Vexing exhaust leak

[robert b]

I have 76 280z.  Datsun Spirit did a STR 2.8B rebuild for me and I installed an MSA 3-2-1 header with a plug in the EGR connection at the header and intake manifold.  After about 200 miles, the car developed an exhaust leak.  It started with a ticking sound and progressed to a loud banging.  I could feel exhaust pulsing out the side of the engine.

 

I removed the intake manifold and header and found that the center top bolt was stripped and thought that led to the leak, which was confirmed by black streaks on the header and gasket.  I put in a helicoil, fixing the stripped bolt, installed a fresh gasket and reinstalled every thing.  Started the motor up and the loud banging was still there.  I could feel the exhaust pulsing out from between the number 5 and 6 cylinders.

 

I am now on my fourth go around of changing gaskets, reinstalling everything and the leak persists.  I took the headers to a machine shop and was told that while there was a slight crown in the middle, there is now way it is causing the leak because torquing the headers down would bring everything into alignment.  

 

I did notice a black streak on the header flange between the number 5 and 6 cylinders.  I also noticed that the gasket appears clean with no discoloration that might be associated with a leak.

 

Could the exhaust be leaking from some place else, like the plug for the EGR tube at the header?  Any ideas?

[NewZed]

Maybe a weld broke.  Headers are just bent tubes welded in to a steel flange.  Could be you're looking at the wrong spot.

 

Have you torqued the header flange down on to something flat to confirm that the "bow" straightens out to a perfectly flat surface?  Maybe it just unbows to a new bow shape.  And don't forget that heads get bowed too.

[robert b]

I'll try tightening the headers on my Workmate and see what happens.  I certainly hope it is not the heads that are not flush.  Thanks for the tips