Adapting tub to old cast iron toilet drain

I am working on installing a clawfoot tub into a c. 1915 building with exposed 4" cast iron drain/vent pipes. The upstairs bathroom it's going in used to have two side by side toilets, as shown in the attached image (viewed from the bathroom below). The tub will be over where the left hand drain pipe is, the toilet will stay over the right hand pipe, and the far right vertical pipe goes up through the roof as a vent pipe (and down to the sewer below).

I'm a novice, but am trying very hard to do everything in this bathroom myself (including replacing the entire floor last week). My plan was to come down through the floor with the 1.5" tub drain and put the P trap below the level of the existing iron pipe and then go back up and down into the iron pipe using a rubber doughnut, like this: http://www.yourepair.com/howtos/images/kitchenbath/cast-iron-hub-donut.jpg

I thought this was the best solution, as you wouldn't see the rubber doughnut from below (I'm trying to preserve a somewhat clean "original" appearance for the exposed piping downstairs by avoiding rubber sleeves, plastic piping, etc). But, I'm having trouble finding a doughnut that adapts from 4" ID pipe to 1.5" OD pipe. I've thought about drilling a hole through a large rubber stopper...

Anyways, the larger concern is that after reading through some other forum posts, I'm realizing that my idea will basically resemble an "S" trap, which can be a code violation because of the issue with losing water in the trap due to suction issues. Should I be concerned about this, and is there any similar solution that won't have the same issue? My original thinking was that because I was adapting relatively small pipe into a much bigger pipe that had it's own vent that this wouldn't be an issue (plenty of relative airflow), but I have doubts now and could use a second opinion!

Any thoughts would be very much appreciated...I'd like to do this as "right" as possible. There is also a sink drain on the other side of the room, but because of it's layout with relation to beams/vents/windows, it would be a real challenge to plug into that as well. This old toilet drain is right where I need it to be!

if your pipe is 4'' cast iron. you can buy a ''Mission transition band 4''cast x 2''plastic''. then, use a 2'' x 1 1/2'' bushing

that will get you from 4 to 1 1/2 cleanly


can you post a picture of the area wher the tub goes?

i will use it to draw some pipes

Thanks for your response Frodo. I posted a picture of the upstairs...not much to see though! I just got finished putting the subflooring down and will tile soon, but wanted to work out plumbing placement first. I also attached a picture of the 1923 standard clawfoot that my better half just stripped the exterior paint off of and refinished for reference of what's going to go there.

In hopes that it might be more helpful, I got a bit nerdy and drew up a measured version of what's there. The pipe is 4" cast iron (~4.5" od), but right now the bell has the old lead from the previous toilet still in it, which I cut off cleanly and kept because it has a pretty much perfect 4" ID (+/- 1/16").

I also made a sketch of what I was thinking, and imagine that I could use a pipe and a drillpress to bore a 1.5" hole in a rubber stopper if that makes any sense (it prevents me from having to cut the cast iron or do any lead joining, and also would keep the rubber adapter out of sight from below). Does that solution seem like it would work without venting issues?

The fitting you mentioned also looks like it could do well, but would that require my cutting the iron pipe and plugging in from the side (around the outside of the pipe)? I've never cut cast iron before, but do have a sawzall...
Might not be so pretty, but I guess function might be better than form in this case...

*bump*

Any ideas on a "proper" way of doing this? Should I cut the cast iron and use a rubber sleeve, coming in from the side? Or does that stopper idea look like it would be fine?

Many thanks for any suggestions...

As you have already noted, the way you propose to attach it will create an S trap. The pipe exiting the Ptrap should remain horizontal with 1/4" per foot of fall until it reaches a vent.