Don't forget your plugs
"One of the jobs frequently assigned to the inexperienced airplane mechanic is the cleaning and overhaul of spark plugs. While this job may appear to be unimportant, such is by no means the case. The spark plug is truly the heart of the engine and if it fails to function properly, the engine will likewise fail."
1939, Daniel J. Brimm and H. Edward Boggess
It's a tough job
Spark plugs are the great equalizer. It doesn't matter whether a pilot flies a 65-horsepower Aeronca Champ or a 760-horsepower twin-engine Beech Duke, spark plugs cost the same. If aviation spark plugs seem expensive, consider the conditions each plug must endure between normal service intervals. Plugs must provide a dependable spark under outside air temperatures that vary from 20 degrees Fahrenheit below zero to plus 120 above, at combustion chamber gas pressures of up to 2,000 pounds per square inch and combustion temperatures as high as 3,000 degrees F. Voltage at the terminals can range up to 18,000 volts and, between 100-hour inspections, each plug will fire approximately 6 million times.
Typical massive-electrode-style aircraft spark plugs — which can be likened to the Chevrolet or Ford of plugs — can be purchased on the street for between $16 and $20 each. The Cadillac of plugs is the iridium spark plug, also known as the fine-wire spark plug, which is available from aviation parts warehouses for between $45 and $65. The effective life of a massive-electrode plug ranges from 200 to 250 hours when used in high-performance engines and up to 500 hours when used in low-stress, low-compression engines. Fine-wire plugs have an effective life of at least 1,200 hours in high-performance engines and can provide good service up to 2,000 hours and beyond when used in low-compression engines.
It's pretty well known that fine-wire spark plugs are superior because they require less frequent service, have a better resistance to fouling, and provide a better economic value than massive-electrode plugs. RAM Aircraft, of Waco, Texas, has published flight-test data proving that fine-wire plugs also reduce fuel consumption by being "around 2.2 percent more efficient" than massive-electrode plugs. RAM attributes this to the fact that the size of the electrodes in massive-electrode-style plugs shields some of the spark energy from the surrounding fuel/air mixture. Because of the long life, increased efficiency, and absence of high-altitude misfire reported by its customers using fine-wire plugs, RAM recommends them to all its customers. Study after study has proven that fine-wire plugs offer a cost-per-hour savings over massive-electrode plugs. Yet, few general aviation pilots seem willing to spend the up-front cash needed to equip their engines with these plugs.
Short- and long-reach factors
There are four variables that play into choosing the correct plug. There's the massive-electrode/fine-wire choice mentioned previously. Then there are a few others.
Aviation spark plugs are referred to as either long reach or short reach. Before I explain what those terms mean it's important for you to understand that aircraft engine cylinders vary widely. High-compression cylinders used on high-performance engines must withstand higher combustion pressures during operation so the aluminum heads of these cylinders are beefier and thicker than those of lower-performance (low-compression) cylinders. Therefore, the spark plug boss — a boss is a steel-threaded insert that's installed in the aluminum head to prevent wear in the head — must have more depth than the boss in low-compression heads. Typically, the threads on the electrode end of a long-reach spark plug are five-eighths inch deep and the threads on the electrode end of a short-reach plug are three-eighths inch deep.
However, like most rules there are exceptions. Textron Lycoming Service Instruction 1042X — Lycoming's service publication on spark plugs — warns some operators: "The same engine model may use short- or long-reach spark plugs." Lycoming engines that require long-reach spark plugs can be identified by a patch of yellow paint on the cylinder fins between the spark-plug hole and the rocker cover. Short-reach plug applications do not have paint identifying marks. The latest Teledyne Continental Motors (TCM) service information on spark plugs is in Service Information Letter 03-2B.
Installation of the wrong reach plug will create big problems. If a long-reach plug is installed in a short-reach cylinder, the nose of the plug will project down into the combustion chamber, where it will quickly overheat, prompting preignition.
If a short-reach plug is installed in a long-reach cylinder, the threads in the plug boss will become contaminated with carbon and lead deposits. This crud will have to be cleaned before the proper reach plug can be installed.
The heat-range puzzle
Aircraft spark plugs also are rated by heat range. A high number in the part number signifies a hotter plug than does a lower number. Spark-plug and engine manufacturers print service instructions specifying which plugs are approved for installation. In some cases there may be more than one plug on that list — in almost every case the only difference between the plugs is the heat range. The heat-range number signifies the ability of the plug to transfer heat from the firing end of the plug to the cylinder head. A hot plug has a longer insulator tip than does a cold plug. The longer tip makes the heat travel farther before it's transferred through the threads of the plug to the cylinder head. Cooler-running engines use hotter plugs.
Colder plugs (those with lower numbers in the part number) are used on higher-performance engines, which typically achieve their higher output by turbocharging or with increased compression ratios. For instance, the Champion plug specified for a turbocharged TSIO-520 engine is an RHM 32E and the Champion plug for the carbureted TCM O-470R engine is the RHM 40E. When the compression ratio was raised from 7.0-to-1 in the O-470R to 8.6-to-1 in the O-470U engine, the plug was changed to the colder RHM 32E.
