97 z28 vert 90,000 on the car 40,000 on the motor

i changed the o2 sensors when i first got the car when i took the left sensor out the threads on the manifold came out with the sensor. so i replaced the exhaust manifold at first the two last bolts (the two closest to the fire wall) came loose about 6 months later. i snuged them up but now there coming loose every couple of weeks. can someone tell me why these two bolts come loose i have swaped the bolts around and there is no visable cracks on the head. maybe i can use a set of locking header bolts to keep them in any thoughts on this matter.

 

i would put a small amount of high temp rvt or soft setting thread lock on the bolt threads and reinstall. the aluminum threads in the head are most likely worn or partly stripped. last thing you want to do is let them continue to get lose and retighten them.