X screws will be horizontal and probably about 2900+ long. I forgot about nut torque and bolt stretch, doh! I can't imagine screws get particularly hot, but it would be temperature differential between the steel and frame steel screw that might reduce tension, and then vibration potential if the nut is near the ends, I.e. not supporting near the middle. I don't have a feel for temp differential, but I read a figure of 3 degrees the other day, gives a large change in tension, so maybe 3 degrees is the number to base a nut torque calc on, but that still leaves 1st bending natural frequency, which I also don't have a feel for the permissible lowest natural frequency for a screw. It's not going to be much different between a 25 dia or a 20 - they are both long and slender at nearly 3 metres.