I guess I missed this post before. The slip flange will seal without problems if the first pipe is a tight fit on the header and if 4 1" cuts are made in the header end of the first pipe. Muffler shops don't like to install the first pipe like this I guess because they don't understand. In the old days all exhaust was installed this way. Now mig makes it too easy to just weld the connections but it sucks if you want to remove the exhaust. If your having a problem with my headers not sealing please give me a call. I need to know these things.