December 2012
Spark plug replacement and cleaning is a task that aircraft owners and pilots can easily do to maintain engine efficiency. I encourage aircraft owners to get involved in the maintenance of their aircraft, as a pilot aware of the mechanics of his or her aircraft is a safer pilot.
A component of maintaining an aircraft engine in good condition includes the regular removal, cleaning/inspection or replacement of the spark plugs as allowed by FAR 43 Appendix A (“Major Alterations, Major Repairs and Preventive Maintenance”).
Spark plugs wear out but can also just become dirty with use; they can become lead-fouled as well. Cylinder and engine problems can be identified by just looking at the tip of the spark plug.
Spark plugs, to be maintained in the optimum condition, need to be cleaned and re-gapped at no more than 100 hours (or as needed), tested and replaced when necessary. Some aircraft engines prone to lead fouling may require more frequent service to remove lead deposits.
THE RULES OF THE GAME
According to the regulations governing aircraft maintenance it is necessary to always follow accepted techniques and practices such as tool inventory before and after a job, cleanliness and taking the time to do the job right the first time.
In addition to these general practices it is a requirement to adhere to the manufacturer’s instructions regarding service, approved replacement spark plugs, necessary tools, and specific values (such as torque values) during installation, use of anti-seize compound on the threads, and the proper gap. This requires referencing the technical data before you start the job, and the data can be obtained for free on the Internet. (References and resources are listed on page 31. —Ed.)
In addition, all maintenance—whether preventive maintenance performed by a pilot, or regular maintenance by a licensed mechanic—requires a logbook entry identifying the work, what was done and when (tach time and date) and by whom, to include a signature, license number and license type (“PPL,” “Com,” etc).
DECISIONS AND NECESSARY TOOLS
First, you will have to decide if you are going to just remove and replace the spark plugs (sending out the used set for cleaning and testing), or actually service the spark plugs yourself. It can be money well spent to have a set of spare spark plugs on hand so that one set can be sent out for cleaning, testing and re-certification, and the other can be used in the aircraft so you can continue to fly.
Aircraft Spark Plug Service of Van Nuys, Calif. (see listing on page 31) can clean, inspect, test and recertify aviation spark plugs and has for sale used certified spark plugs as well. (The company does not perform a resistance check on your spark plugs, so you will need to do one yourself. —Ed.) Initially you may want to purchase a set of used spark plugs in good condition, or you may choose to purchase a set of new plugs to start the process.
The time required for complete servicing and the investment of the equipment necessary to properly clean, gap and test a spark plug can be considerable. Aircraft flown less than 100 hours a year might benefit economically from a remove-and-replace option, with cleaning sent out rather than done locally.
The basic tools necessary to remove and replace the spark plugs would be a 3/4 or 7/8 inch open-end wrench (to remove the ignition leads from the spark plug) and a 7/8 inch six-point deep socket and either a ratchet or a breaker bar used in conjunction with the socket.
Some engines have limited space in the engine compartment so check carefully just how much room you do have to maneuver before purchasing tools only to find out they won’t work. You may need some combination of universal joints, adapters and extensions as well.
While you may be able to remove some if not all the spark plugs with an open-end wrench, you can easily damage the cylinder by forcing a wrench into contact with the cylinder. Use the proper tool to minimize the possibility of expensive engine repairs. The spark plug can also be unknowingly damaged by contact with a wrench or socket that is not evenly fitted to the circumference of the spark plug at every point on the hex.
Rough handling or dropping a spark plug on a hard surface can destroy a spark plug. Never reuse a spark plug that has been dropped even if the plug tests okay, as a cracked insulator may not be revealed in testing.
A spark plug tray (or something similar made at home) will work but it is important to keep track of each spark plug according to what cylinder and what position—lower or upper—for several reasons. Knowing what cylinder an oil-fouled plug came from can assist in corrective action.
Rotating the spark plugs between cylinders and top to bottom every 50 hours can prolong the life of the spark plugs. You may want to invest in a set of gloves if you intend to handle spark plugs removed from a warm engine. In addition to the open-end wrench and socket and ratchet, you will also need to use a torque wrench to reinstall the spark plugs.
