Maintenance Matters

Spark plugs-A window into how your engine is running.


Spark plugs can tell an operator much about how an engine is running. In return they need some regular care to keep them working well. They are at the working end of the ignition system that will include magnetos or electronic ignition systems, wires, and the plugs themselves. Plugs get blamed for many problems, but they often only provide the visible indication of a problem, rather than cause it.

Types of Spark Plugs

Most aviation spark plugs have 18mm threads that screw into the cylinder heads, although some come with 14mm threads, a size more common in automotive spark plugs. The use of actual automotive spark plugs that thread into adapters as parts of electronic ignitions by companies such as Light Speed, E-Mag, and others is becoming much more common in the Experimental world. These plugs are constructed somewhat differently than aviation spark plugs and are much less expensive. Auto plugs are preferred by electronic ignition makers because of their ability to run at wider gaps and thus handle higher voltages.

Massive electrode aviation spark plugs. The standard plug is on the right and the extended nose ‘BY’ plug is on the left.

Massive electrode plugs are best gapped with this special tool available from many aviation vendors. The screw device on the top presses the ground electrode closer to the positive electrode.

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Among aviation spark plugs there are massive electrode plugs (most common), extended reach (a type of massive electrode plug), and fine-wire or iridium. Plugs are further distinguished by how long the threaded portion is (reach), heat range, and the threads on the barrel (where the plug wire attaches). Champion distributes a publication called Aviation Service Manual AV6-R that explains their numbering system and contains much useful information about their spark plugs and oil filters. It is usually available for free at any airshow where Champion is present, or they will send it to you if you contact them. The main thing to remember is to run the recommended spark plug in your engine. Using spark plugs of different heat ranges or thread lengths can cause problems, some of them serious.

A common plug type is Champion’s REM38E. The R stands for resistor, the E stands for a 5/8-inch 24 thread on the barrel, the M stands for -inch reach (length of threads into the cylinder head), 38 is the heat range, and finally the last E stands for two massive electrodes. A higher heat range number would be a hotter plug, and a lower number would be colder. If the last E were an S, it would be a fine-wire or iridium plug, and if it had BY at the end instead of E or S it would be a massive electrode plug with an extended core nose.

Normal spark plugs recently removed from a smoothly running engine.

Tempest uses the same numbering system except their numbers have a U added at the beginning. Thus a Tempest UREM38E is equivalent to Champion REM38E. Of course, both manufacturers insist that their plugs are better than, rather than equivalent to, the other brand. I’ve had good luck with both brands, the main difference being the cost advantage held by Tempest.

This combination spark plug tester and cleaner by Aircraft Tool Specialties runs about $600. It uses compressed air from your shop compressor to test the spark plugs under pressure. This is considered the definitive test of whether or not a plug is good. The plug cleaner portion is on left.

Maintenance Time

Spark plugs should be cleaned and gapped at each yearly condition inspection or every 100 hours if you are fortunate enough to fly more than 100 hours per year. They should also be checked whenever you fail to get a good mag check during runup. Plugs should be replaced when half or more of the electrode has been worn away by sparking and corrosive combustion gasses. An easy way to see this with a massive electrode plug is to note the oval shape of the center electrode. When the oval is half as wide as it is long, it is time for a new plug. You can also buy a gauge that will take any guesswork out of the process. With a fine-wire or iridium plug, it is time for a replacement when the wire is worn halfway through or more. BY plugs are finished when the ground electrodes are worn to a knife-edge point and the center electrode has a noticeable wear ring below the tip.

Plugs should also be replaced if a center electrode insulator is cracked or after they have been dropped on a hard surface, which may cause internal insulator damage. A word to the wise: place something soft under your engine when you are removing your plugs, especially iridium plugs, just in case you get a case of the butter fingers. It is a bitter pill to swallow to throw away an iridium plug ($70 to $95 each) just because you dropped it.

Mechanic Ed Zaleski uses a Champion plug gap gauge to set the gap on a spark plug.

Needless to say, if your plugs are not firing well, even after cleaning, no matter what the wear, it is time for new ones. A spark plug tester that places the plug under pressure is the best way to test a plug, but a cheaper way is to measure the resistance through the center electrode with a standard ohmmeter. Any plug with resistance over 5000 ohms should be replaced according to Tempest, with 1000-1500 ohms being common readings for good plugs. Champion does not recognize this as a valid test, saying that the resistors in their plugs are voltage dependent. That means that a resistance reading at the voltage a typical multimeter would provide is not the same as the 10-20,000 volts an ignition system would make. Unfortunately Champion’s spark plug tester and cleaner costs $1,790 retail. Even a low-cost copy costs about $600. Unless you have a shop where you are cleaning and testing plugs all the time, the ohmmeter test will have to do.

