Thread size for thermostat vac. switch

[mustang68mustangs1]

I need to plug the port in the thermostat housing where the old vacuum tree would go. I cant seem to figure out what size threads they are. It seems like a straight non tapered thread. I was gonna try an AN plug with the o ring. Hope someone knows the the correct size.
Thanks Bob

[69F100]

I think mine was 3/8'' pipe tread on the thermostat housing but have seen some that was 1/2'' pipe tread.

I got it backwards ^ mine is 1/2'' went out checked this morning the one on the motor home thermostat was 3/8.

[bbf-falcon]

Most are 1/2,atleast mine are.  

[mustang68mustangs1]

Thanks guys ill give it a shot. It just seemed like i couldnt get a npt plug to thread in to it.
Bob

[GTmustang]

It should be 1/2" straight thread instead of a tapered pipe thread. Just run a 1/2" pipe tap in and it will clean up the threads to npt which will allow the use of a common pipe plug.
Gary

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