How important is a torque wrench in this application? It is necessary because the manufacturer requires it. Too much torque can damage the threads and under extreme cases (or over a period of time) can allow the spark plug to become disconnected from the engine. Not enough torque can have the same result.
The torque value varies depending on the engine manufacturer (and some models may even be different from each other); refer to the proper manufacturer’s service information for the specific torque values.
Installations require, at a minimum, checking the gap of each plug, installing an unused or annealed copper gasket, and the use of anti-seize compound on the threads that are inserted into the cylinder (not on terminal end nuts).
TIPS AND PRECAUTIONS
As mentioned earlier, dropping a spark plug on a hard surface can easily destroy a spark plug internally by cracking the insulator. Even if inspected visually and no cracks are seen, it is not safe to use. Replace it.
Do not use a wire wheel on the tip of a spark plug as the abrasion is excessive and the force may crack the center insulator. In addition, protective coatings on the threads and outer body will be removed causing the spark plug to corrode or rust.
When using anti-seize material, use it sparingly and do not use the compound near the tip end of the threads. One popular product is Tempest’s T556 anti-seize. It will not bead up, applies smoothly and dries quickly.
Terminal sleeves are easily contaminated with dust, grease or even oil from your hands. Clean the insulators with methyl ethyl ketone (MEK), wood alcohol or naphtha and look over the internal components such as conductor spring for arcing or burn marks.
Replacement, repairs or adjustments to ignition system components other than spark plugs require the services of a licensed mechanic. Excessive torque on the terminal nuts can damage the spark plug or terminal connector. Hand-tighten the terminal connector, and then go one-eighth to one-quarter of a turn more to keep it from backing off.
Always check that the plug you are ordering or using is the correct one for your engine AND for the ignition harness/terminal ends. (Note: there are two different types of ignition leads: the larger one requiring a 7/8 inch wrench for removal is the all-weather type and has ¾-inch diameter threads. The smaller type, which is more common, is the 5/8-inch diameter barrel requiring a ¾ inch wrench for removal.) When ordering new spark plugs or reconditioned plugs, know which type of ignition lead you have.
In addition to the size, there are many different brands of spark plugs. Each engine manufacturer has a complete listing of the plugs approved/allowed for use in specific engines. Know the exact engine number (including dash number) so you can be assured you’re using the correct spark plug.
CLEANING, GAPPING AND TESTING
Removing lead deposits and testing each spark plug should not be rushed. Before starting the cleaning and gapping routine, check that the spark plugs are not worn to—or beyond—the limits. There is a handy go/no-go gauge that is available from many of the aviation parts supply houses identifying when the plug should be replaced.
While sometimes you can be successful with just picking out the lead and re-gapping the plug, pressure testing after service is part of the job. A dental pick works well for picking out minor lead deposits; however, the center conductor is easily damaged by picking if lead between the center conductor and the shell is very hard (which is usually the case).
There are electric-powered vibratory tools that do an excellent job of removing lead and this tool may be necessary depending on the design of plug you have and the amount of lead to be removed.
Grit blasting is also done on a routine basis as a finish cleaning of the spark plug tip, but it must be accomplished using a short blast of the grit as the grit can erode the plug and dramatically shorten its life. Tempest recommends using Aircraft Tool and Supply’s cleaning abrasive P/N 78B and a setting of 80 psi or below.
Occasionally grit particles get between the center and outer electrode and short out the plug, making the plug unusable. Check carefully for this before installation. (Please see the sidebar on page 29 for detailed information about cleaning a spark plug. —Ed.)
Checking and adjusting the gap of the spark plug is important, as this is what makes the spark and starts the combustion process. Improper gap can impair engine efficiency and cause ignition problems.
There are tools designed for setting the gap as well as wire-type gap gauges for checking the gap. The normal gap is within the range of .016 to .021 for most spark plugs and applications. However, if you gap below .016 by mistake, you’re okay down to .012. (Always check current technical data to ensure the gap is correct. —Ed.)
Be careful in setting the gap, doing a little at a time and each side equally, checking with the wire gauge each time you adjust the gap closer. Gap that is set too close renders the plug un-serviceable as there is no approved method of widening the gap—and an attempt at widening the gap may damage the center conductor.
