I don't understand the concern about the plug falling into the crankcase if pushed in. The point of the plug is to keep coolant in the engine but to provide a pressure relief if the coolant starts to freeze and expand? With coolant behind the plug, how can the plug (or chips from drilling the plug) get into the crankcase without the coolant also ending up in the crankcase?
I still suggest that care be used if drilling since chips could contribute to radiator blockage or water pump damage.
