Perhaps I am the poster in question. When I finished a recent rebuild for a B20 I was having an overheating problem for which I could not find the cause. A guy at an auto parts store convinced me to put a release valve into the upper most hose of the cooling system. It's a plastic heat proof spigit type of mechanism which is installed into the highest part of the system, allowing one to safely let the air out after the motor is warm and the thermostat has openned.
In a closed system air can get trapped in 2 places, as far as I know. One is below the thermostat if it's not properly installed (small air release hole situated at the highest point). The other is at the top of the cooling system. That trapped air can lock the system up altogether, even if the thermostat is open.