Last fall when I installed my stove I used Simpson double wall stainless. I noticed while we were putting in the T that it had a slight seam gap in the upper right hand side, inside pipe. It was about a quarter inch wide and 1 inch long. There were no other problems in any more of the pipe. This looked to be an area that when pipe was assembled it just did not make the complete seam. There is insulation within this crack and of course an outer wall. It really looked fine to use so we used it with no problems at all. I also called Simpson company and ask them if there was a problem and they said there was no problem whatsoever with this piece and that this slight gap was sometimes seen in these T's. I am going to clean my pipe soon and plan to look it over to make sure it still looks good. Does anyone think I need to put a little stove crack filler in this small gap (what kind ?) while I got it opened up ? Thanks. Robbie.