6 to 5 Duct Reducer-Ductwork-Heating Duct-Air Duct-Ventilation Fittings 6 Inch to 5 Inch Duct Reducer Duct Increaser. Sheet metal Ducting Reducer. Duct Reducer in stock - fast shipping. Small (Male) end is crimped although picture may not show crimp. Easy to install air duct reducer.