For the last few days I haven't been able to turn off the hot water in the bathtub - I can turn it the faucet all the way to the 'off' position but water still gushes out. I've been turning the water off downstairs at the hot water tank. Today I took the faucet assembly apart, removed the stem, and found that the old rubber washer was in pieces, and I had to remove it and the screw and the little metal cup bit from the pipe with some needle nose pliers. Anyway, I went to Lowe's and got replacement washers, and thought when I got home that I'd do the cold water faucet too, since it had been dripping a bit. So I got the stems put back together, and put them back in and turned the water back on, and now the water positively gushes out - both on the hot AND the cold. I've tried putting the stems back on with the end bit screwed all the way down, and I've tried it not screwed all the way down - either way, water keeps gushing out. So now I have to have my MAIN water supply shut off, unless I want a constant gush of water running in the tub. Any idea what I'm doing wrong?

Thanks so much in advance :)

Remove the stems again and inspect the washers. If damaged replace. While you have the stems out, flush the valve by turning on the water momentarily. Re-install stems while in the open position. If you installed the stems in the close position you may have damaged the washers. If you have an access panel you may have stop valves so that you can turn off the water to the tub with out turning off the entire house.

Thank you!

I removed the stems again, washers looked good. By "open" position, I assume you mean "as loose as can be" i.e. the end bit not screwed down on the big hex nut at all. I did that with both sides and now the hot water turns off fully -- hooray! The cold water is still leaking quite a bit. I've taken the cold water stem off and put it back on a few times trying to get it right, to no avail. Am I over-tightening, maybe? FWIW, the screw at the end that holds the little metal cup and the washer on has usually loosened up when I remove the stem. Don't know what that's about, or if it's the source of my problem. For tonight I'm going to call it good, at least the hot water's not leaking and I can take a shower. If you have any further suggestions for me, a very novice homeowner, I'm most grateful. Thanks again.

Inspect the seats for damage. Some are removable, some are not. But you can re-mill the seats.

Any possibility, you got the wrong washer, cone vs. flat. Installed the cone upside down?

Which part is the "seat"?

Although believe me, I've been at this blinkin' project all day and I feel I have inspected every part of this I could...

I'm pretty sure I got the right type of washer; it was the same as the old one, anyway. I took the whole assembly with me to Lowe's when I bought the new washers. And the hot water one seems to be okay.

Now I know why plumbers charge so much.

It's what the washer "sits" in. A "seat".

If that has burrs, or sharp edges, then it jut cuts into the washer. Generally what the washer seals against needs to be perfectly flat.

This link: Faucet and Seat Removers and Reamers Product Info and Reviews | DoItYourself.com

Has a number of tools, namely the seat wrench for removable seats and the seat reamer which restores the surface the washer has to seal against.

BTW: Torx stainless screws work really nice instead of brass with a little (something in a tube that you put on the threads that makes disassembly easier). Mind escapes me now. A little silicon grease keeps the seat from being grabbed.

With plumbing, you never know what to expect.

Ahhh.. "Generally what the washer seals against needs to be perfectly flat." -- I think that's my problem. On the cold water faucet, the little metal cup got bent at some point today -- probably when I first put the stems back on, unfortunately in the closed rather than open position. It's a very thin metal cup -- pretty bendable, I'd say, between the washer and the screw at the end. So I do believe you've solved the mystery, and hopefully tomorrow I can solve the problem. Thank you very much for your help!

Implicitly, the "seat" is typically on the faucet side, but your right, you do have "stem seats" and "faucet seats".