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Vanishing Coolant of Rotax 912 Engine

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Rotax 912 engine: upper coolant elbow with flange, cylinder #3. The liquid seen on the bolts is Corrosion X added after installation. By 2017, our Rotax 912 engine had run 1,815 hrs over 22 years.  During that time, the engine always had some small loss of coolant on long trips, as was evident from a slightly lowered coolant level in the overflow bottle.  We attributed the change to evaporation.  So during each oil change, we added a bit of coolant to that bottle.  In recent years, the level in the overflow bottle dropped somewhat more rapidly, and we had to add coolant after each extended cross country tip. We could not see any obvious leaks:  The water pump did not show any leaking coolant in the bleed hole or at its four elbows, all hose connections were dry, and the upper and lower coolant elbows of the cylinder heads showed no loss of coolant. Coolant elbow at bottom of cylinder #1 Then came a surprising change.  We had flown from Aero Country, our home base nea