This 5/8 x 1 steel heater hose reducer is used in our RMW Subaru conversion.  The bottom nipple on the coolant expansion tank is 1" and the heater hose used in Vanagons is 5/8".  Use this reducer for this transition.

You would install this reducer if you are sticking with the original coolant expansion tank rather than upgrading to our 5/8 x 5/8" Aluminum Expansion Tank for Subaru Conversions.