Leaking headers!
 

I just installed a set of mac long tube headers on my 89 5.0. They are the 1 3/4 primaries with 2.5" collectors. The passenger side header is leaking, possibly the drivers side too, but it sounds like its coming from the passenger side. The headers are torqued down correctly. What's the problem? I've heard that the headers have a break in period where they expand from heat, and have to be periodically tightened down. Is this true or is something else wrong.

I check the tightness of my Mac 1 5/8 long tube headers once a month,never had a problem with leaks.

Take and and check the bolts again and tighten if necessary.If you still have leaks,you better pull them and check the gasket make sure that it's seated properly.

Definitely check the gasket, but also check the flanges. I had a set of bbk shorties that the flanges were tweaked on (one was slightly bent up and another slightly down), it was a mfr defect and Summit replaced them. I checked them by placing the flanges flat against the wall and noticed 2 of the flanges did not sit flush with the wall and the other 2 did. It's not the most scientific method but it was effective for me. The Macs are notorious for warping. Check the seal between the flange and the header tube too, I had a set of longtubes where the flange and tube weren't welded properly (there was a gap), another mfr defect and Summit replaced those too.
Another thing you can try is to double up the gaskets.

Here's a quick one, what if the flange is just really used on the pipe and doesn't want to seal anymore to the header. Besides trailoring it to a muffler shop to fix it, besides won't they just heat it up or something to fix it? any tips?