Can I glue a PVC flange to a cast iron sewer pipe?

[Gastronomique]

Hi, new user here. I'm remodeling a basement bathroom including replacing the toilet in a 1939 era house. When I removed the old toilet (really old with all metal parts inside the ~5 gal tank), I found the old flange was virtually nonexistent. The only thing holding the upsidedown bolts that held the toilet in place was a thick, flaky layer of plumber's putty.

So I need to connect a flange to the 5" inside diameter cast iron sewer pipe. I've never heard of a 5" inside diameter sewer pipe, but see the photo for yourself. The top of the pipe is flush with the concrete floor. There is what looks like another pipe inside the 5" one and that interior pipe ends about 5" from the end of the bigger pipe (and the floor).

It seems I have a couple options. 1) I could chisel away the surrounding concrete and use a rubber transition to connect a PVC flange to the outside of the sewer pipe - if I can find one that fits a 5" pipe. That would be a lot of effort. 2) I could use PVC flange and insert it directly into the sewer pipe. By luck, it looks the the 4" inside diameter flange is about 4 7/8" in outside diameter, which may fit just right (or with a bit of sanding/filing) into the sewer pipe. See photo. If this works, it would avert the need to break through the concrete.

My questions:
- Does my description and options make sense? Am I missing anything important?
- What is right way to address this situation?
- If the right way is too onerous, would my option 2 work? Could I use PVC primer and glue to glue the PVC flange to the inside of a cast iron pipe? If not, is there a different adhesive I should use (how about silicon caulk)? My intent is to screw the flange crown into the concrete floor to add stability and take pressure off the glued joint.

Thanks for your help.

[johnjh2o]

You can't glue PVC to cast iron. What you have there is a 4" cast iron hub. There is a flange that should fit your needs. It's made in PVC or ABS. Either one will work if it's long enough to reach below the hub and into the pipe. It has three bolts that compress a rubber gasket against the pipe. Just be sure that you screw the flange to the floor. This is a link to the flange you need. It's the one they call a expansion flange.

http://www.plumbingsupply.com/toiletflanges.html

John

[Gastronomique]

Thanks v much johnjh2o. That helps. I found the expansion flange on the the website you sent. Unf, when I called them to check on the depth of the flange, they said its only 3" or so deep. Since its a good 5" from the end of the 4" cast iron hub to the floor surface, that wouldn't be long enough. However, they recommended I just add a length of 4" pipe and attach it to the existing hub to extend it to the surface of the flooring, or whatever length is best.

Does this sound reasonable? If so,
- What type of pipe should I use and how would I attach it to the end of the existing hub? Should I solder new cast iron piping to the old one?
- Since adding piping would allow me to control the depth or height of the end of the pipe relative to the floor, what would be the ideal height for the new piple extension. Is flush with the floor surface (after retiling it) about right? If so, I could use a simple 4" inside PVC flange with metal collar and bolt it to the floor.

Thanks, I appreciate any feedback. Mark

[johnjh2o]

Mark if you caulk a piece of 4" PVC into the hub using oakum and lead wool then a standard flange would work. The one you have pictured would be a good choice. Just be sure it's screwed to the floor after you glue it into the 4" PVC.

John

[Gastronomique]

Thanks again John. I'm going to give your suggestion a try. - Mark