Jim:
A few points:
- You're right. It can be impossible to tell if a small valve has a washer and seat or a gate unless and until you take it apart. Sometimes it's apparant, but not always.
- You should be aware that you may not even need to cut your pipe. If your existing valve is soldered into place, most plumbers would simply:
a) drain the line of water and ensure the valve and outdoor spigot are open,
b) heat up the valve with their torch on the upstream side of the valve and slide the piping outward. Wipe the molten solder off the end of the pipe with a dry rag.
c) then heat up the valve on the downstream side and knock it off the pipe (if there isn't a finished floor that would get burned by the hot valve) and wipe the molten solder off the end of that pipe too.
Now, as long as the tinned ends of the pipes will fit into the valves, you can just treat those soldered ends as the new surface of the pipe. Sand it to remove any lead or tin oxide on the solder's surface, and flux that tinned end just like it was your copper pipe. Brush out the sockets on the new valve and flux them too. Assemble everything and solder it together. Ensure both the valve and the outdoor spigot are open when soldering. It's capillary pressure that sucks the solder into the joint, and if your piping is closed and the air expands as it's heated, that can be enough to overcome the capillary pressure and prevent solder from flowing into the joint.
To be clear: You don't need to completely remove the old solder from the pipe. As long as the pipe end will fit inside the valve, you're OK. Just treat the surface of the old solder the same way you would a piece of copper pipe.
Ditto if you wanted to re-use that old valve. You wouldn't have to remove all the old solder out of the sockets; just enough so that the new pipe would fit into the valve.
(If it's a gate or globe valve, it's best to take the stem out of the valve before soldering to protect the rubber washer and packing material from the heat. If by chance you muck up the little fiber gasket that goes between the stem and valve body, you can buy something that will work better at any place in your area that sells O-rings to the pneumatics and hydraulics companies. Ask that place for a "teflon back-up ring" the same size as the fiber gasket. Teflon back-up rings come in all the same sizes as O-rings do and are quite expensive, costing between $1.50 to $5.00 each, depending on size.
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- If you cut the pipe, you'll have to use a slip coupling and a little bit of extra pipe to replace the length of copper pipe you cut out. Where I live, it's difficult to get slip couplings because most stores only stock "dimpled couplings" like this one:
There's lots of bad advice out there, and something you might hear is to convert a dimpled coupling into a slip coupling by FILING off the dimple on the ID of the dimpled coupling, and this is terrible advice. Often those couplings are dimpled so deeply that you can have a zero or near-zero wall thickness at the dimple if you file it off on the inside. If you can't get a slip coupling, then use a 1/4 inch drive deep socket (or small 3/8" drive socket) as an anvil and pound the dimple out of the coupling with a hammer. That way, you're not reducing the wall thickness of the coupling at the dimple. And, if someone tells you to file out that dimple, tell them to tell people to pound it out with a hammer instead so this bit of bad advice is finally killed.
Hope this helps.