Recently it has been discovered that a spark plug’s resistance is a good indication of performance, with values of 500 to 3,000 ohms being good and with values up to 5,000 ohms being acceptable. Tempest offers a handy resistance tester that can quickly determine the serviceability of a spark plug.
Final spark plug testing in a pressurized test unit is necessary after servicing to be sure that the plug does operate under real-time conditions.
KEEPING THE LEAD OUT AND EXTENDING SPARK PLUG LIFE
Lead fouling of spark plugs can be minimized to some extent with the use of some preferred operating techniques and the use of the newest style BY series spark plug (if approved for your specific engine).
Lead fouling is the result of engine operations that don’t heat up the plug tip to a high enough temperature to allow the lead to be scavenged out the exhaust, rather than deposited on the cylinder head and spark plug tip. This can be the result of prolonged idling, extended descents at low power (or even cruising at very low power settings), or idle mixture set excessively rich.
Poorly maintained fuel injection and carburetor units can cause an overly rich fuel mixture and may promote reduced combustion temperatures and excessive amounts of lead in the combustion chamber.
Spark plug life depends on many factors including the overall condition of your ignition system and the fuel delivery system. Contrary to popular belief, magnetos—a mechanical device—do need regular maintenance; this is recommended at five-year or 500-hour intervals. This is not an “if it ain’t broken, don’t fix it” situation. Regular maintenance of magnetos at recommended intervals is necessary.
A good hot spark not only makes the combustion process more efficient, it helps keep the spark plug tip clean. Frequent changes to power and rapid acceleration and deceleration of the engine (i.e., rapid temperature changes) also reduce spark plug life and contribute to lead fouling.
The ideal situation for lengthy spark plug service life would be full-power takeoffs, cruise flight at recommended power settings and leaning according to manufacturer’s recommended procedures. Partial power descents with regular adjustments to the mixture would also be desirable.
If your aircraft exhibits an excessive mag drop when you’re performing a pre-departure engine runup, check it out and get it resolved before flight. Excessive mag drop is telling you something is wrong, so don’t just take off and hope the problem will go away.
The problem might be a fouled plug, but it could be something more significant such as a bad cylinder fouling the plug with oil, or an ignition system issue.
WHERE TO GET INFORMATION AND HELP
Your local FBO/mechanic can be your best resource when it comes to specific information and assistance. Safety is paramount here; plus, it is almost always cheaper to ask a pro if you’re in doubt than it is to make costly mistakes. In any case, get the problem resolved before you fly.
A search of the engine manufacturer’s website can get you some, if not all of the information you need such as approved spark plugs, torque values and spark plug gap. Tempest has a wealth of information on its website. Sacramento Sky Ranch also has information on spark plugs and servicing. Aircraft Spark Plug Service of Van Nuys sells used plugs of all types, including used fine wire/iridium plugs, and services spark plugs at a reasonable price. Spark plug servicing by an aircraft owner can be very beneficial and allows for a better understanding of how your plane works. Learning how to change your spark plugs and keeping some spark plugs on hand may someday save the day, should you have a fouled or otherwise uncooperative spark plug or two.
Michael Berry, a former aircraft repair shop owner, is a multi-engine rated ATP (757/727), an A&P/IA mechanic and airplane owner. Berry has 15,000-plus pilot hours. Send questions or comments to editor@www.piperflyer.com.
RESOURCES
Aircraft Tool Supply Co.
aircraft-tool.com
Sacramento Sky Ranch
sacskyranch.com
Tempest Aerospace
tempestaerospace.com
REFERENCES
Approved plugs – major manufacturers
Lycoming
Service Instruction No. 1042Z
lycoming.com/support/publications/service-instructions/pdfs/SI1042Z.pdf
Teledyne
Service Information Letter 03-2B
tcmlink.com/pdf2/SIL03-2B.pdf
Further reading
Tempest Aviation Spark Plugs
Installation and Maintenance Manual
tempestplus.com/pdf/AA-1710A%20Maintenance%20Manual.pdf
Aero Accessories’ Tempest
spark plug application guide
tempestaerospace.com/Portals/0/PDFs/PlugElegibility062711.pdf
Shell Oil article on lead fouling
https://www.shell.com/home/content/aviation/cq5ex-leftnav_files/technical_talk/techart18_30071600.html
