The top hose barb fits 3/8" hose, and is screwed into the tank w/ NPT threads & sealant to prevent leaks. This hose barb could be removed and replaced w/ another size if needed, as long ans it is resealed again to prevent leakage. There is a tube inside the tank that extends to the bottom of the tank and this allows the coolant to be pulled back in the cooling system. This top hose barb is connected to the radiator w/ a hose that attaches near the radiator cap (on most radiators - this is the only connection to the radiator and this hose works for overflow & recovery of coolant). This info is included with description
