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Frequently Asked Questions

How do I get hold of the Rotax approved documentation for my aircraft engine?
Visit http://www.flyrotax.com and select your aircraft engine to download the approved documentation.
How Often Should I Change the Spark Plugs?
The Line Maintenance Manual stipulates that the spark plugs should be changed every 200 hours. It further states that the plugs should be changed at 100 hours if you are using 100LL more than 30% of the time. This is because you will get lead oxide buildup on the plugs with the use of leaded avgas. The 100 hour checklist also, however, requires the cleaning, inspection, and gapping of the spark plugs. Since the price, for a set of 8 spark plugs, is less than $25, we recommend just changing them instead of spending all that time messing with the old ones. About the only way to reasonably clean the plugs is with a sand blasting box, and with the surface that process leaves on the plugs, they’re just going to load up more quickly after being cleaned.
A couple hints on installing the plugs:
Always use heat transfer paste (Rotax pn 897 186) on the threads. This helps transfer heat from the plug to the head and prevents the plug from becoming a hot spot that could cause pre-ignition. A small amount – about quarter the size of a pea – applied 2 or 3 threads from the end works well. Leave the first couple rows of threads clean, and there’s no need to spread the dab of paste – spinning the plug into the head will spread it quite adequately.
Always spin the plug all the way in by hand. Then tighten with a torque wrench. People who put a wrench on the plug, without spinning it in first, are risking damaging the threads on the head. The only Rotax authorized repair is replacing the head – and you don’t want to know how much that costs.
Always return plugs – that have already been used – to the same hole from which they came.
Never torque spark plugs into a warmed head. Always allow the cylinder head to return to room temperature before torquing the plug. To do otherwise is to take great risk with the threads in the head the next time the spark plug is removed. Allow me to remind you of the comment two paragraphs up.
What Oil should I run in my Rotax 4-stroke engine?
The oil that you want to use depends, to some extent, on what fuel you are using. If you are running exclusively automotive fuel, you can use a synthetic oil and go 100 hours between oil and filter changes. If you are running 100LL some or all of the time, you will want to change the oil you run based upon the 100LL use. Synthetic oil does not do a good job of suspending the lead and getting it from the engine to the oil tank.
If you are running automotive fuel most of the time, and go on a cross country, 3 or 4 times a year, that is long enough that you need to top off with 100LL to get home, you needn’t worry about changing the oil you use just for that. If you are running 100LL 30% of the time or more, Service Instruction SI 912-016 stipulates that you drop your oil and filter change interval to 50 hours. It would also be wise to change to an oil that is at least partially petroleum based.
If you are running Avgas 50% of the time or more, the same service instruction recommends that you drop your oil change interval to 25 hours. At that point you may want to go to an entirely petroleum based oil just because of the cost factor.
The oils that we recommend the most for those three categories are Mobil 1 Racing 4T (fully synthetic), AeroShell Sport Plus 4 (semi-synthetic), and Pennzoil Motorcycle Oil (a petroleum based oil).
SI 912-016 lists the requirements for oil quality, and also has some specific recommendations for oil brands and products. In all cases, and in all conditions we recommend that you stick with a motorcycle oil. Motorcycle oils are formulated for use in engines that have an integral gearbox and wet clutch – and that’s exactly what you’ve got in your Rotax. The only exception to that is AeroShell Sport Plus 4.
Sport Plus 4 was developed in an effort by both Rotax and Shell to produce an oil whose formulation was stable and well suited for the Rotax 4-stroke aircraft engines. It is, by the way, the only AeroShell product that is suitable for these engines. Do not use any other AeroShell product in your Rotax. The others are all intended for traditional aircraft engine types and will not treat your Rotax well.
What Fuel Should I Run In My Rotax Engine?
Rotax aircraft engines have all been designed to be run on automotive fuel. The lower compression ratio engines (503, 582, 912UL, 912F) can all be run on 87 AKI fuel or better. The higher compression ratio engines (618, 912S, 912ULS, 914UL, 914F) should all be run on a minimum of 91 AKI fuel. The 914s are not actually high compression ratio engines, but they are boosted by the turbocharger so they will experience some of the same issues.
