oh, thats an easy one. they are 3/8 NPT.
i use a 45 degree adapter and put radiator petcocks in there that way they point down.
easy to drain to change a thermostat or a hose with out alot of mess
npt is measured by the inside diameter of the pipe they go on...so really with plugs hard to tell unless you recognize them or have a chart showing their actual sizes
here..
