In this installment of Cessna Flyer’s series on owner-performed preventive maintenance, A&P Jacqueline Shipe looks at the servicing and replacement of aviation spark plugs.
Aviation spark plugs need to operate while subjected to the wide temperature ranges that are possible in an aircraft engine. A spark plug with a 0.020-inch gap must be able to handle around 14,000 volts and fire reliably during its lifespan.
Regular cleaning, gapping, and rotation of spark plugs helps ensure that the longest and most reliable service life for each plug is obtained. Regularly pulling and inspecting the plugs also helps diagnose cylinder health.
Under Appendix A, paragraph (c) of FAR 43, the items “spark plug cleaning, gapping and replacement” are on the list of maintenance items an owner can perform on their own aircraft.
Anatomy of a spark plug
Aviation spark plugs have a positive center electrode that is connected to the ignition lead terminal through a resistor. This center electrode assembly is housed in a ceramic insulator, which prevents the high voltage electrical current generated by the magneto from grounding out against the metal outer shell, which contains the negative electrode(s).
These plugs are designed to withstand severe operating conditions and typically provide a long service life if they are properly maintained.
Removing the plugs
The first step in spark plug maintenance is removal of the plugs. Once the engine cowling is removed to the extent necessary so that access to all the plugs is achieved, the ignition leads can be disconnected from the spark plugs.
The inner part of the lead needs to be held stationary as the outer nut is removed to prevent the lead from being twisted as the outer nut is turned. The leads should be gently pulled straight out and not cocked as they are removed from the plugs.
A good deep-well six-point 7/8-inch socket is required to remove the plugs. Aviation spark plug manufacturers, including CFA supporter Tempest, makes and sells a specialized aviation spark plug socket that works well. Be sure the socket is properly seated on the plug before attempting to break it loose.
It is important to keep track of which position each plug is removed from. This helps for diagnosing cylinder health and for plug rotation during the reinstallation.
Homemade spark plug trays with marked receptacles for each plug are easy to make, or plugs can be laid out on a piece of marked cardboard. Tempest highly recommends using a spark plug tray to keep plugs from rolling of the workbench and to assist with proper plug rotation.
Avoid laying a plug on top of the cylinder, or any place where it could roll off and hit the floor. Dropped plugs often have cracked insulators or damaged resistors—and even if they pass a resistance check afterward, they could still have defects that can result in malfunctions and misfiring later on. Any plug that is dropped should be discarded.
Inspecting the spark plugs
Plugs should be inspected after removal for excessive wear and general condition.
Any bottom plugs that are wet with oil aren’t a cause for concern, but if the top and the bottom plug in a cylinder are wet with oil, it can be a sign that there is either excessive piston ring wear, the ring gaps are lined up and/or the plug is malfunctioning. It wouldn’t hurt to take a compression check on the cylinder in question.
Plugs that are misfiring will be oil-soaked simply because they aren’t firing enough to clean off any deposits; a top oil-soaked plug could simply be the result of the plug itself malfunctioning.
Oil-fouled plugs should also be inspected for cracks and/or chips in the core nose insulator, according to John Herman at Tempest. Cracks or chips here may indicate a broken ring, which may result in cylinder damage from the broken piece of ring scoring the cylinder wall during piston cycles.
Cylinders with insulator plug damage should be borescope inspected to be sure the cylinder has not been damaged or there is no evidence of foreign object damage or debris (FOD).
Taking note of buildup
Normally, any removed plug has a deposit residue of some sort on it and will be a little sooty just from the normal combustion process in the cylinder.
Plugs that have virtually no deposits on them (i.e., too clean) or that have a slight reddish-brown tint on the insulator are indicative of a cylinder that is running too hot, or too lean, or both.
If this is noticed only in one cylinder, the intake gasket and tube should be inspected for leaks. A partially clogged fuel injector on fuel-injected engines can also cause a cylinder to run lean.
