Cast Iron Main Stack connection problem

[nastymage]

I have a 50 year old house and the drain lines are 1-1/2" galvanized steel with cast iron elbows and tees. All drain lines run to a cast iron main stack. I need to replace one of the 1-1/2" galvanized steel pipes that enters a 3" ID leg of a "Y" that looks like a sanitary tee in the main stack. The old 1-1/2" steel pipe was shoved into the 3" ID opening in the tee and the space around it was filled with lead (I guess to hold it in place and seal it). I thought it was going to be a simple job to replace the steel pipe with ABS, by just butting a 4" diameter ABS pipe up to the cast iron "Y" collar and connecting it with a Fernco coupler containing a SS sleeve, but when I looked into the tee, I saw the ID reduces to about 2" right before it turns downward (see attached photo). I'm worried that if I just connect the ABS pipe directly to the end of the cast iron collar, the ID reduction will form a small lip inside that will catch debris and have the potential to clog. The old steel pipe was inserted all the way up to the reduction point in the "Y", which probably helped the flow, but I wasn't planning on inserting the the ABS inside the cast iron because I don't think that is the proper way to connect the two types of pipes. What do I do in this situation? Do I just connect the large ABS to the CI with the Fernco connector as planned and not worry about the little lip that sits inside the "Y" at the point where it enters the main stack?

[HayZee518]

a fitting made for the purpose of joining plastic to cast iron is called a DESANKO fitting. it fits into the hub of the CI pipe. OAKUM is packed into the joint and "CAULKED" or tamped into place with a flat bottomed chisel and a hammer. this is followed by a hot lead pouring or a two part expanding epoxy "CAP" - all during the procedure the desanko should not be allowed to move. When water or waste water goes through the pipe and fitting, the oakum expands, sealing the joint.

[pushkins]

I also think there is a special fitting available for this as well. It works like Hayzee mentioned but has a rubber gasket that is used as the seal.
You should be able to get it at plumbing supply stores.