We are remodelling our bathroom and installing a new toilet. When we took the existing toilet off, we found the cast iron flange was broken, brittle and in need of replacing. We could maybe get away with a spanner on the completely busted side, but the other side isn't very solid so we think we should just replace it. The problem is, the drain pipe is PVC. I know how to replace a cast iron flange if connected to lead pipe and a PVC flange if connected to PVC pipe, but not cast iron that is connected to PVC. Any words of wisdom? Also, it's a basement bathroom so no access to under the floor.
Check out some of the fittings and repair parts from Oatey. Not sure what sizes of pipe you have. But if you have a 4 inch pipe, 43539 is supposed to slide inside of it. It has a rubber piece that expands using 3 socket cap screws.
Link to Oatey site product page for the fit inside, expanding rubber flange
I have been repairing my own toilet bend, which is a lead piece coming from a cast iron drain stack. I would have used that part above, but I think I'm going to go with a 3x4 repair flange instead because this non expanding flange was shorter and fit better. The "3x4" part means it will fit over a 3" pipe or fit inside a 4" pipe.
I hope this helps, every toilet flange problem is different. I had to buy many of the repair parts and test fit them before I found the right piece.
Don't get too hung up on "no access to under the floor" if the other approach does not work for you - standard techniques for plumbing in basements often involve chisels and hammers to gain access to pipes. Concrete is not untouchable. Bash a hole, access the pipe, pour some new concrete, move on with life.
What all that OSB is doing on a basement floor is a question, but I suppose it's something like Dricore on top of the concrete. Depending on how much you are remodelling, consider changing that out in the bathroom for cementboard/tile backer - OSB in a bathroom floor is a disaster waiting to happen, in my experience.
If renovations are more minor, plot out the edge of the new toilet on the floor, and stay inside that if you need to cut down to access the pipe.
There are toilet flanges with a special compression gasket that fits inside the pipe and seals the waste pipe. They are widely available at home improvement stores and plumbing supply houses. If your pipe is well secured from below (as it likely is almost certainly since it's in your basement floor), you can use this kind of gasket.
First you will need to remove the cast iron flange. Remove any bolts or screws and try to pry up somewhat gently so you don't break the pipe. You may find that the pvc pipe is itself a toilet flange with a compression gasket. If you can't pry it up, you can take a cold chisel and break the flange off.
If you are tiling the floor, you definitely want to wait until the tile is done before installing the new flange. When you're ready, slide the new flange into the pipe and secure it well to the sub-floor. That's it.
It's obvious, but because I have overlooked this in the past, I'd like to remind you to cover or plug the hole until you install the toilet to minimize the amount of sewer gases that escape into your house.