I'll try to be as detailed as possible without making this a long story. The mainline for the house runs directly from the sewer and straight to the toilet which is then vented up through the roof. After years of water leaks caused my toilet to literally fall through the floor I had to replace the plumbing to the toilet with new ABS. PROBLEM: The cast iron vent does not fit properly into the ABS hub. Everything works fine but the vent is only sitting on the hub. There is no smell of sewer gasses which are pretty identifiable.
CONCERNS: I have almost no room to cut the cast iron vent without damaging the wall behind it and making a HUGE mess so I wrote off putting in a short span of ABS and coupling it to the vent. If I have to cut it and do damage I will. So that leads me to my questions...
QUESTION: What will happen if I leave it this way? I don't intend to but I'd like to know the risks and consequences cause its good to know. How do you recommend I fix this? And lastly I'd like you to weigh in on two suggestions I received: 1- I was told that I could use duct tape. Their logic was its used by HVAC techs to stop air from escaping the system so it should work just fine to stop sewer gases from escaping the gap. 2- I was told I could wrap fiber glass tape around the vent and hub and apply ABS glue to it similar to fiberglass patching. I don't know what to think so PLEASE weigh in on this one and don't hold back. The more details the better, if it goes over my head then I'll Google it :D
Thanks!

Cut off ABS hub, extend ABS pipe and use fernco coupling to attach ABS to cast iron pipe.

Be sure to support cast pipe so it does not drop down.

I am not a plumber but my understanding is that vent goes through roof to get any gases away from indoors. If u have a problem with main house trap u would let gases in.

Duct tape deteriorates over time and leaves it porous.

Chuck

I agree with Chuck. Cut the hub out and connect the two with a shielded coupling, (see image).

my understanding is that vent goes through roof to get any gases away from indoors
Vents are there to relieve the suction caused when something discharges or drains. Vents pull air into the system they don't expel sewer gas out. Good luck, Tom

I agree with above posts. It is a routine repair. The ProFlex coupling is the fix. Milo