Rich,
Yes I am using it for a coupler. Due to the requirements of the tuned pipe there must be a gap between it and the header for adjustments of length. I put a short section of 1" aluminum tube inside of the teflon so the flame will not have direct contact. When adjusting the header length I just use different lengths of tube inside the coupler. The problem is on the header side. I use a compression clamp to hold the seal and where ever the teflon contacted the header it was melted. And not just soft but melted away. I am unable to find Viton in 1" size which is what I need. I have some 3/4" silicone from Macs but would prefer to use 1". McMaster Carr has some with a 1" ID that has a braid in it to give it a higher pressure rating but it is rated only at 350 deg. I will continue to look and if I find something I will post it here. There are some commercial manufacturers that I could buy through from work. Got to do something quick as pattern season is on us.

Dennis