The Rotax installation manual recommends that a vent line be installed in the cap of the Overflow Bottle. This is in case the coolant overheats. There is no mention of this in the Searey manual. Following the Rotax diagram I quickly drilled a hole in the cap and installed exactly what they recommend, with some parts purchased at the local plumbing store, including the little 'anti-freeze notch' in the hose end.
