Hey Willy...just to be clear, the 3/16" hole drilled in the t-stat plate has nothing to do with the overflow tube, unless your calling the Upper Radiator Hose the Overflow Tube??? The hole just allows trapped air to bubble up from inside the block when the t-stat is closed (engine not running), as you stated. As soon as the t-stat opens the little hole is a mute point...
Mike B
