Universal diesel thermostat housing petcock valve thread size

Jun 11, 2004
1,812
Oday 31 Redondo Beach
Does anybody happen to know the thread size for the petcock valve in the thermostat housing on the universal M-18 or M-25 engine (they are the same)?
I want to order a real ball valve for this to make "burping" the cooling system easier. I'm guessing it is 1/4 or 3/8 but am not near the boat.

Thanks in advance for any help.