Hiya Paul. If I understand you correctly, you're talking about the large metal pipe that's like a drain trap or a U that threads into the head at the t-stat box. Anyway, I just used a standard pipe thread - I think 3/4" NPT - that you can get at most hardware stores. It might be 1/2" NPT but I think that's too small. Just clean the threads in the head good and wrap it with pipe tape or silicone and you should be good to go. Howard
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