Hi there - first time poster here. So - I'm in the process of renovating a kitchen, and when I had all the old cabinetry out, I found out why the previous sink/disposal did not drain properly - the wall outlet for the drain was ~24" off the ground, rather than the usual 10-12". I guess this would explain why the disposal was always full of water. Anyway, I figured that the only thing to do was to open up the wall, cut out the old sink outlet and reestablish a new one about a foot lower down. As you can see from the pic, the original T is in very bad shape - the hub above the "T" is badly cracked, with several segments missing - so it could certainly do with changing. As I understand it, I can cut out a ~18" section of the stack, including the T and the damaged hub joint, replace this with PVC and join top and bottom with no-hub connectors.
Here's the first thing - the vent above the T is 2" OD threaded iron. However, the pipe below is 2 5/8" OD (2.6") cast iron. I measured this by taking the circumference (8 5/16") and calculating the diameter. I can't find any reference to cast iron pipe of this size, and Home Depot certainly didn't carry any connectors of this size (2" and 3" only). My plan is to replace the cut out section with 2" PVC pipe, with a 1 1/2" T to the sink drain. I make the upper connection with a 2" connector, and - if I could find such a thing - make the lower connection with a 2 5/8" to 2" connector. I can get a 3" to 2" connector, but I fear that this slightly-too-large fitting may not make a sufficiently tight seal with the cast iron pipe. Any ideas on how to use a 3" fitting, or any sources for odd sized connectors?

Second thing - the previous drain connection was located centrally between the hot and cold supply lines. However, this required a ~12 horizontal run to the vertical stack, and cutting a 2 1/2" notch in the intervening wall stud. I was thinking, when I establish the new T, just having it extend a couple of inches from the vertical stack, thereby avoiding another notch in the stud, and locating the drain to one side of the supply lines. Since the sink will be draining through a garbage disposal, which can be rotated to deliver in any direction, it shouldn't matter that the drain connection in the wall is offset, should it? Any specific reason - other then symmetry - for the drain to be located between the supply lines?

And when you figured the size of the cast iron did you subtract 0.25" for the pipe walls. Looks like 2" to me. Is the pipe the same size under the tee as it is on top? Measure it and tell me. If this were my job I Would take a set of cast iron ratchet cutters and make a cut about 14" above the floor, take everything down and convert to PVC with a 2" or 3 X2"No-Hub Coupling. Good luck, Tom

I'm not sure what you mean about subtracting 0.25" for the pipe walls. Neither of the pipes is a standard size. I understand that the actual OD of a nominal 2" cast iron pipe is 2 1/4". However, the actual external diameter of the lower pipe in this case is 2 5/8" - measured directly and also calculated from measured circumference at two separate points along it's length. Given that this pipe is wider than "standard" 2" cast iron, isn't the actual OD the value what I need to be working with when it comes to matching this with a connector? As I understand it, I need to match the internal diameter of the no-Hub coupling with the external diameter of the pipe it's coupling to (I'm talking actual dimensions, rather than nominal). The external diameter of 2" (nominal) PVC is 2 3/8". Presumably the internal diameter of a 2" (nominal) No-Hub coupling is a similar 2 3/8". Will a 2 5/8" OD cast iron pipe fit into a No-Hub coupling designed to take a 2 3/8" OD PVC pipe? If I have to grease the coupling to get it onto the pipe, will that compromise the seal, or am I likely to damage the rubber if I have to stretch it over a pipe which is 1/4" wider than the intended pipe? The actual OD of a 3" PVC pipe is 3 3/8", so I'm pretty sure a 3" coupling will be too big to seal effectively with the 2 5/8" OD cast iron pipe.
The upper pipe is threaded with an actual measured OD of 2 1/16". The is close enough to the OD of 2" threaded (2 1/4") that a 2" coupling should work. Thanks in advance for any advice and info you can provide. DC.

Do some careful measuring. 1 3/4'' galvanized exists. I have cursed 2 1/2'' and 3 1/2'' black iron (not cast).

Hi there - I lightly clamped each pipe with a 3" G-clamp, then measured the gap in each case. The upper threaded pipe measures 50 mm external diameter (= 1.96"). The lower cast iron pipe measures 65 mm (=2.55"). The question is - what size No-Hub coupling will connect the cast iron pipe to a 2" PVC pipe at the bottom, and 2" PVC to the threaded pipe at the top. Assuming that there are no odd coupling sizes between 2" nominal and 3" nominal, will a 2" coupling stretch to go over the larger cast pipe and compress sufficiently to seal onto the slightly small threaded pipe? Thanks in advance. DC.