Different heat-range spark plugs are needed to maintain the desired spark-plug insulator nose temperature. The center electrode of aircraft spark plugs is surrounded by a special super-hard ceramic material. The correct heat-range spark plug maintains insulator nose temperatures above 900 degrees F (when idling) and below 1,300 degrees F (at takeoff power). This prevents the formation of deposits that cause lead fouling at idle speeds, and overheating at high power settings.
A visual inspection of the firing end of a spark plug is the best method of determining if the plug heat range is correct. If the end of the plug has small, hard glassy-looking globules (which consist of lead from the fuel) between the center insulator and the outer shell, this may mean a colder plug is needed to raise the idle temperature of the plug tip. But this could be a red herring. If the pilot doesn't know how to lean the mixture during ground operation and/or the idle mixture is set too rich, the first step is to counsel the pilot on ground-leaning procedures and adjust the idle mixture setting before changing the heat range of the spark plug.
If the plug has a very white-colored insulator tip the plug may be too cold. In very extreme cases the plug may even have become hot enough to slightly melt or soften the edges of the electrodes. Again, the plug may not be the problem. Make sure that the fuel-injection-system takeoff fuel pressure settings and the ignition timing are correct before changing plugs. As an aid to diagnosing spark-plug heat-range issues, Champion Aerospace manufactures a visual aid called Check-A-Plug (Champion part number AV-27). These are usually given away by Champion at the big airshows, or can be obtained by contacting a Champion Aerospace area sales representative and asking for one.
This whole heat-range equation can be summed up by looking at a special spark plug created for the Lycoming O-235 L2C engine developed for the Cessna 152. When first introduced, this engine kept technicians very busy cleaning spark plugs. The engine had the unprecedented characteristic of extracting what seemed like all of the tet-raethyl lead in the fuel before depositing it on the firing end of the recommended RHM 40E spark plug. Champion quickly developed the RHM 37BY, a colder plug with an unusual-appearing extended tip. This plug was designed specifically to deal with the lead-fouling problem in that particular engine. It has done that job so well that it's now approved for installation in many low-compression engines that need help preventing lead fouling.
The last of the spark-plug variables is the barrel style. This is most easily identified by the size of the wrench needed to loosen the ignition harness nut. There are two sizes. The smaller of the two has a five-eighths-inch-by-24-thread nut and requires a three-quarter-inch wrench. The larger has a three-quarter-inch-by-20-thread nut and requires a seven-eighths-inch wrench. The larger of the two is called an all-weather plug, but both types have the same sealing provisions.
Servicing tips
Since the spark plug is at the heart of the engine, it must be serviced correctly. There are some nonnegotiable rules that govern plug maintenance. That insulator material that's hard is also very brittle. If the plug is ever dropped from any height don't even think about using it. Just throw it away. Better yet, destroy it with a hammer before throwing it away so no one else will be tempted to use it. Cracked insulators compromise the heat-transfer mechanism for the center electrode. When the center electrode gets too hot it causes preignition of the fuel-air charge — preignition means the fuel-air charge is prematurely ignited by something other than the spark plug. This ignition always takes place before the piston has reached optimum position on the compression stroke. The result is always a loss of power that has the potential to lead to serious cylinder damage.
Both TCM and Lycoming service data state that new copper sealing gaskets must always be used when installing spark plugs. These can be purchased from supply houses. These gaskets provide one of the most important heat-transfer paths for the spark plug. With use, these gaskets harden and deform, which compromises spark-plug cooling. Before installing plugs, lubricate the threads with a small amount of anti-seize compound, making sure that the compound is not applied to the thread closest to the firing tip.
The threads of the spark-plug boss in the cylinder head should be clean before installing spark plugs. The best way to test for cleanliness is to screw the plug into the boss by hand — if the plug can't be run almost all the way down into the boss, the threads are dirty and a special spark-plug thread-chaser tool should be used to clean them. Another alternative is a stiff-bristled toothbrush. Apply a little grease to the toothbrush first so the debris that's scrubbed off the threads sticks to the grease instead of falling into the combustion chamber. And always use a torque wrench when installing a spark plug. Lycoming specifies a spark-plug torque of 420 inch pounds and TCM recommends 300- to 360 inch pounds. Over-torquing results in the threads of the plug barrel extending down into the combustion chamber. Overheating and preignition can result.
Spark plugs wear during operation when material is transferred from the center electrode to the shell electrodes, or vice versa, during each spark event. Because of the peculiarities of magnetos and the firing order of the spark plugs, the spark polarity at each spark-plug lead is always the same. The direction of transfer depends on the position of the plug in the engine. To get the maximum life from a plug, this transfer has to be reversed by rotating the plugs at regular intervals. One method is to transfer each plug to the next position in the engine firing order, and move it from the top to the bottom plug hole, or vice versa. For example, the firing order of a TCM O-200 engine is one, three, two, four. Rotating these plugs would require the top plug of cylinder number one to be installed in the bottom plug hole of cylinder number three, and then the bottom plug of cylinder number one to be moved to the top plug hole of cylinder number three, and so on.
Old-time auto mechanics say that the act of buying and installing a set of spark plugs is the least expensive tuneup available. Airplane spark plugs are a little different because of the cost — substitute regular maintenance for buying a new set and the saying that prevention is the best cure is still valid.
E-mail the author at [email protected].
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