An array of tools and materials needed to service spark plugs. Included are: spark plug tray, torque wrench with plug socket, spark plug anti-seize compound, fine-wire plug-gapping tool, copper spark plug gaskets, spark plug gap gauge, and a massive electrode gapping tool.

Spark plugs clean up nicely with media blasting, either in a blast cabinet or by using one of the popular cleaning and testing units available from your favorite aviation parts and tools vendor. The main thing to remember with media blasting is to take it easy. Excessive blasting will erode the insulator around the center electrode, leading to premature failure.

The best way to measure the spark plug gap is to use a wire-type spark plug gauge such as the Champion CT-450 or a similar one by ATS. Most aviation spark plugs should have a gap of somewhere between .016 and .021 inches. Thus the .015 wire should fit between the ground and center electrodes, but the .022 wire should not. Be sure to consult your plug manufacturer’s literature for the correct gap for your engine and spark plugs. The Champion Aerospace Aviation Catalog AV-14 has an extensive table showing spark plug gaps. It is available online at: or in paper form through Champion or Amazon. Do not use machinists’ flat feeler gauges to gap massive electrode spark plugs because of the curved gaps they employ.

Champion and ATS make special tools for adjusting spark plug gaps. While it is possible to adjust gaps with ordinary tools, it is not a good idea, because it is very easy to damage plugs with improper tools and techniques. Fine wire plugs are especially prone to damage from improper gapping due to the brittleness of iridium used in their electrodes. Aircraft Spruce and other aviation vendors have spark plug gapping tools starting at around $10.

Changing and Reading Plugs

Removing spark plugs is simple enough, but there are a few steps you can take that make the process better than just grabbing a wrench and taking the plug out. It is common for a little dirt and debris to collect around the base of the top plugs, so a little squirt of compressed air can help to remove this material and keep it out of your engine. When removing the plug wire, you typically use a -inch wrench to loosen the cap, but if you take the time to also grab a 7/16-inch wrench and hold the center portion, you will save some wear and tear on the wires. A spark plug tray is a must for keeping track of which plug came out of which cylinder. This is important because plugs need to be rotated at each changing to even out the wear caused by different polarities in the ignition system.

These plugs show signs of lead fouling. Note the tiny balls of lead inside the plug between the body and the insulator. A dental pick is the best tool for removing lead balls such as these.

With all the plugs out, it is time to look things over and see what’s happening in each cylinder. Plugs should be a caramel color with the edges often somewhat darker than the tips. There should be no signs of oil, lead, or carbon fouling, no cracked electrodes, and no excessive gaps between the center and ground electrodes. What is more common in the real world is that some plugs will show signs of carbon or oil fouling and others will look pretty good. Some or all plugs may show signs of lead accumulation down inside the plug between the center insulator and the body of the plug. You will have to decide how much of each of these problems you are willing to live with. Less is better, but as an engine ages, it is pretty common to see more and more of these things show up on your spark plugs. Lead fouling is almost a universal problem unless you have access to unleaded auto gas without ethanol. Few people are so fortunate.

These plugs are oil fouled. It is pretty obvious when plugs are oil fouled because they are oily. It is less obvious what should be done about it.

Carbon fouling is a sign of running too rich. Aggressive leaning will usually cure this problem completely, unless the plug is simply not firing well. With a carburetor, where even exhaust gas temperatures are difficult to achieve, it may not be possible to cure the problem 100%, but aggressive leaning during taxiing and proper leaning in cruise will help a lot. Running lean-of-peak will usually produce a spark plug color that is lighter and cleaner than more traditional rich-of-peak operation. Excessively rich running can cause carbon build-up in the cylinders and contribute to sticking valves, in addition to simply wasting fuel. For these reasons it should be avoided.

A mechanic uses a dental pick to remove lead from a spark plug. Carefully pick out lead fouling and never try to remove lead with just blast cleaning.

Oil fouling is more common on older engines and especially on the bottom plugs. It is caused by oil getting into the cylinders through worn rings and cylinder walls. It can also come in through worn intake valve guides. The only sure cure for this is to overhaul the offending cylinder, but if the engine is running well and plug fouling is manageable, it may be acceptable to wait until it gets worse. If you are really worried, a borescope examination is the next step. With that information you will be able to make a better decision. This becomes a judgment call that you must make yourself. A consultation with a trusted mechanic may be in order. The spark plug can tell you about the problem, but obviously, a clean plug will not cure it.

