At NBO we use brass fittings to connect to the heater hoses in our conversions. These are heavy weight and strong. I personally have seen (3) "hi temp. Plastic tees and (2) hi temp plastic elbows fail in the last 6 months. The kind you can buy at the auto parts store. I feel very strongly that you must not compromise the coolant system with weak fittings, so we use these rather than the cheaper automotive ones. Mercedes 123 use (1) 3/4" tee (1) 3/4" elbow (1) 5/8" tee (1) 5/8" elbow and (1) 5/8"-3/4" reducer. The heat exchanger we sell are 3/4" that’s the same as the supply lines 19mm. If you are gong to heat the tank on a 123 you will need (2) 5/8"-3/4" reducers. For the Mercedes 126 use (2) 3/4" tees, (1) 3/4" elbow. If you use tank heat in a 126 you will need (2) 3/4" tees, (2) 3/4" elbows, (2) 5/8"-3/4" reducers. No S&H on Brass fittings when order with heat exchanger or 12 volt heaters.