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Spark Plug Servicing

Spark Plug Servicing

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According to FAR Part 43 Appendix A, “Major Alterations, Major Repairs, and Preventive Maintenance,” aircraft owners are permitted to service the spark plugs in their engine. The following should help owners get more acquainted with this task.
The short list of removal and reinstallation tools include a six-point 7/8-inch deep socket; a ratchet wrench and extensions that fit the socket; either a ¾-inch or 7/8-inch open-ended wrench to remove and reinstall the spark plug high-tension leads; a torque wrench to insure that the plugs are tightened properly during re-installation; and some anti-seize compound. Additional tools will include a spark plug rack and replacement copper gaskets.
During removal, insert each plug in the corresponding position (e.g., “1T” for #1 spark plug top; “2B” for #2 spark plug bottom—use the spark plug numbering system for your engine) in the rack. You need to know which cylinder each plug was removed from so you can “read” the plug symptoms and trace the symptoms to the correct cylinder and so the plugs can be rotated before reinstallation.
Fancy metal racks can be purchased, or a homemade one can be built by boring holes into a suitable board. All that’s required for 90 percent of the GA engines is two rows of six holes—one row for the top plugs, and a corresponding row beneath the top row for the bottom plugs.

After removal, the plugs should be inspected for damage, carbon fouling, lead fouling, (which appears as small, hard, glass-like balls); oil fouling and extent of electrode wear.
Many technicians and owners are unaware that some ignition problems are being traced to out-of-tolerance internal resistors in individual spark plugs. To detect these resistors each plug must be tested for electrical resistance between the internal center contact (down at the bottom of the barrel) and the center electrode. Plugs with a resistance of more than 5KΩ are out of spec and must be replaced. This is best done with an analog (moving needle) type of meter—digital meters don’t pass enough current through the plug internal resistor to effectively conduct this important test.

Plugs that have excessive internal resistance may cause burning of the primary points in the magneto that fires the affected plug, and can cause engine damage due to ignition misfires.
Carbon-fouled plugs indicate that the fuel injection or carburetor idle mixture setting is too rich, or that the pilot is not leaning the mixture during idle and taxi power settings.
Oil fouled plugs indicate a worn cylinder bore, loose valve guides, or annealed rings.Oil fouling may be lessened 
by installing a slightly “hotter” spark plug if one is approved for installation.
Lead fouling can also be lessened by leaning the mixture during taxi, idle and other low-power engine operations.
Learning to interpret the “tale of the plug” is not difficult. A primer for learning this skill is the Champion Spark Plug Service Manual AVR-6 and from Tempest Plus, makers of Tempest spark plugs. (The Tempest Plus manual is in production but should be available by the time you read this or soon after. —Ed.)

When I set up my aircraft maintenance shop in Alaska I purchased specialized spark plug tools including a Champion CT-907 S “Plugmate” 7/8 socket. This socket has a magnetic plate that prevents plugs from slipping out of the socket. Plugs that are dropped must be replaced since it’s impossible to determine if the porcelain insulator inside the plug has been cracked or damaged after a drop.
The Champion socket retails today for about $65. Approach Aviation (www.approachaviation.com) and Aircraft Tool Supply Co. (www.aircraft-tool.com) also sell plug sockets with magnetic holders.

Do I Clean, or . . . . ?
Once the plugs are removed and inspected there’s a decision to be made. Should money be spent to purchase spark plug cleaning tools, or should the money be spent sending the plugs to a plug cleaning service?
Aircraft Tool Supply Co. (ATS) has made this decision easier by developing and marketing a wide range of spark plug servicing equipment. Its version of the Champion magnetic spark plug socket sells for $25. Its spark plug cleaning and testing unit costs a bit over $545 compared to the Champion cleaner/tester that retails for just over $1,800. ATS also sells what it calls a spark plug field service kit (SPK001) which is sufficient for an owner’s needs for a little over $160. Its deluxe spark plug service kit (SPK-DX) which is used by many shops costs $700.

Spark plugs can also be sent to Aircraft Spark Plug Service Inc. in Van Nuys, Calif. for reconditioning. According to Jeanne, the owner, each plug gets cleaned and gapped, then tested to check for a hot spark under a 200 psi air pressure—which is a much higher pressure than is available in most shops or from small air compressors—before being painted and shipped with a new gasket.
You get your own spark plugs back; this is not an exchange service. The cost is $3.75 a plug. Total cost (less shipping) for a four-cylinder engine is $30; a reconditioned set for a six-cylinder costs $45.

