When your radiator coolant gets too hot, it will blow through your overflow spout and all over your engine bay unless you have an overflow tank. Aeroflow's new overflow tanks are available in polished billet aluminium or anodised black with 3/8" barb fittings for a simple installation. They will allow the coolant to expand and then recover as the temperature drops.
Instructions can be found here.
Instructions can be found here.