Ed

How many copper washers you using?? I put in two and tighten it really tight and dont have a problem with it. I will usually get 500 flights before the little flange starts to come apart, but very rarely do they come loose on me. One thing to try is to get the pipe more rigid, could be that your pipe is moving more than your engine transmitting that back to the header causing it to come loose, cant hurt to try it. Other than that man, I am not sure what would cause it.