There were leaks in my kitchen sink drain pipe in two places:
-- Immediately below the kitchen floor (in crawlspace)
-- Horizontal 1-1/2" galvanized steel pipe just before it reaches the wye and turns down into the sewer drain located at the floor (is this considered a stack?)

I removed the old, galvanized steel pipes in the crawlspace and replaced them with 1-1/2" PVC. In the process, the rusty vertical pipe attached to the sewer drain broke off right at the top of the drain. So now, I am faced with replacing the vertical pipe and sewer drain (stack?) too. It appears that the sewer drain is made of cast iron. There is also what appears to be a cleanout right next to it in the floor. I've been searching the web for several days, but haven't been able to find any information on how to replace the cast iron drain/stack with PVC. Can someone please help me?