How big is the pre-drilled hole fixed? Are they suitable for m4, m5 or m6 screws?
The holes in the HGR 20 rails are for M5 Allen screws
Is the material suitable for outdoor use, i.e. weather resistant? Stainless metal?
Yes, they are made of high quality stainless steel, so you dont have to worry about rust when you use them outdoors.
What size is the bearing cushion Block Top Hole Thread?
The pitch is 5mm
What screw size and distance do the holes in the rail have?
The rails accept M5 socket head screws and the holes are 60mm center to center starting 30mm from the end.