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"Spark Plugs and The Rotax Engine: Part 2" Sport Aviation / Experimenter "Technically Speaking" Article March 2016

Spark plugs and the Rotax Engine Part 2

In part 1 of this article, we discussed the theoretical aspects of the spark plugs installed in the Rotax engines. In this article, we will take a more in-depth look at the practical aspects and the “how to” of the spark plug in the Rotax engine.

Spark Plug Removal. When removing the spark plugs, during an annual inspection or any time for that matter, keep in mind that there is a lot of information to be had by “reading the spark plugs.” A spark plug rack is a useful way to keep track of the cylinder position of the spark plugs as you remove them. The spark plug rack should be labeled with both the cylinder number as well as top or bottom position. (Figure 1)

If you don’t have a spark plug rack, you can simply make one from a cardboard box by cutting a couple of holes in an X pattern and labeling them with a magic marker. Place the spark plugs with the electrode end facing up so that you can “read” them. In part 3 of “Spark plugs and the Rotax Engine” we will take an in-depth look at  troubleshooting the Rotax engine by “reading” the spark plugs. Keeping track of the spark plug location is not just for identifying potential problems within a particular cylinder, but for more practical reasons as well. We want plugs located within a single cylinder to have nearly identical wear pattern so that the redundant ignition systems will have a similar spark profile. If one spark plug is worn significantly more than the other plug within that cylinder, the ignition system with the worn spark plug will have to work harder than the cylinder with the new spark plug. This will produce an uneven ignition event within the cylinder. If the plugs are to be reinstalled into the engine, they should always be placed back into the same cylinder from which they were removed, and should be kept as a set. Replacement of one spark plug should warrant the replacement of both spark plugs within that cylinder.
“Drop it once, drop it twice,” the second time should be in the garbage can. Never use a spark plug which has been dropped. The potential for micro cracks occurring within the insulator after being dropped is significant enough that it’s not worth the risk. The cracks may not initially be causing a problem, but after several hours of operating time, the heating and cooling cycles may cause the cracks to propagate to the point of causing a plug failure. If you’re removing the spark plugs on a Rotax 9 series engines for the purpose of doing a compression check, remember the engine is going to be hot.  Always wear gloves. The most common reason for dropping a plug is that it’s…  well…  HOT!


Spark Plug Replacement. For the Rotax 9 series engines, Rotax recommends inspection, gapping, and cleaning at 100 hours and replacement at 200 hours. With the exception that replacement is recommended at 100 hours if you use 100 low lead more than 30% of the time. For the Rotax two-stroke engines, the Rotax spark plug application chart lists the NGK BR8ES Spark Plug. (Figure 2) Rotax also recommends the NGK B8ES spark plug that does not contain the internal RFI suppression resistor. If your aircraft contains any kind of avionics or a radio system, you would probably not want to use the non-resistor type of spark plug. However, it is important to recognize that the resistor type spark plugs do reduce the voltage to the spark plug electrodes and typically, you will find that you will have an improved spark with the use of a non-resistor plug. We have seen anecdotal evidence of improved starting performance on two-stroke engines in very cold weather, and with engines mounted inverted when using non-resistor type spark plugs. The two-stroke engines require replacement of plug every 25 hours.

Spark Plug Cleaning. You may be familiar with the spark plug cleaning routine normally associated with spark plugs used in Continental and Lycoming type engines. It is not uncommon that we can use a set of spark plugs, in these type of engines, for 1000 hours of operating time. It’s also not uncommon that we will need to clean these spark plugs on a fairly regular basis. Every 50 to 100 hours of operating time would be a normal interval to be inspecting and cleaning spark plugs on a typical general aviation, training aircraft. The spark plugs used in these type of aircraft can cost anywhere from $30 up to well over $100 each. With these costs, spark plug cleaning becomes a necessary routine. However, when we’re talking about the automotive type spark plugs, which we use in a Rotax engine, it is not considered normal practice to clean these type of spark plugs especially with an abrasive blaster. The cost of the NGK spark plugs are in the neighborhood of $2.50 to $3.50 each. Even the cost of labor to properly clean and test these spark plugs makes it an impractical exercise. There is some anecdotal evidence, which suggests that the use of abrasive blasting media to clean these type spark plugs increases the surface roughness on the ceramic and lessens the spark plugs ability to burn off the carbon build up on the insulator. The Rotax manual is very clear on the subject,  “Attention: Never clean spark plugs with an abrasive cleaner.”  We do know that the use of media blasting does help to round off the edges on the electrodes. It is the sharp edges of the electrode that promote a good clean spark. Cleaning, if at all, should be relegated to the task of spraying with carb cleaner and then blowing them out with an air nozzle.  If you have a spark plug condition in need of more than this, you would probably benefit from new spark plugs. All of the debate about cleaning automotive type spark plugs in the Rotax engine is really kind of irrelevant.  A properly set up, maintained, and operated Rotax engine should never see any problem with spark plug performance in-between the required spark plug inspection or change interval. It is only with engines that have some significant deficiency that require extraordinary spark plug, maintenance.


