ex leak,mnfold gasket migrated out @#6 exh

Just replaced exhaust manifold on '89 xj, 4.o, auto, w/ 235k miles. Original manifold cracked at the usual place for these motors. Anyways after a couple of hundred miles after replacement, the #6 exhaust port portion of gasket migrated out toward the firewall. You can see the metal ring part of gasket that should surround exhaust port has moved out of the mating surface and hence exhaust is coming out of the gap. I used copper coat on a Felpro gasket before install and torqued all bolts to specs, <30 ft/lbs I believe, with original bolts and no thread locker in the suggested pattern. Off the bat the fsm torue value seemed low since your torquing a non oem manifold that may be slightly out of spec (Chinese made manifold bought on Ebay, never again!) What did I do wrong, should I have torqued more, threadlocker, rtv sealant instead of copper coat on gasket, different brand gasket, abraid mating surfaces??? I didn't check the manifold mating surface for trueness, could that have aggravated the problem.I would like to get it right this next go around. TIA

I had that problem with a Dorman header. Pull it off and take a straight edge to it. I ended up ordering 4 manifolds before I got one in spec. Basically, the mounting surfaces on the two end flanges were out of square and wouldn't sit flat on the cylinder.

On another manifold we put in my buddies (I think it was the Chinese one), the same thing happened, but in that case it was because the ears on a couple of flanges were oversized and interfered with the intake manifold flanges. 1st install, we pushed things into place. 2nd install, we did a bit of file work and now no leaks. The manifolds should mate to the head without interference.