Silicone Heater Coolant Hose 5/8" 16mm. that is the text from an ebay ad. Silicon hose can stretch more than rubber hose. It is likely auto speed shops have it.

There is also plastic or nylon coupling/union that have one size larger and the other end smaller and you would use 2 lengths of hose that are different inside diameters.

__________________
84 300D, 82 Volvo 244Gl Diesel