A secondary heat exchanger subassembly, such as a transmission oil cooler, is disposed in a metal tank of a radiator and has a pair of metal fluid fittings. Either a connector ring engages a fitting extending through the opening in the tank or a connector nut extends into the opening to threadedly engage the fitting whereby the metal fittings are held to the metal tank and inserted into a furnace for being brazed into sealed relationship with the metal tank.