New Closet Bend and Flange Question

First off, thank you for the forum help! It is so nice to have a place to go for answers.

I am putting a new toilet in by basement under the slab of course. I am running it to a 3 inch ABS drain pipe. I have the cement broken up and have the hole all done and am ready to run the pipes.

I found a closet bend 4 inch (input) to 3 inch (output) at Home Depot.

My question is Home Depot has two flanges. One fits inside the 4 inch closet bend and one fits on the outside of the closet bend.

The one that fits inside sort of slants in and reduces. The hole seems bigger on the one that fits outside but maybe too big for the wax ring???

Does anyone have any idea if I should use the inside fit or the outside fit?

Also, I assume I want to fill in the hole with cement before putting on the flange so I am sure the flange is on the floor level but if I use the flange that fits on the outside how to I pour the cement and leave room outside for the flange?

Thanks for all of your help with the plumbing questions! And if you have a computer question I'll be glad to answer it!!

You can put a sleeve around the pipe before you pour the concrete and remove it after if you go that way. Also, keep in mind that if you are going to put anything on the concrete like tile, the flange should be installed on top of the finished floor and anchored through it to the (in this case) slab. You don't need 4", the whole toilet flange and bend can be 3" if that helps. With 4", though, you should be able to get a flange that mounts internally, so the outside of the pipe could have the concrete up to it; you need an outside mount flange for 3". 3" will be potentially a little shorter and give you a little more leaway for the slope. As to which is best, I'll leave that to the guys with more experience...

bend

The 4x3 is much shorter than a 3" quarter bend, and the outside one it the better installation. Even though the opening seems larger, once it is on the pipe, the pipe's opening will determine the size. The wax ring will fit perfectly.