I was replacing a washer on my stem in the tub and noticed the seat was just sitting in there not attached to anything. There was nothing to hold it to. Can this be fixed without replacing the whole thing?

The seat has to be screwed into the body of the faucet. You need to use Seat Wrench - an inexpensive tool you can purchase in Home Depot for five bucks.

Check the seat for nicks or rough spots. If you find any you must replace the seat. Good luck, tom

I have the tool and a new seat. There just isn't anywhere to screw the seat.

I have the tool and a new seat. There just isn't anywhere to screw the seat.
The seat screws into the hole the water comes out of. Good luck, Tom

Use flashlight. Look inside so you can ID the threads. Insert seat onto the correct end of Seat Wrench, insert it in and start turning the wrench CW.

The thing is that the opening for the seat is too big and it won't screw in because of that. It feels like there are threads on the opening in there but the seat is too small. I got a new stem and seat from a plumbing supply place. He said he has never seen where that area would have been worn down or broken. It is really old plumbing. Could I perhaps get a larger seat to go with the stem I have or somehow rig it where it will go in?

I believe that what is going on is that the stem and seat that has been used in this tub have been the wrong sizes all along and since I have never had to replace the seat and only replaced the stem, I have never taken it out. This time when I went to change the washer the seat was just sitting in there loose. So I have to put one back in. I think maybe someone has rigged it to make the smaller seat fit and that rigging gave way. I am going to Home Depot tonight and see if there is a stem that will fit that is bigger on the end with a larger seat than the one I have. There is a place for the seat to go, but the seat I have is just too small for the opening. I will let everyone know how that works out.