Coolant expansion tanks are necessary for proper air bleeding in applications that utilize a rear mounted radiator or a low mounted radiator such as a V-Mount. However, expansion tanks are also valuable if integrated in a common radiator setup. To allow heat expansion, traditional coolant systems require air to be present in the top portion of the radiator during low temperatures. A coolant expansion tank permits the radiator to be 100% filled with coolant in all conditions.  The coolant expansion tank features two internal chambers. These chambers are NOT isolated from each other. There is cross-over passages at the top and bottom. This product is not a coolant overflow tank. Instead, it is designed to part of the pressurized coolant system. The Radium Engineering coolant header tank also has an internal swirl feature, as shown. Hot pressurized coolant from the cylinder head is sent into the vortex chamber's inlet tangentially and accelerated to a high rate of rotation. This is to help aid in deaerating the system. Threaded Ports Side (Inlet): -6AN ORB (9/16"-18)Bottom: -10AN ORB (7/8"-14) Instruction PDF