In a recent thread regarding a stove that would not shut down (https://www.colemancollectorsforum.com/post/new-restoration-tank-not-shutting-down-10676554?pid=1312842417), I suggested, ‘The valve has double-start thread. Back the packing nut out and unscrew the valve being careful to feel when the stem releases from the female thread. Turn the valve stem 180 degrees and thread it back in. I've had this happen twice, once on a stove that wouldn't shut off, and once just a couple of days ago on a lantern that wouldn't shut off.’
Rob (Dubblbubbl) asked in response, ‘Help me out here, starting the valve stem 180 out does what exactly? Is this a work around for a worn valve tip?’
In answer to Rob’s question, I say, “I don’t know.” A search for double start thread on this site reveals discussions of double-start thread in the past, primarily explanations of the how and why of double-start threads on the valves. So, my question is the same as Rob’s. In both of the cases I’ve dealt with, the needle didn’t look bad – I couldn’t get a good look at the seat, but if it had a nick, how would a 180 flip of the needle help it seal? Do the female threads terminate within a half-turn of the valve closing? [Edited: This last question, I think now, is unreasonable. The threads start at the same elevation so they'll come to a stop at the same depth, right?] I suppose one thing I should do is take a properly functioning valve, turn it 180 degrees and see whether the valve then refuses to shut off. Any of you engineers care to comment? Thanks.