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.
It should be noted that on occasion, carbon will build up on the end threads of the spark plug making it difficult to loosen. If the plug is seized, never apply excessive torque or use an impact wrench to try to remove it because the plug may snap off. One way to remove seized plugs is to start the engine and run it until the cylinder head temperature reaches its operating range. The spark plugs then can usually be easily removed immediately after engine shutdown.
Segregating the plugs
The plugs should be removed in pairs from each cylinder. As they are removed, they should be placed in a tray, arranged by cylinder number and top or bottom position, with the electrodes up. Not only does this protect them from damage, it is helpful during inspection to see if any telltale signs are present on certain plugs that would indicate problems in the engine. Knowing where they were removed from is essential for initiating corrective action.
By inspecting and analyzing the spark plugs once they are removed, possible problems with the engine can be identified. The appearance and their signs follow.
Normal plug – A normal plug will have a brownish gray deposit with some slight electrode wear.
Carbon fouling – Dry, fluffy black deposits are indicative of carbon fouling. This can be caused by a rich fuel/air mixture, excessive ground idling, mixture too rich at idle or cruise, or faulty carburetor adjustment. As mentioned earlier, it can also be caused by a plug with a heat range that is too cold to burn off combustion deposits.
Oil fouling – Oil fouling is indicated by black, wet deposits on the bottom plugs in flat, opposed cylinder engines. Oily deposits on the top plugs may indicate damaged pistons, worn or broken piston rings, worn valve guides, sticking valves, or faulty ignition supply. In new engines, this may simply indicate that the piston rings have not yet properly seated.
Lead fouling – Lead fouling, in mild cases, shows as a light tan or brown film or a build up on the spark plug firing end. In severe cases, lead fouling appears as a dark glaze, discolored tip, or as fused globules.
In order to remove residues from the spark plug firing ends, they can be cleaned using an approved cleaner like unleaded gasoline, naphtha, or Stoddard solvent. After they have been thoroughly cleaned, they can be blown dry with oil-free, low pressure shop air. In addition, if necessary, the firing ends can be given an abrasive blast cleaning to remove minor deposits. During the cleaning process, it is important to keep the plugs in their respective spots in the spark plug tray to ensure proper rotation during installation.
Gapping the plugs
After cleaning the plugs, they should be checked for gaps. It is best to set gaps to the lower limit called for. For example, if Lycoming recommends a 0.016- to 0.021-inch gap setting for the spark plugs on your engine, go with the 0.016-inch setting. As the plug electrodes erode through normal use, their gap increases. Setting the gaps at the lower setting ensures that they remain within tolerance between inspection cycles.
Generally, it is recommended to swap plugs from the top and bottom position on the cylinder that they were removed from. Some even recommend rotating plugs from top to bottom and to the next cylinder in firing order in an effort to change polarity (see sidebar on page 18). Whatever method you choose, rotating the spark plugs can lead to increased service life. Effects of excessive torque on spark plugs.
A. Stretching occurs over thread lengths.
B. Combustion gas seal is opened.
A thin coat of anti-seize compound should be applied to the lower threads of the spark plug. Be careful not to get any of the compound anywhere on the first two threads in order to prevent possible compound seepage into the plug and contamination of its electrode. Perform a final inspection of the plug before installation to ensure there is no damage or debris present.
After installing a new gasket, screw the plug into the cylinder head by hand until it is seated on the gasket. If you are not able to install it most of the way by hand, remove it and clean the threads as necessary to allow for a clean fit.
The spark plug then can be tightened to the specified torque using a deep well socket and torque wrench. If a torque wrench is not used, the spark plug may be undertorqued, allowing the hot gases to blow past the threads. This can result in a loss of compression as well as burn damage to the cylinder. If the spark plug is overtorqued, then the plug can be overstressed and damage to the seal can result.
Finally, the terminal connector can be reinstalled. Clean the terminal sleeve with a lint-free cloth moistened with an approved solvent. Also ensure the inside of the spark plug shielding barrel is clean. Once they are clean, don’t allow them to get contaminated. Even the oil on your hands can cause contamination. Carefully insert the lead into the shielding barrel of the spark plug and tighten the coupling nut to recommended torque, while holding the coupling elbow to prevent twisting.
Final operational check
After all of the spark plugs are installed, the engine should be run and a complete ignition system operational check should be performed.
Always refer to the manufacturer’s inspection and maintenance procedures during spark plug maintenance. With a little care and attention, you can keep your spark plug’s fire going for millions of cycles — providing years of trouble-free service. The Source
Champion Aerospace Inc.
1230 Old Norris Rd.
Liberty, SC 29657 USA
Teledyne Continental Motors
PO Box 90, 2039 Broad St.
Mobile, AL 36615 USA
652 Oliver St.
Williamsport, PA 17701
7575 Baymeadows Way
Jacksonville, FL 32256 USA