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Threaded Galvanized

As you can see above, I've got three bathrooms with 1/2" galvanized pipe multi-turn toilet valves I'm wanting to replace with quarter turns. I originally was planning on just breaking loose the old valve but I've begun to worry a little bit about creating a leak on the other side of the drywall in the process. Is that reasonable?

So I moved on to the idea I'd use a pipe cutter (I would like the stub out shorter anyways) and rethread. But I guess that still leaves me with the potential of creating a leak. I don't mind taking my time and utilizing a pipe wrench to try and combat force on the other hidden end but do I need to just bite the bullet and cut an access and replace from the joint I'm sure is on the other side of the drywall?

Never done this type of work before and I know it's not an atypical or "hard" job, even my upstairs bath already had the multi-turn replaced but that was more than likely done by an experienced plumber. What's the SoP for this type of thing? By the way the escutcheon is pulled away from the wall a bit so it makes the pipe look shorter than it is.

Thanks for any help in advance!

EDIT: Sorry forgot to update. Couple weekends ago I hit it with PB Blaster and was able to work the old valve off while holding the nipple dead still. Got the new 1/4 turn on with some tape and putty and got the toilet back and mounted. Plenty of room in the back with the 1/4 turn now so glad I didn't bother shortening the nipple. Thanks again for the help everyone.

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A pipe wrench holding the exposed portion of the nipple should allow you to remove the valve without loosening the joint in the wall.

But be sure to inspect the condition of the pipe. This appears to be galvanized steel pipe which rusts over time. If the pipe appears to be rusted severely or is filled with scale, you may want to replace it and even the pipes in the wall.

Note, you may wish to employ a helper to hold the pipe wrench while you unscrew the valve.

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    Yes, I mentioned in the post it is galvanized. No rust in the water but of course won't really have an idea of the inner until it's cracked. Eventually I'll look into re-plumbing with PEX but the home was built in the late 70's and I've seen no signs of pipe issues, thus far in my 6 month ownership. Good idea about a helper, will re-threading put a lot more pressure on the opposing joint because I'd like to shorten?
    – jaxwithanx
    Mar 14, 2020 at 18:06
  • Cutting and re-threading will probably put more stress on the opposite joint than just removing that nipple and replacing it with a shorter one. Personally, I wouldn't mess with it outside of changing the valve and escutcheon. +
    – JACK
    Mar 14, 2020 at 18:30
  • You would never be successful with trying to re-tread a pipe like that in-place. Successful threading in the cutting direction places a huge amount of stress on the pipe and then it is often necessary to back off the threader tool in the opposite direction to break off the accumulated metal shavings.
    – Michael Karas ♦
    Mar 14, 2020 at 18:56
  • Thanks. I'm going to just replace the valve and hold in place with pipe wrench. I was wanting to shorten because I'm installing a one piece toilet with the smooth sides that don't give great access but I should be fine with the 1/4 turn valve.
    – jaxwithanx
    Mar 14, 2020 at 19:29
  • @jaxwithanx It will be darned near impossible to thread a pipe in place like that one. First of all I don't think there is enough sticking out of the wall, second I don't think the die handle will turn that close to the floor, and lastly you run the risk of splitting the elbow in the wall due to the torque needed to thread the pipe. If you really want to rethread it, remove it and replace it but I'd just use a new piece rather than fool with that one.
    – jwh20
    Mar 14, 2020 at 21:19

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