my brothers 69 chevelle had headers on it with fel-pro gsks and when the motor was rebuilt with the new headers, every time before he drove the car and every time after he shut it off he would re tighten the header bolts. he did this for about a week and then the bolts finally quit tightening and he put on over 30,000 miles so far and hasn't had a gasket leak yet. the motor has around 380 hp so the headers do have some stress also.