Air Purge Valve
The bolt has a small air passage through the center from the bottom going to a couple of vent holes in threads. When you drain the system, remove the bolt to allow coolant to fully drain from the TStat bypass. When refilling, you can leave the bolt out or screw it in leaving the air passage holes exposed. This will allow air to escape as the coolant fills the system. Trapped air causes hot spots. (The air will eventually purge through the overflow, but it is far better to get it out through the purge valve and avoid the hot spots.) When you see coolant beginning to flow from the purge valve, tighten the bolt down to seal the system. Don't overtighten and use two wrenches (one on the bolt and one on the tube) so that the tube doesn't twist.
One more thing... When you drain and fill the system, it is a good idea to have the heater setting as high as it will go to get the coolant flushed from the hoses and heater core.
