Questions about replacing old toilet flange on cement floor

Hi, I am remodeling a bathroom in the basement and after removing the toilet, found the flange was very corroded and old. I tried scraping what I believed was a wax ring away to see where the flange began and pieces of the flange itself came apart (metal). The pipe has an interior diameter of about 3.5 inches. It's hard to tell what type of metal it is but I know another shower drain project (right next to it) revealed copper drain pipes.
My question is, how difficult is it to replace this flange? Does the flange have to be sealed to the original pipe via some type of welding or can I simply use a plastic flange, fit it inside the original and screw it into the concrete with anchor bolts?
Thanks
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david wrote:

You can use a plastic flange if it will fit. Apply silicone sealant to the flange extension to seal to the pipe. Put anchors in the floor to screw it down using stainless bolts.
If you can find them, there is a plastic flange which has a rubber expansion ring on the extension. It is made for just the problem you have.
http://www.oatey.com/apps/catalog/showprod.asp?ctg &subctg=3
Check that the diam will fit.
Jim
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I have used that expansion ring Oatey part and it worked great (to cast iron). Bill

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Check with a local plumbing supply store. They will have what you need.
The plastic may work. If not there are retro fit pats that will.
Colbyt
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If the toilet is on a concrete floor, from a practical viewpoint, you don't really even need the flange. Our house was built in the early 70's and the downstairs bathroom on a concrete slab has the 4" cast iron soil pipe flush with the fllor with no flange on it. A master plumber told me this was fine and met code at the time. As long as you anchor the toilet to the floor so it doesn't move around, you can just use a wax ring w/plastic sleeve and you should be fine.
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Lowes and Home Depot sell a PVC flange repair part. You cut the old flange even with the floor and useing a hammerdrill, attach with concrete bolts. The stub fits down into the old pipe and seals by a rubber piece attached to it.

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