Feeding your Rotax 4-Stroke Aircraft Engine

by Michael A. Anderson

Feeding your Rotax 4-Stroke Aircraft Engine

Fuel | Engine Lubrication | Oil Filters | Cooling Liquid | Spark Plugs


Octane rating

The Rotax 912 A/F/UL engines require a minimum octane rating of 87 AKI, commonly referred to as “regular” while the Rotax 912 S/ULS/ULSFR and 914 F/UL require a minimum of 91 AKI (“premium”).

Some may have noticed that the Rotax manuals mention ratings of 90 and 95 respectively. These ratings are calculated based on the RON standards used in Europe and are equivalent to a rating of 87 and 91 under the AKI standards used in Canada.

In all cases, we recommend using as high an octane rating as possible, since fuel evaporates and quickly loses its octane rating by osmosis when it lays in your aircraft’s fuel tank or in a plastic jug. A “premium” fuel will see its octane rating reduced to unusable levels after as little as three weeks. Fuel with a lower octane rating would obviously have an even shorter usable life.

Too low an octane rating will cause pre-ignition and detonation which can damage the piston ring grooves, skirt and crown.

Where to buy

It is recommended to buy gas at the busier gas stations of the major oil companies, since their tanks are renewed often allowing the fuel to stay fresh and clean.

Aviation Fuels

It is possible but not recommended to use 100LL AVGAS, since the the lead content is like cholesterol to your engine: it will accelerate wear on the valve seats, create deposits in the combustion chamber and sediments in the lubrication system and gearbox. Increased maintenance is necessary to compensate. Unlike “conventional” aircraft engines, lead is absolutely not essential to the proper lubrication and operation of a Rotax 4-stroke aircraft engine. The increased octane rating also has no marked advantage for the operation of your engine.

To be avoided:

  • “Regular” fuel except if used in the 912 A/F/UL and burned entirely on the day of purchase;
  • “Premium” fuel which is more than 3 weeks old
  • AVGAS except when the required automotive fuel is not available


Engine Lubrication

Oil Specifications

Rotax recommends using a high quality, major brand, 4-stroke motorcycle oil with gear additives and “SF” or “SG” API classification.

The gear additives are required to withstand the high stresses in the reduction gearbox. The “GL4” or “GL5” specification is recommended.

Oil Types

Users running mostly unleaded fuel can opt for full-synthetic or semi-synthetic oils. Users running leaded AVGAS more than 30% of the time should only use mineral or semi-synthetic oils, since a full-synthetic oil will sludge and create residues when used with leaded fuel. Oil changes and other important maintenance tasks have to be performed more often when using leaded fuel, as described in the maintenance tasks table.

Engine Lubrication

Figure 1. Selection of oil viscosity based on climatic conditions.


A multi-grade oil is recommended. Refer to figure 1 to select the appropriate viscosity for your climate.

Further Reference

Rotax has published a Service Instruction document titled “Selection of Motor Oil and General Operating Tips (SI-18-1997 R5)”. It contains a list of suitable oils and unsuitable oils. Bear in mind that these tests were based on a selection of oils available in Europe. These oils may not be available in Canada, or the oils available under the same designation in Canada may not use the same formulation as their European equivalents. Also, oils only available in North America are not included in this list.

  • SI-18-1997 R5, Selection of Motor Oil and General Operating tips for Rotax Engines Types 912 and 914 (series)

To be avoided:

  • Synthetic oil if you run leaded fuel more than 30% of the time
  • Oils with friction modifier additives (“antifriction”), will cause the slipper clutch to slip
  • Oils designed for “conventional” aircraft engines
  • Oils designed primarily for diesel engines as they generally have insufficient high temperature properties and additives which can cause friction clutch slipping


Oil Filters

Only the Rotax-recommended oil filter (part 825 701) should be used. Only those filters assure the correct pressure for bypass valve operation. Other filters have less filtering surface and are not made to withstand the high oil pressures the Rotax 4-stroke engines operate at.

To be avoided:

  • Any other filter than Rotax part 825 701. There are no equivalents or substitutes!
  • Contrarily to what some rumours would like you to believe, Rotax does not authorize the use of any other filter!


Cooling Liquid

Type selection

The 2004 release of Service Bulletin “Change of coolant specification (SB-912-043 / SB-914-029) has created some confusion among owners. The instructions it contains are fairly simple but require referring to a few manuals for full comprehension. Here is a more concise explanation:

Figure 2. Cylinder heat temperature limits based on type of coolant and radiator cap in use.

The selection of your cooling liquid depends on the efficiency of your cooling system (radiator size and installation, etc).

Cylinder head temperature (CHT) is directly related to the efficiency of your cooling system and to the dangerous presence of vapour bubbles. Therefore this temperature is measured instead of the cooling liquid temperature.

If in all situations your cylinder head temperature is inside the limits of the “hot” or “normal” ranges of the first two columns in figure 2, you may use the ethylene glycol and water type.

You may notice that systems using a 1.2 bar / 18 psi (standard on recent engines) radiator cap have a broader range of allowable temperatures than those using a 0.9 bar / 13 psi cap.

If however your cylinder head temperature reaches the “hot” range of the third column, the use of Evans NPG+™ non-aqueous (waterless) liquid is mandatory.

Ethylene glycol type

Rotax recommends a mix of 50% long life antifreeze concentrate without sulphates and phosphates, with anticorrosion additives designed for aluminium, and 50% distilled or demineralised water.

Do not forget to renew this cooling liquid every two years.

Non-aqueous type

The Evans NPG+™ non-aqueous cooling liquid is mandatory under certain circumstances, but we recommend it for every engine since it offers more efficient cooling, an extremely high boiling point, a very low freezing point, corrosion prevention and unlimited life (no need to renew every two years). Also notable is that it operates at no or minimal pressure which greatly increases safety in case of an in-flight leak. It is available from us.

To be avoided:

  • Antifreezes containing phosphates and sulphates, even those which are low phosphate and sulphate
  • Water which is not distilled or demineralised
  • Mixing ratios other than 50:50
  • Ethylene glycol type of coolant if the appropriate limits can be exceeded

Oil FiltersSpark Plugs

Plug Type Socket Electrode Gap
912 A/F/UL NGK DCPR7E 16mm 0.7-0.8mm/.028-.032”
912 S/ULS/ULSFR NGK DCPR8E 16mm 0.7-0.8mm/.028-.032”
914 F/UL NipponDenso X27EPR-U9 12mm 0.6-0.7mm/.024-.028”

To be avoided:

  • Other spark plug models, including those with a different heat range code, and other manufacturers’ equivalents
  • Unverified spark plug gaps. Always check gap before installing!
  • Cooling LiquidThese informations and much more can be found in our Engine Maintenance Logs.