Rotation is the Name of the Game
Each time a spark plug fires, material is transferred from the ground electrode to the center electrode, or vice versa. This is due to electrical polarities produced in the magnetos. To compensate for this transfer, and to equalize wear between the center electrode and the ground electrode(s), it’s best to rotate the plugs whenever the plugs are removed, or every 100 hours. This is called rotating the plugs.
An illustration in the Champion Spark Plug service manual mentioned earlier gives the following formula for rotation. 1 Top (T) to 4 Bottom (B); 2T to 3B; 3T to 2B; 4T to 1B; 1B to 4T; 2B to 3T; 3B to 2T, 4B to 1T. That’s it for four-cylinder engines. The proper plug rotation is illustrated on every box of Tempest spark plugs as well.  

There’s an easier way if you’re using a rack. Commercially-produced racks have numbers for the plug position in the engine stamped in the metal—plugs removed from the top hole in each cylinder are put in the holes above the numbers and plugs removed from the bottom holes are put in the holes below the numbers. As each plug is cleaned and tested, put it back in the same hole in the rack.
Ready to rotate the plugs? Pick up the rack and rotate it 180 degrees. This puts the #4 bottom plug in the #1 top position if the rack hadn’t been rotated. Ignore the numbering on the rack by imagining that the plug in the top left position is the #1 top plug, and so on. It works every time—even for six-cylinder engines.
New Gasket, or Anneal?

A soft copper gasket at the base of each plug provides a seal. This gasket is installed on each plug before installation. Used gaskets become work-hardened and don’t provide a good seal. New gaskets can be purchased for around 60 cents each. Existing gaskets can also be reused after they are annealed. Annealing a gasket consists of heating it to a dull red color—slightly cooler than cherry red is sufficient—and immediately quenching the hot gasket in water.
A video by the Experimental Aircraft Association (EAA) shows the steps of this process. View it at www.eaa.org/video/homebuilders.html?videoId=57437956001. (Tempest does not recommend annealing gaskets and recommends all gaskets be replaced each time the plug is removed. —Ed.)

If a cylinder head temperature (CHT) gasket-type thermocouple is installed on a plug, do not install a copper washer on that plug.
After installing new or annealed gaskets on each plug, paint a small amount of anti-seize compound on the second and third threads up from the firing end of the plug. Don’t overdo it; a little swipe is all that’s needed.
After screwing the plugs into the cylinders, use a calibrated torque wrench to tighten each one to the proper torque. Spark plugs installed in Continental engines are torqued to 300 to 360 inch-pounds (25 to 30 foot-pounds) and plugs installed in Lycoming engines are torqued to 420 inch-pounds or 35 foot-pounds.

Plug Leads
Clean each plug connector with acetone or rubbing alcohol. Inspect the contact spring for any signs of erosion or contamination. Insert the connector straight into plug barrel and tighten the nut finger-tight. The Champion service manual suggests tightening each nut an additional 1/8 turn. Lycoming service information advises 80 to 90 inch-pound torque for 5/8-24 lead nuts and 110 to 120 inch-pounds for the ¾-20 nuts. Always use a torque wrench to insure proper torque values. Over-tightening can damage the insulator.

The Logbook Entry
All preventive maintenance by owners must be logged. An example of a good log entry for this plug removal, inspection, cleaning, gapping, testing, and reinstallation process would contain the details spelled out in FAA Advisory Circular (AC) 43-9C.
For instance: “5-25-2010 – Removed, cleaned, gapped, tested, rotated and reinstalled all spark plugs in accordance with Champion Spark Plug Service AV6-R, revised November 2004.”
Although it’s not required, it’s a good idea to also note the tach (or Hobbs) time with the entry. Then sign your name below the entry with your pilot’s certificate number and your rating—“PP” for private pilot; “CP” for commercial pilot and “ATP” for airline transport pilot. That’s all there is to it.

Steve Ells has been an A&P/IA for 38 years and is a commercial pilot with Instrument and Multi-Engine ratings. Ells is the owner of Ells Aviation (www.EllsAviation.com) and the proud owner of a 1960 Piper Comanche. Send questions and comments to .


Aircraft Plug Service, Inc.
Phone (818) 787-5680

Aircraft Tool Supply Co.

Approach Aviation

Champion Aerospace, LLC

Tempest Plus Marketing Group, LLC

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