This box style overflow tank was made to affix to a fan shroud for a customer short on space.  

Should the system pressure exceed the cap’s rating, the spring will compress and fluid will flow through the upper tube to fill the chamber.  

In the event the recovery tank reaches capacity, the fluid will find the second tube and exit to the street.  A vacuum will draw the coolant back into the cooling system as the engine cools down.

Made  to order here in the USA