I've had my headers on for a couple years now.... and just cam back from exhaust shop to fix a leak at the collector. He fixed the collector leak, but told me there is another leak as well. He found it when he was inspecting his work with the stethescope.

Guess where? Right between where the three pipes meet, in the center of hte 4-5-6 header. He said to fix I'd have to take the header off, chop off the collector, weld up the area in the middle of the three pipes, then weld the collector back on.

I figure if they are machine welded, and mine is leaking, chances are others, from the same batch at least, are as well. So check yours to make sure.