Replacing cast iron pipe in garage - where to cut?

[SDmark]

My 1950s home in San Diego has a 2" cast iron drain pipe running across the ceiling of the subterranean garage. It comes out of the slab on one side and connects to a toilet drain just before going through the cement wall on the other. The exposed cast iron leaks rusty goo onto any car in the garage.

I'd intended to just cut the 2" cast iron at both ends and replace with ABS, but the bathtub above the garage complicates matters. From bottom to top, I see a combo (if that's what it is called), a sanitary tee, a 45, and a vent pipe, all cast iron. The cast iron above the ceiling is in pretty good shape.

Can I (and should I) replace just the horizontal pipe with ABS and somehow connect it to the combo? How would I do that--cut the combo an inch below the upper bell? Or do I need to open the wall behind the tub, cut the vent pipe, and replace _all_ of the cast iron fixtures with ABS?

Thanks for your thoughts...

In the next photo, the horizontal drain pipe above the copper water line goes to the tub drain (last photo below).

Here's the same drain pipe in the tee, with a 45 in the vent, which is parallel to the long side of the tub. It looks like a 2x6 wall. I do have a non-tile wall in a bedroom behind the tub, i.e. I think I could cut through buttonboard from the bedroom to get to this 45.

Many years ago I put in an ABS p-trip, I think to fix a leak.

 

[Jeff H Young]

Total judgement call on your part if for any reason you want this fixed now and fixed right you might be willing to go the extra mile something you didnt mention is the penetration through the dry wall with abs and fire ratings .old home and likley or possibly exempt . I think it would be wise to keep the living spoace somewhat firesafe from garage .
Id like to rep0lumb the entire tub waste as well , the vent coming off the top of that santee is it wet ? serving a lav perhaps? too much work to run all new abs and build a soffit for the piping . might not be too big a job but a bit of wall patching .
I guess at a minimum you can just replace the 2 inch cast from the combo down stream.

 

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[SDmark]

@Jeff H Young, thanks taking a look and for your reply.

If I remember correctly, the plumbing was exposed when I replaced the p-trap (1999?). An inspector checking electrical didn't like that the open garage ceiling due to risk of carbon monoxide rising into the dwelling. So I "sealed" the ceiling it with drywall and patching compound. I think he was happy after that. No mention of firewall. Would that be different with ABS? Maybe I need to get a permit and ask the inspectors.

I will check but I don't think the vent is wet. There's no lav on this wall and the tub hasn't been used in weeks. Probably a shadow in the photo.

The horizontal bell that the combo plugs in to is leaking rusty goo. I have no skill or desire to try to unseal the cast iron joint. So I think the combo has to be replaced as well, though I wondered about cutting it below its vertical bell.

I *think* if I dig into the wall behind the tub, I could get a Sawzall in there, cut the vent, and drop all three cast iron fixtures. Then it would be all ABS up to the diagonal vent. It occurred to me that there is an open attic above the bathroom, so I could theoretically cut the vent up there, but then running the ABS all the way down the wall could be tricky.

 

[Jeff H Young]

I think you are ok with abs since thats not fire rated