Hi,
measure the distance from bed to top on all 3 rods, it should be almost the same (it's not THAT critical, when you're calibrating, the endstops set the correct values). And then measure the diagonals (from left rod bottom to right rod top and vice versa). This should be the same everywhere, because it ensures that the angles are correct (90° ;-)).
I've found the kossel to be very rigid, regarding the small parts used. But if it isn't in your case, maybe you can span some wires between the rods (again from left top to right bottom and so on), that would get you a lot more stiffness.
measure the distance from bed to top on all 3 rods, it should be almost the same (it's not THAT critical, when you're calibrating, the endstops set the correct values). And then measure the diagonals (from left rod bottom to right rod top and vice versa). This should be the same everywhere, because it ensures that the angles are correct (90° ;-)).
I've found the kossel to be very rigid, regarding the small parts used. But if it isn't in your case, maybe you can span some wires between the rods (again from left top to right bottom and so on), that would get you a lot more stiffness.