Hi Ejner,
Use a Rotax compatible coolant (eg Castrol Radicool SF).
https://legacy.rotaxowner.com/si_tb_info/serviceinfo/si-912-016-r12.pdf
Concentrated coolant should be mixed at the ratio 50:50, concentrate to pure water (deionised /distilled/ reverse osmosis/ etc).
Fill the cooling system slowly via the central small tank (Rotax call it an Expansion Tank).
When full, repeatedly squeeze the larger radiator hoses. The movement of coolant will encourage air pockets to be expelled.
Eventually there will be no more air expelled.
Top up Expansion Tank until coolant at bottom lip - ie adding more coolant will just overflow into Overflow Reservoir.
Put the Radiator Cap on the Expansion Tank - make sure it's secure.
Fill the Overflow Reservoir to about 1/2 full.
Start the engine and bring it to full operating temperature (do not run engine below 2200 rpm).
Allow the engine to cool (best overnight)
Check coolant level in Expansion Tank & top up with coolant if necessary.
Check coolant level in Overflow Reservoir - bring level to between 1/3- 1/2 full.
Check coolant level before first flight of the day - top up if required.
Fly the aircraft as normal but keep a check on Coolant temperature for the first few hours of flight (especially in Climb) after servicing the cooling system.
Hope this helps.😈
