot) engines to prevent plug overheating, dissipating the excess heat quicker to keep it within the optimum temperatures. The bottom line is to use only those spark plugs approved by the manufacturer. Use of incorrect plugs can seriously affect the spark plug’s life and engine performance. Damage to the spark plug’s shielding barrel insulator and ceramic lead terminal can occur if improper lead removal techniques are used.
Spark plug removal
Spark plugs should be removed at regular intervals per manufacturer’s recommendations for inspection and servicing. Before removing the plug, the ignition harness lead must be disconnected. The spark plug to elbow coupling nut should be carefully loosened and removed. The lead then can be pulled straight out and in line with the centerline of the plug barrel. Applying a side load while pulling out the lead can damage the barrel insulator and the ceramic lead terminal. If the lead cannot be removed easily , the collar may be stuck to the shielding barrel. It can be broken loose by twisting the collar as if it were a nut being unscrewed from a bolt.
Once the lead has been disconnected, a deep socket can be used for the spark plug removal. Applying steady pressure with one hand on the ratchet handle while holding the socket in alignment with the other hand is the best method to remove the spark plug. Not holding it in alignment by this method can cause the socket to cock to one side and damage the spark plug insulator or connector threads.
Holding socket in alignment during removal is essential.
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