GG and Shelterman I spaced the 1 1/4" pex lines with 1" sheet (pink) foam pieces about two inches long. One piece between pipes every two feet. Then i put a piece about 1" by4" by 4" on the outsides of the two pex pipes, all four sides. (You will have to experiment with foam size so that it has some space when sliding in the drain pipe.) Then duct taped the sandwich all around. This was done every two ft of pipe as i tried 4 ft but got to many sages.( so i thought) I wanted to make sure the pex was as close to center of the 6" flexible black pipe as possible. I tied the black pipe to a telephone pole and then pulled the pex in with a rope, i had someone at other end feeding it in. It was about 30degrees out when we did it so pipe was a little stiff. Worked a lot better the second time i did it when pipe was warmed up from the sun, 60 degrees. Foamit Up