Just a quick word on couplers before you close the book though, try to keep the gap between the header and the pipe as small as possible to reduce the exposed area of the silicon. I also use a slightly larger coupler than i need, so if I'm in a rush I can just slide the coupler over so the charred bit of silcon is now safely behind the header or the pipe. Also I like to flare the ends of my pipes by stuffing an appropriate sized socket into the hole and rapping it lightly with a hammer. this stops them coming off so easily.