QUOTE: I notice from cold to normal temp (the arrow in the middle), the coolant temp from bottom radiator is very still cold compared to top radiator hose.
That's the whole idea of the cooling system.
The coolant circulates from the pump, under the thermostat, down the by-pass hose/passage, into the bottom of the block, up through the block, into cylinder head (getting pretty hot now) and back to the pump and thermostat. Round and round, and pretty quickly.
Once the circulating water gets close to the temperature of the thermostat setting, the thermostat SHOULD start to open just a little, and a little very hot water goes into one end of (or the top of) the radiator. As this hot water goes in, some cool water is displaced out the other end of the radiator and MIXES with the flow in the by-pass that is heading for the bottom of the cylinder block. That means the water going back to the engine is a tad cooler, hopefully just enough cooler to maintain the correct running temperature. This (if it is well designed) will self-regulate, AND it should operate gently, without causing any thermal shocks to the system.
Thermal shock is what happens (especially on a nice cold frosty morning) when the engine gets nice and hot, but there is NO flow into the radiator, and then (due to bad design) when the temperature reaches "normal" the bypass closes suddenly as the thermostat opens up, and the entire VERY COLD contents of the radiator get thrown into the cylinder block and head. The sudden cooling effect causes differential contraction of parts to take place, and (again due to bad design) this has the effect of loosening the clamping effect holding down the cylinder head - and the head gasket fails.
Drilling a few holes in the thermostat ring means that as soon as the engine starts, some flow into the radiator takes place, and very gently the radiator will warm up. The engine may take a little longer to heat up, but chances are there will be no thermal shock effect, or not enough to do in the head gasket.
