Keeping the Fire Going
Some spark plug maintenance tips
By Joe Escobar
October 2001
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.
Simple looking 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.
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.
Telltale signs
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.
Cleaning
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.
Effects of excessive torque on spark plugs.
A. Stretching occurs over thread lengths.
B. Combustion gas seal is opened.
Installation
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
Additional resources....
Champion Aerospace Inc.
1230 Old Norris Rd.
Liberty, SC 29657 USA
(864) 843-5400
www.championaerospace.com
Teledyne Continental Motors
PO Box 90, 2039 Broad St.
Mobile, AL 36615 USA
(334) 438-3411
www.tcmlink.com
Textron Lycoming
652 Oliver St.
Williamsport, PA 17701
(570) 327-7278
www.lycoming.textron.com
Unison Industries
7575 Baymeadows Way
Jacksonville, FL 32256 USA
(904) 739-4000
www.unisonindustries.com