AKI (anti-knock index) is the performance number that you will find on the pumps when you pull in to fill your automobile. The AKI is actually an average between the RON (research octane number) and the MON (motor octane number). The RON and MON are just the results from standardized testing that is done on the fuels to determine the fuel’s ability to resist knocking (detonation) in certain conditions while being burned in an internal combustion engine.
All these engines will run just fine on 100LL avgas. The presence of tetraethyl lead in avgas, however, will cause the need for adjusted maintenance intervals as there will be resultant lead oxide deposits in various parts of the engine.
What Do I Do About Ethanol Fuel?
According to Rotax Service Instruction SI 912-016, you are allowed to use a fuel with up to 10% ethanol (E10) in your Rotax aircraft engine. The caveat is that you check the airplane to be sure that it can deal with the ethanol. You may have to check with the manufacturer, but the fuel tanks, the fuel lines, fittings, gaskets, fuel filters, all must be able to deal with the ethanol. The engine doesn’t have a problem with ethanol fuels of this percentage, but if components in the aircraft do, those problems can float downstream and cause problems with your engine.
An advantage of an ethanol blended fuel is that small amounts of moisture will not cause problems. The ethanol will absorb the moisture, and the water will flow through with the fuel. It will have no impact on the combustion process and you won’t even know it’s there.
If, on the other hand, water gets into the fuel from a leaky filler cap, or access panel, or due to condensation in a tank with a low fuel state, you can get enough water to get what is called “phase separation”. Over a certain percentage of water, the ethanol has more affinity for the water than it does for the gasoline. Both, then, fall out of solution with the gasoline. If phase separation occurs, you will be able to see it when you sump the tanks on preflight. You can shake the wings and keep sumping until the separated material is gone. The good news is that the water is gone. The bad news is that the alcohol is gone as well. This will mean that you no longer know the octane performance number of the fuel that remains, because the fuel blender will invariably have counted on the octane boosting capability of the ethanol when blending the fuel. You will need, in this case, to drain the fuel from your aircraft, put it in your SUV – because it doesn’t care, and start with a fresh fuel charge in your airplane.
Is this common? No. Of all the people we’ve trained on this engine (well over 400) we’ve had two people that have experienced phase separation. On the other hand, you want to be aware of what kind of fuel you have in your airplane so that, should you find water in your sample tube, you know whether or not the octane performance number on your fuel has changed.
Can My Engine Go to the 2000 Hour TBO?
Technically, all the Rotax 4-stroke engines can go to 2000 hour TBO. But to do so, all the applicable service bulletins must have been complied with – and that’s not economically practical on most engines. Service Bulletin SB 912-057 is the document that stipulates the conditions for the 2000 hour TBO. It states the requirement that the engine have the latest-design engine case. The latest case design (part number 892 654) was introduced in July of 2006. Unless your engine was manufactured after that date, or the case has been changed for some reason, your engine is not eligible for the 2000 hour TBO. The good news is that the overhaul kits, that we now receive, all include a short block. So if you send an engine in for overhaul, it will come out as a 2000 hour TBO engine. You can check the serial number of your engine against the list in SB 912-057 to see if your engine qualifies. If you are in doubt, the best thing to check is the serial number of the case. To do so, look on the side of the case near the bottom of the engine (figures 1 & 2).

Figure 1 - The 2/4 side of the engine

Figure 2 - Old serial number format
If you have a 5 digit sequential number that is stamped into the case, such as this one, you have the old style case and the engine is not eligible for the 2000 hour TBO.

Figure 3 - New style case  

Figure 4 - New format for serial number

If you have a number that is laser etched into the case – and has this 2-digit, decimal, 4-digit format – you have the new style case. The first two digits are the year in which the case was manufactured. The four digits are the sequential number for the cases that were produced that year. These cases are eligible for going to 2000 hours TBO provided they meet the other requirements of SB 912-057.