We spent years searching for these fittings and finally had to have them specially manufactured. Custom made in the USA for Greasecar by Parker, these fibreglass reinforced nylon fittings feature 3/4" x 5/8" x 3/4" barbs which allow you to connect 5/8" coolant hose into a 3/4" circuit.  Made specifically to tolerate the heat and chemicals found in automotive cooling systems. (sold in pairs)

3/4"x5/8"x3/4" Fibreglass Reinforced Nylon Barbed Tee Fittings