I just bought my pop open cap from NPD early this year for my '69. I have never had a problem with it popping open by itself, or even if I bump it. It clicks hard when it's closed and doesn't open unless you push the latch all the way down. Admittedly, there's probably only been a couple of full throttle launches with it on there, but there has been a couple, and the car puts out 416 ft. lbs. of torque at the wheels.
The problem I had is that it didn't seal right. There was not enough throw in the spring/cap assembly to form a tight seal at the top of the cap, so gas would blow out under one of those hard launches. There is no adjustment really to be made, as they come in the box, but I modified mine. I put a spacer behind it, which pushes the cap out further. Since then, it's worked like a charm.
The problem I had is that it didn't seal right. There was not enough throw in the spring/cap assembly to form a tight seal at the top of the cap, so gas would blow out under one of those hard launches. There is no adjustment really to be made, as they come in the box, but I modified mine. I put a spacer behind it, which pushes the cap out further. Since then, it's worked like a charm.
