We love this Vintage Air Airport Bulkhead because it cleanly installs in a 2 1/2" diameter hole in the firewall, in the toe kick area or even in an inner fender panel. No additional fittings are required as the hoses just slip through the bulkhead. It is designed to handle three #10 Reduced Barrier or E-Z Clip hoses and one #6 Reduced Barrier or E-Z Clip hose. When using this bulkhead, you will want to use #10 Reduced Barrier or #10 E-Z Clip hose for your heater hoses. 5/8 heater hose is too big to fit through this bulkhead. Includes one 5/8" heater hose x #10 E-Z Clip or Reduced Barrier adapter. We recommend a second adapter (part number 361403) so that a transition to conventional 5/8" heater hose can be made inside of the car and keeping the ugly heater control valve out of sight in the engine bay.