How to attach 3" waste pipe to existing 4" flange?

I have a bit of a tricky situation:

I'm remodelling a bath room where I moved the toilet away from its original location, to build a shower on that spot. The old flange is embedded into the concrete floor. The concrete is insanely tough! Since I can't take out the concrete, I need to run the new 3" toilet drain into the old flange which has an inside diameter of 4".

The tricky bit is that it looks like there is no such thing as a 4" to 3" adapter that fits inside a 4" pipe end. I can only find such an adapter to fit in the female end of a 4" pipe, but I basically need one that fits inside the male end of a 4" pipe. I tried prying off the top of the flange, but it all seems glued together tightly.

The solution I have in mind is this: the horizontal 3" pipe will run towards the flange with the proper slope, and drop down into it with a 90 degree elbow. The female end of that elbow fits quite nicely into the 4" hole. However: there is a little bit of play, about a 1/16th of an inch. Therefore, I would need a good gap-filling, non-brittle, shock-absorbent epoxy to connect the elbow inside that flange. (Anything you can recommend in particular?)

My question: is there a better solution? If I were to do this, were I to violate plumbing code? I have studied the plumbing code inside out and I know how to meet all the various rules with regards to venting and what not, but this detail I am not so sure about.

I am really looking for a way of tapping into that flange without taking out the concrete. Any tips are really appreciated...

Thanks so much,

- Erik

flange

In just a few words, "forget it". There is no "legal" or even guaranteed long term solution for what you are trying to do other than remove the concrete and the flange. There is no concrete that is that hard. Either get a larger sledge hammer or rent an electric concrete breaker.

May have found another solution: just got a PVC flange from Home Depot that fits inside a 4" pipe and takes a 3" pipe. I just have to cut the top bit off, so I can drop the cilinder in the current flange as an adapter, cement that in place, and then cement the 3" pipe into it.

What do you think? I have to check first if it's a good enough fit, but if it is, then that should be good, right?

- Erik

flange

If you are comfortable doing it and then pouring cement over it, then I guess it will be alright. But a plumber would not do it that way, nor would I ever advise a customer to do it either.

I won't poor cement over it actually.

As it turns out, a little deeper inside the flange the pipe is indeed exactly 4" and the piece I got at Home Depot fits in there perfectly tight without any play. I just fitted it in there, and it's a perfect fit -- there's no play at all.

So, I don't have to fool around with any gap-filling epoxy or what not. I can just clean it, prime it, and cement it, exactly like this piece is meant to go inside a 4" pipe

Just measured how deep inside it fits, and the overlap of this adapter with the narrow (4") diameter of the pipe below the flange is 1 1/2 inches if I push the 3" elbow on top all the way down into it. So, luckily I have the right amount of overlap as well to cement it in place. It's a really tight fit too, I had to pull it quite hard to get it out...

- Erik