I'm replacing the drain stack in my kitchen. I have the wallboard removed and exposed is the cast iron stack from the basement. At the top of that stack about 19" above floor level (inside the wall) is the connector or hub that receives a vertical “T”. Entering the “T” horizontally was the old P trap and vertically at the top of the "T" is the vent ( galvanized pipe).
I'd like to replace that stack inside the kitchen wall to somewhere above the basement floor. I can connect new PVC inside the kitchen wall and down into the basement to that cast iron with the shielded connectors that I've read about here.
I know that the sink tailpieces and P traps will be 1 ½”, I'm not sure what sizes of PVC I should use elsewhere. The current cast iron stack is 2 1/2 " O.D (4 1/4" O.D. hub). That hub connects on top to the 2/14" O.D. " O.D. " (3 3/4" O.D. hub) that received the old P trap and galvanized vent pipe.
Also, I'd like to know if I should install a vertical clean-out where I connect just above the basement floor while I'm at it.
Hope that explains what I have here. Thanks in advance.

I know that the sink tailpieces and P traps will be 1 ½”, I'm not sure what sizes of PVC I should use elsewhere.
Go 2" all the wy from where you converted to PVC. The 2" "P" trap will have a gasket to attach the sink tailpiece to.
Cleanouts are always good. You just can't have too many. Good luck, Tom

Thanks for the advise speedball. I thought that 2" was the I.D size of the cast iron stack, I just hadn't cut it yet to confirm.
I'll have to find an adapter to fit the 1 7/8 O.D. galvanized vent into a new 2" PVC double sanitary T as I think I have to put double traps under the sink (county code).

Thanks for the advise speedball. I thought that 2" was the I.D size of the cast iron stack, I just hadn't cut it yet to confirm.
I'll have to find an adapter to fit the 1 7/8 O.D. galvanized vent into a new 2" PVC double sanitary T as I think I have to put double traps under the sink (county code).

Use a 2 X1 1/2" slip to a 1 1/2" female threaded reducer and simply thread the galvanized in one end and glue the other end on to a short piece of PVC that will be coming out of a 2" sanitary tee that you will use in place of a double san tee. Once stubbed out of the wall install a wye and tun your traps off that. Good luck, Tom