My problem occurred when I drained warter seperator 4 times & drain plug stripped.
I have changed water seperater still does same thing. What could cause 6.0 not crank after warm up.

On diesel engines that will not restart, after they are warmed up, it's often a sign of a tired fuel pump. As they heat soak, inside pump head clearances increase and it will no longer develop sufficient fuel pressure at cranking speed. While it's running, fuel helps cool the pump by carrying the heat away. If this sounds like your situation, install a new Motorcraft fuel pump and under frame fuel filter.