The most common deposits on spark plugs are lead and carbon. Lead buildup forms hardened balls that can eventually bridge the electrode gap and cause a plug to not fire. Carbon is jet black and sooty in appearance.
Excessive lead and carbon buildup on several plugs is a sign that an engine is being run too rich and not leaned properly. A good practice, endorsed by the folks at Tempest and others, is to lean on the ground any time the rpm is below 1,000. Always be sure to richen the mixture prior to takeoff.
Cleaning the plugs
Once the plugs are removed and organized as to which position they came from, the next step is to clean the plugs.
Lead deposits can be very built up and hardened, making them difficult to remove. Safety glasses, a dust mask, and chemical resistant gloves should be worn to protect eyes, lungs, and hands during spark plug cleaning.
Champion makes a machine that uses two cleaning prongs that vibrate at a high frequency to break loose the lead and pulverize it into fine particles that can be shaken out. Avoid breathing any of the dust generated from this process, as it contains lead particles.
These two-prong machines can be a little pricey, but there are handheld single-prong models that retail for a little over 20 dollars. (See Resources for a list of CFA supporters that sell the handheld spark plug vibrator cleaners. —Ed.)
In addition to getting the lead out of a plug, some shops clean the firing end of a spark plug in a sand or glass bead blast cabinet.
Tempest does not recommend glass bead blasting on its plugs because some of the glass bead residue can become lodged between the center electrode and the ceramic insulator. As engine temperatures heat up, the glass beads melt into a conductive coating which can cause the plug to misfire.
If a plug is to be blasted, Champion and Tempest both recommend using an abrasive grit that is made specifically for cleaning plugs. These companies advise lightly blasting only the tip of the plug; excessive blasting erodes the electrodes, causing premature wear.
Some mechanics don’t recommend any kind of abrasive blasting to clean plugs due to the electrode erosion it can cause, especially on fine wire plugs. Tempest doesn’t recommend abrasive cleaning for its fine wire spark plugs for this very reason.
If plugs are oily, a little solvent (e.g., Varsol or other traditional mineral spirits) works well to clean the residue out of the firing end. Note: the plug should not be fully immersed in the fluid; it should only be used on the firing end.
For stubborn lead deposits on the firing end, a good gun cleaning solvent, such as Hoppe’s #9 Bore Cleaning Solvent, is recommended by Tempest.
A swab soaked in Methyl ethyl ketone (MEK, or butanone) works well to clean the insulator and ignition lead contact in the opposite end of the plug. Note: never use Tetra ethyl chloride on the terminal well area of the spark plug; rubbing alcohol will work just fine, according to Tempest.
The threads on the firing end can be cleaned using a wire brush; just be sure not to clean the electrodes with the wire brush, as this can damage them.
Gapping the plugs
Once the plugs are cleaned and dried, they are ready to gap. There are a few different styles of gapping tools, but they all essentially work the same.
The plug is threaded into a receptacle on the tool, and a prong is pressed or screwed against the ground electrodes to move them closer to the center electrode. The recommended gap varies according to the plug and can be located on the spark plug manufacturer’s website.
A wire-style feeler gauge is used to measure the gap between the center and outer electrodes. Care needs to be taken to not close the gap too much, as the electrodes can’t be spread back apart.
Do not leave the feeler gauge between the electrodes when setting the gap. This can put a load on the insulator and cause it to crack.
Fine wire plugs typically don’t require re-gapping too often. Champion makes a specialized gapping tool for use on fine wire plugs if they do need to be reset. Tempest doesn’t currently have a similar tool, but is in the process of expanding its spark plug tool product line.
Bench testing the plugs helps to detect and prevent reuse of a faulty plug.
Both Tempest and Champion recommend the use of a bomb test to check a plug’s ability to fire under pressurized air. These types of testers are expensive and are usually found only in an equipped maintenance hangar, but it should cost only a few dollars to have the shop do the checks for you.
