In working with the fuel tank petcock on my ’45 Chevy 1/2 Ton, it was, for me anyway, was so counterintuitive, it might help somebody to discuss it here. Of course, the range of years this or similar petcock was used is probably wide.
The petcock is, as a whole, a shut-off valve only. It will let fuel flow or stop it. To use it as a drain, you have to undo the stem all the way to remove it.
The counterintuitive part is, when you open it to allow gas to flow, there is no “stop”. When you first start opening it by twisting counter-clockwise, it leaks. As you proceed twisting, there is no obvious place to stop, however it does seem to quit leaking. Eventually the stem will unscrew out completely, allowing fuel to drain.
As I concluded, as you are unscrewing to let fuel flow, after a revolution or 2, the threads start to give resistance, as if they are expanding against each other. I think THAT IS what is happening. So, you end up unscrewing for about 4 revolutions, until you feel confidant that the threads are tight enough to not leak, and leave it there.
I was uncomfortable with this arrangement, so wrapped 1 rotation of the gas rated orange thread tape. This was not easy as the orange tape is kinda thick and stiff. Also, there is no shoulder to wrap over for an easy start. Cutting the ends of the tape on an angle did help. A heat gun would have been nice, but way too risky. I was not testing for leakage with water
In hind sight, given the thread tape challenge, next time (?) I’ll try trusting the enlarged threads. I can always keep adjusting until I find the setting that I think stops leaking best. It’s a Chevy thing, so it must be good.
My truck is not home right now, but I think a supplier photo would be best anyway. See attached. I'm not nessesarily describing a problem. If the opperation I describe is the way the petcock is supposed to work, I think you can see why I describe it as counterintuitive. But perhaps mine is not working as designed. At the very least, this thread could help some of us. I guess I could buy a new one and find out that way, but mine is working perfectly now.
Most petcocks I've seen are designed to be fully open or fully closed and have packing to keep them from leaking when fully open. Minor leakage around the threads while opening or closing can happen.
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I think this has been a productive conversation. My impression of counterintuitive is directly related to your vetran's opinions. However, I think my shortcommings prevented me from going to the next stage of questioning further, the packing. I had disassembled, inspected and reassembled the packing, torquing to what I still think was the correct tightness.
As I backed the stem out and started feeling resistance, my mind got fixed on visualizing the threads expanding. But holy cow, what a machining nightmare that would be! It didn't occure to me, something about the stem diameter in the packing could expand and tighten as I unscrewed it.
At this point, I think that is somehow the most likely scenario. This suff is all part of this game we play. I hope kicking it around here does not seem like a waist of time. I certainly would have benifitted from this thread & gotten a lot less gas on my face.
I'm getting into the new-found habbit of Googling predicaments like this, taging on VCCA to my question. That automatically brings up THE TOTAL ARCHIVE of related conversations. It's fantastic!! In this case, nothing of value came up. I am going to promote this practice, as I am here. Give it a try! If you agree, you might help spread the word. I'd be suprised if 1% of the membership knows of this short cut to knowledge. AND, IT'S QUICK ENOUGH TO DO WHILE YOU'RE IN THE THICK OF IT!!
I've encountered shut-off valves like the one you have as well. They have a straight needle which can be turned completely out. I just turn them out a few turns, or more if it's a higher flow application, and leave it at that.
I've come to the point now where I loosen the packing nut a bit before I turn a shut-off valve either in or out, and snug it up again when done. The packing seems to last longer that way. If I'm changing diesel fuel filters for example, I loosen the packing nut, turn the valve closed, change the filters, open the valve, and snug the packing nut. Normally I like to tighten the nut until I can JUST turn the valve, in case of an emergency and I don't have a wrench handy.
Haven't had to replace a valve, and they don't leak after I've been turning on them. Hope that helps. 🙂
Now here is a reply with some real finesse! It takes a farmer, who deals with this stuff beyond hobby. Your loosening and retightening makes total sense. Also, your amout of torque. Thanks for your confirmation, some shut-off valves have a needle/stem that unscrews completely out to create a drain.
Hey Guy who knows how to look this up when you need it, congratualtions! You've hit GOLD!!
I'm not sure that valve was intended to be used as a drain, by removing the stem. It would never have occurred to me to use it that way. By screwing the threaded part of the stem out through the packing, I think the packing would suffer, as a result. I guess it depends if the threads are rolled onto the stem, making the diameter larger, or cut into the stem, making it a bit smaller. I think a few turns out is for full flow open and closed is closed. To drain, remove the line. Finding the spot where packing prevents a leak but still allows for easy turning, can be a challenge. For me, turning the fuel on/off at this location is so infrequent that I have not given much thought to your situation and would fully expect it to be a P.I.T.A. when required.
I believe you are correct, they were never actually intended to be used as a drain. And if they were used that way, they would be AWFULLY slow. Also agree in that I always thought the threads would damage the packing if backed all the way out as well.
I have used shut-off valves quite a bit though when changing filters. And when changing a fuel pump on a '46 truck with a full tank of gas, a surprisingly large amount of gasoline is in the pail under the truck when done, even though the tank is under the seat. 🙂
