I don't know about thread specs so I don't know if the threads we have are NPT or not, but 1/8" sounds small to me. My system uses 22mm copper pipe and my automatic air vent is 3/8" (unknown thread type). There is no science behind that sizing though; I installed it myself and that is just the valve size I picked up at the store. All I can say is that it works so is big enough.
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