Lburou, I used 11/4" couplings which gave me just under 11/2 " OD for the 270 Watt by 11/2" ID by 11/2" long band heaters which can be obtained from Plastic Process Equipment. I do not know if you will be able to stretch them to 1.625" OD of your couplings. My couplings are only 2" long so if you can make an aluminum plug you could go as much as a 1/2" into your coupling although 3/8" could be enough. You would need to machine the aluminum to the correct dimensions then heat the copper up some and cool the aluminum when you press it in. I build mine by putting the 11/4" coupling onto a flat piece of copper and then silphos welding them on the inside of the coupling then trim the excess flat off the bottom with metal shears and finish of with a disc sander. I have just checked the band heaters I use and the max they will go around is 1.567"