For example" 1.5" 38.1mm - 1.75" 44.5mm is a 1.5" 38.1mm section swaged one end to fit over 1.5" 38.1mm OD tube and the other side is swaged to fit over 1.75" 44.5mm OD tube. Stainless Steel Tube & Pipe Joiner.
eBay