A resistance test can be performed in addition to the bomb test, but it’s not a replacement for the bomb test.
Tempest recommends using an electrical multimeter to check the resistance value between the ignition lead terminal in the upper part of the plug and the center electrode. The electrical resistance should not exceed 5,000 ohms on Tempest plugs. Any plugs with readings higher than 5,000 ohms should be discarded.
Reinstalling the plugs
After the plugs are gapped, they are ready for reinstallation.
The copper gasket that seals the plug against the cylinder head hardens as engine temperatures heat and cools the gasket over a period of time.
A hardened gasket does not seal as well as a soft gasket does, and can also keep the plug from properly seating against the cylinder head. Therefore, copper gaskets should be replaced before reinstalling the plugs. Spark plug manufacturers recommend that the gaskets are replaced each time the plug is removed and cleaned.
Plugs that have thermocouple gaskets attached to CHT monitors do not require a copper washer in addition to the thermocouple washer.
Before the plug is threaded into the cylinder, a thin coat of a high-quality anti-seize material should be brushed on the threads.
The first two threads closest to the electrodes should not be coated to prevent the conductive anti-seize compound from getting on the electrode and causing a misfire.
Champion and Tempest make specialized anti-seize lubricants that they recommend for use on their plugs.
A high-quality graphite- or copper-based anti-seize works well also. Aluminum-based anti-seize lubricants typically don’t work well because they don’t hold up under the severe heat. (Per Lycoming Service Instruction No. 1042AH "Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound.")
Rearranging the plugs
Aviation spark plugs should not be reinstalled in the same location they were removed from.
Ignition leads are polarity-sensitive on all magnetos (other than some of the dual magneto models); this means that the north and south poles of the spinning magnet in the magneto generate a negatively-charged spark that is sent down one lead, alternately followed by a positively-charged spark sent down the next lead.
Plug electrodes wear in predictable ways. The plugs connected to the positively-charged leads always fire from the positive center electrode to the negative electrodes, eroding the center electrode. The plugs on the negatively charged leads always fire from the negative electrodes back to the center electrode, eroding the outer electrodes.
Keeping the plugs rotated so the positive and ground electrodes wear evenly will double spark plug life. They should also be rotated from top to bottom, as the bottom plugs usually incur more deposit material. A rotation that a lot of mechanics use is top-to-bottom, and next in firing order.
Proper torque values should be used when reinstalling the plugs. Lycoming recommends 30 to 35 foot-pounds (420 inch-pounds); Continental recommends 25 to 30 foot-pounds (300 to 360 inch-pounds).
The ignition leads should be installed with care, and the leads should not be allowed to twist as the outer nut is tightened.
Mag check and troubleshooting
An engine runup and magneto check should always be performed to ensure that all of the plugs are firing properly. A smooth runup and magneto check indicate a job well done.
A rough-running engine during the magneto check is most likely indicative of a little debris or excess anti-seize on the electrodes of one of the plugs causing it to misfire or not fire at all. Take note of which magneto the engine is running rough on.
Once the engine is shut down and cooled off, check to see which plugs are fired by the magneto in question by visually following each ignition lead from the rough magneto all the way out to each plug.
These plugs can then be removed, and any debris can be gotten out with a small pick. Any anti-seize lubricant that has gotten on the electrode can be cleaned off with a little degreaser.
Over the last seven months, I’ve given you some general tips and step-by-step ways you can work on your own aircraft according to what’s allowed in FAR 43, Appendix A, paragraph (c).
This DIY series, along with guidance from a trusted mechanic, should give you a better understanding of preventive maintenance on your airplane—and might even save you a little money in the long run.
Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.
Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send questions or comments to .
Aviation spark plugs
– CFA supporter
Champion Aerospace, LLC
Spark plug vibrator cleaners
Aircraft Spruce & Specialty Co.
– CFA supporter
– CFA supporter
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