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Purpose of the overflow bottle in 912s


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Can someone explain to me the purpose of the overflow bottle in the 912 cooling system. Why can't the coolant after passing over the cylinder heads, into the expansion tank, just cool in there and then pulled back into the radiator via the water pump? 

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There's nothing in the coolant system as far as I am aware that will "burst" until pressures get a lot higher, but there's no need to design a system to take more than is necessary plus some safety margin to tolerate the pressures needed to cool the engine without boiling.

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Without the overflow bottle when the coolant expands as the engine heats up it would be pushed out into the engine compartment. Think old cars spewing coolant out of their radiator caps. The design of the radiator cap allows for the coolant to move to the overflow bottle as it expands while the cap regulates the pressure within the cooling system.  As the engine cools after shut down it allows coolant from the overflow bottle to be pulled back into the system, always keeping it full if everything is working as it should. Most automobiles with a liquid cooled internal combustion engine uses a similar system.

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