Lead fouling will be with us until we get lead out of aviation gasoline. Little balls of lead can short out plugs and cause them to fail. It is best to remove these lead deposits with a dental pick that will fit down into the tight space between the center insulator and the plug body. Carefully pick out each piece and then blast the plug clean. Do not try to remove lead fouling by blasting alone. If lead fouling becomes a real nuisance, a switch to extended-nose BY plugs may provide some relief. Your dentist will often have used picks that will work for you but are no longer suitable for his work. That can be the silver lining in your next teeth cleaning experience.

This chart shows Champion’s recommended rotation for spark plugs in four- and six-cylinder engines to minimize the effects of polarity on the spark plug wear patterns.


Once the plugs are properly cleaned and gapped, it is time to reinstall them. Every time spark plugs are changed they need new copper gaskets. These gaskets are inexpensive, so get a bag of them and store them with your spark plug tools. Champion says that annealing will not fully restore these gaskets, so it is best to just toss them out after they have been used. Plus, threads need to be treated with anti-seize compound before installation. Do not forget this important step. A seized spark plug can be a real headache to remove and potentially damage your cylinder head. With anti-seize and a new gasket in place, the plug should be threaded by hand into the head. If it does not go in easily remove it and find out why. The threads on the plug and in the cylinder head need to be clean and undamaged. You can use a small triangular file to touch up the threads on a spark plug, but you will need a thread chasing tool to clean up cylinder head threads. The thread chasing tool is much preferred over a machinist’s tap for this work. The idea is to simply straighten out damaged threads, not cut new ones. If spark plug threads are extensively damaged, you will need to get a new spark plug. If cylinder heads will not clean up with a thread chaser, you will need some professional help. In any case, do not force a spark plug into your cylinder head. Fix what you can, but also have the good judgment to know when the problem is bigger than your ability to solve it.

If a spark plug will not easily thread into the cylinder head, it may help to restore the threads in the cylinder head with a spark plug thread chasing tool. This is much preferred over a standard tap.

This plug shows 1446 ohms resistance through the positive electrode using a standard multimeter. This is well within the acceptable range. Plugs with more than 5000 ohms resistance should be replaced according to Tempest.

Mag Check

The mag check during runup is the standard test in the field for spark plug performance. After all, no matter what the plugs look like or how they test on the bench, if they don’t work during a mag check, their performance is simply unacceptable. However, not all poor mag checks come from bad plugs. As the name implies, a mag check is also a test of the magnetos. If your mags are approaching or past 500 hours since their last inspection and/or overhaul, you may well have a magneto problem that no amount of new spark plugs will cure. Get out your logbook and see when your magnetos were last serviced or replaced. It may be time to pull them off and give them the once-over.

Minor magneto work such as replacing the points and condenser, and setting the internal timing are well within the capabilities of most amateur builders and can be done without a lot of expensive special tools, but more serious work will be easier to pass off to a repair station.

A multi-cylinder EGT is a great way to track down a spark plug or plug wire problem. When you do your mag check, note the EGTs for each cylinder. A misfiring cylinder will have a lower EGT than the rest. That will save you removing and checking all your plugs. By switching from one mag to the other, you can pin the problem down to one spark plug fairly easily.

Plug wires do not often go bad, but they can. If you have a weak cylinder, but the plugs check out fine, you have two options. Switch the plug you suspect with one that you know is good. If the problem follows the spark plug, you know you have a bad plug. If the problem remains, you most likely have a bad plug wire. You can also measure the resistance of several wires and see if one of them has much higher resistance than the others. If it does, it probably needs to be replaced.

These plug wires show signs of deterioration from years of service and possibly some not-too-careful plug cap removal.

Don’t forget that other things unrelated to the ignition system can also cause a cylinder to run poorly. Clogged injectors, burnt or sticking valves, and intake tract leaks are other possible culprits, but those are topics for other articles.

For more information see:

Champion Aerospace Spark Plugs
Champion publications Aviation Service Manual AV6-R and Aviation catalog AV-14
Tempest Aviation Spark Plugs
Tempest Aviation Spark Application Guide

Dave Prizio is a Southern California native who has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and he is helping a friend build a fourth, an RV-8. When he isn’t building something, he likes to share his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also a member of the EAA Homebuilt Aircraft Council.


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