I have the OEM hard tonneau cover (PT954-35241) and installed it myself on my 24 OR 6' bed. It does lower the factory bed rails about 1/2-3/4" as bed rails are uninstalled and then hung on the tonneau cover aluminum mounting clamps.I have the bakflip MX4, which basically mirrors the OEM install. The rails don't have to be lowered on the bakflip MX4 and I'm mostly sure that the OEM doesn't need to lower the rails either, but I could be wrong. That being said, it is a tight squeeze once you try to get the hardware in there to get the molle panels to fit properly. I had the Trumgeta version and it took some effort. At points I had to unhook the tonneau cover clips to slide in the nuts into the rail.