Thermal Expansion factor?

When water cooling is thermal expansion something that should be taken into account?

If you mean that the loop will have more volume under load, then no the change is tiny.

There are other things that thermal expansion could affect, like the convection in a pumpless system. Water has an unusually low thermal expansion.

When I cap my system I usualy squeeze one of the tubes to make the volume of the loop a bit smaller. This leaves a slight negative pressure in the system and allows for thermal expansion. You could also just stop your radiator fan and let the system get as hot as you dare, then cap it and start the fans again.

I know I can get my coolant levels to fluctuate about an inch in my T line from thermal expansion alone; I have 1/2 inch tubing and I'm using fluid XP.

Ok, so expansion is a factor but it is nothing to worry about.