Reply to "Filling a completely dry coolant system"

The bypass allows coolant to circulate through the block during warm up prior to the thermostat opening. Two important advantages.

First, it reduces the incidence of boiling coolant in hotter sections of the engine before the coolant adjacent the thermostat reaches a high enough temperature to open.

Second, it reduces thermal shock that can cause metal warp or crack. You have noncirculating coolant in the head boiling while the coolant near the thermostat is gradually warming. Then finally, the thermostat opens and the hot coolant rushes out. It is replaced with cold coolant quenching the hot metal quickly. Eventually, the cooler coolant reaches the thermostat and the process repeats, albeit with less of a temp delta until finally the coolant is hot enough to keep the thermostat open.

This tough on all components including gaskets.