Airplane Light Sport Aircraft ROTAX Engine 912: Cooling

Wondering about which coolant to use for your Airplane Light Sport Aircraft ROTAX engine?

Generally, a water-based coolant is preferred because it transfers more heat and is easy to maintain.

However, Evans waterless coolant is used by many manufactures and therefore, must be understood. For some LSA installations, Evans is preferable.

Evans Waterless Coolant

  • Original ROTAX service bulletin (SB-912-043 R1) said all engines had to change over to Evans waterless coolant. Since then, ROTAX has made provisions to use water based glycol coolant if certain provisions are met. (see SB-912-043 R2 and SI-912-016)
  • In S-LSA it is up to airframe manufacturer which type of coolants are allowed.

Advantages and Disadvantages of Coolant Types

  • Conventional water-based coolant can easily be toped off with water
  • When using Evans, the system must be permanently purged of water, and only toped off with Evans
  • Advantage of Evans is that it has boiling temperature of 375 F. This is well above the maximum allowable temperature of the cylinder heads.
  • In general, pusher configurations and taxiing downwind are more susceptible to overheating on the ground and would make them better candidates for using Evans coolant.
  • Conventional coolant has a lower boiling point than the maximum allowable CHT which can lead to coolant boiling out before the maximum CHT is reached.

Changing Coolant

  • How to change from water base to Evans, follow specific instructions ROTAX SB-912-043, SI-912-016 latest versions and follow the specific instruction from Evans at
  • Evans to water-based change: just drain Evans, flush with water, and add water based 50%/50% mix with ROTAX recommended cooling fluids.
  • If published temperature cannot be maintained due to inadequate design for E-LSA, installation may have to be re-engineered.

Water-Based System

  • Maximum coolant exit temperature for 0.9 bar/13 PSI pressure cap is 239 F
  • Maximum coolant exit temperature for 1.2 bar/17.5 PSI pressure cap is 248 F
  • Recommend change to higher pressure cap if coolant temperatures are high
  • Use distilled water, not tap water, to eliminate corrosion from tap water impurities
  • DEX-COOL coolant (marked on manufacturers bottle) is common type to use. DEX-COOL American version is orange.
  • Use only coolants and mixture ratios recommended by ROTAX at

Cooling Tips

  • US coolant should be mixed at a 50/50 ratio with distilled water only. Greater amounts of coolant can lead to gelling in system
  • Some European coolants are more diluted and may come with a placard or sticker saying to mix at 80% coolant, 20% water. Be aware.
  • Water based coolant has high thermal density so engine will run cooler (10 to 20 F below) than Evans waterless coolant.
  • From water based coolant going to Evans will increase operating temperatures.
  • Advantages of Evans allows you to run higher coolant temperatures of the coolant if required (because of the higher boiling point of Evans)

ROTAX Documentation Warning

  • Do not rely on the date on the cover of ROTAX individual manuals.
  • Go “on-line” and get up to date information for specific page for all ROTAX documentation.