Working pressure water system

Which engine?

Sorry, engine 912S

Coolant temperature limits are in the Operator's Manual:      
2.2) Operating limits (912 S/ULS)  - current version, page 2-5

See section 12-20-00 Page 23 of the current version  LMM (Maintenance Manual - Line Maintenance) for information on the coolant system pressure.

All 912 series Manuals are available here: 

https://www.rotax-owner.com/en/support-topmenu/engine-manuals#912-series-engines

5/8" (0.625) hose is going to be a BIG Stretch over the 17mm (0.669) Hose fittings.

Yes, the 17mm hose is pricy and not available locally; But just Bite-the-Bullet and order some.

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The working pressure will be the same as the pressure rating of the Radiator Cap on your expansion tank.   Typically 1.2bar (~18 psi)

The Coolant Circuit is always 100% full with nowhere for any expansion to go.

The Coolant expands more than you might expect.  4 Liters expands over 200cc going from 25 to 100°C.

As the coolant expands and the pressure increases, the rubber hoses take up the initial expansion, but the pressure limit is reached even before the temperature reaches 100°C (212°F);   It has nothing to do with the water boiling.

Your woking pressure is going to be ~18 psi and the Coolant circuit IS going to vent into the overflow bottle on every flight.