Gapping the Spark Plug. It is important to note that the part number on the spark plugs are used for a multitude of different engine applications. As a result, the plugs will, typically, not be correctly gapped for your particular application. It is absolutely essential that you check, and if necessary, adjust the spark plug gap. When adjusting the gap on a spark plug, NGK recommends that the maximum adjustment be no more than .008” from the out-of-the-box setting. Adjusting more than .008” will stress the ground electrode or cause a misalignment between the electrodes. Either of these conditions could contribute to poor spark performance. The recommended procedure for checking the spark plug gap is to use a wire type feeler gauge. (Figure 3) This type of feeler gauge is a bit more accurate when the center and ground electrodes are not parallel. Adjusting the spark plug gap is accomplished by moving, (bending), the ground electrode using a plug gapping tool. Spark plug gapping tools can be obtained at most automotive parts stores. The recommended gapping tool is similar to the one shown in (Figure 4). The slots built into the gapping tool should be placed over the ground electrode and very carefully prying up or down to reposition the ground electrode. Make certain that you do not make contact with the center electrode or ceramic insulator. Prying against the center electrode could result in cracking of the insulator. In cold weather, the Rotax manual indicates that gapping to the minimum dimension can assist in engine starting. You might think that you would want to do this on all of your spark plugs, all of the time, but keep in mind the smaller the gap, the easier it is for the spark plug to become fouled and stopped functioning. Vice versa, by gapping the spark plug larger, to avoid spark plug fouling, you can, in turn, make it difficult for the engine to start as well as increase the potential for misfire during normal operations.


Heat Transfer Paste. All of the Rotax 9 series engines call for the use of heat transfer paste on the threads. Last month, in part 1 of this article, we talked about the reason behind the use of heat transfer paste. This is a case where “a little dab’ll do ya.”  We would like to ensure that we have heat transfer paste 360° around the perimeter of the threads. Then, using your finger, try to remove as much of the heat transfer paste as possible. This will leave just the right amount paste within the threads of the spark plug body. (Figure 5) We also like to make sure that we keep the heat transfer paste away from the firing end of the spark plug as it can foul the spark plug if it gets on to the electrodes. Normally, we do not apply any heat transfer paste to the last three threads of the end of the spark plug.

Spark Plug Gasket. We will always be using the gasket that comes with the spark plug when installing the plug on a Rotax engine. Even if were using a CHT probes for example on a Rotax 503, the spark plug and gasket will be installed through the CHT probe heat sink which will be resting directly against the cylinder head. Remember, we are measuring cylinder head temperature not spark plug temperature. This is different than we would typically see on a Continental/Lycoming type installation where the spark plug gasket is removed and a 1/8” thick copper heat sink thermocouple probe would be installed in its place. When installing a new spark plug, during the torquing process, the gasket will squish considerably before the plug comes up to the proper torque. If the plug is removed and reinstalled, it will only take approximately 1/12th of a turn after it is seated to once again come up to the proper torque. Keep in mind, if you’re troubleshooting a problem on the engine, constant removal and installation of the same spark plug can result in the sealing gasket on the base of the plug to be flattened to the point that it no longer is providing an effective seal.

Spark Plug Installation. The Rotax maintenance manuals are very clear about installing the spark plugs on a “COLD” engine only.  Check the condition of the threads before installation of the spark plug. Ensure the spark plug has been gapped according to the Rotax specifications. Install heat transfer paste on Rotax 9 series engines. Then, when installing the spark plug, screw it in by hand until it is seated against the cylinder head. Next, torque to the proper torque specifications listed in the Rotax manual for your engine. Use a properly calibrated torque wrench. And ensure that the spark plug socket is properly aligned with the spark plug  so as not to damage the insulator.

Solid Terminal versus Threaded Terminal.  This is one of those areas where, it seems, everyone has to learn the hard way.  When purchasing a set of spark plugs for your aircraft, you can have the right part number and still end up with the wrong spark plug. There are solid terminal type and there are threaded terminal type. The part number is the same, but it is the stock number that is different. Ensure, when you order, that you are in fact ordering type that you need. The Rotax 9 series engines use the threaded terminal type which comes with a screw on top that can be used with other engines with different types of spark plug caps. This screw on cap should be removed for the Rotax 912 engines. (Figure 6)   If you accidentally ordered the solid terminal type for the Rotax 9 series engine, they are unusable and will not fit the 9 series caps. With the 2 stroke engines, either the screw on cap, or the solid terminal type will work. However, there are many disadvantages to the screw on aluminum cap. Suffice it to say, it is standard practice to order the solid terminal type for all of the two-stroke engines. There been many an engine failure as a result of the screw on cap failing.

We have provided a broad overview of some of the differences in the application and procedures when it comes to spark plugs in the Rotax engine. The underlying theme with the Rotax engine is always the same. The information contained within the Rotax manuals remains your ultimate source for successful use and operation. Rotax has put a lot of effort into giving you guidance and reference material for successful operation of your engine. If you find yourself straying away from the procedures recommended by Rotax, you are probably on the wrong track.  And the information from Rotax regarding the use of spark plugs is certainly no exception. The good news is, the spark plugs used in the Rotax engines are very seldom the cause of any problem, moreover, they, most often, bare the telltale signs and symptoms of other problems brewing inside your engine. In part 3 of this article, we will look at these telltale signs and discuss how to “read” the spark plugs.

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About Brian Carpenter (255 Articles)
CEO Rainbow Aviation / Adventure Aircraft

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