I was thinking about using this for the inlet of a circulating style block heater. Anyone know what the thread size/type is? I'd like to replace it with an insert (barbed) fitting for 1/2"hose, but ideally I'd have all the parts in hand before draining the coolant.

Alternatively, I could just use it as is, but the current insert (drain) fitting is only about 1/4" and I think that might be too flow restrictive.