Is it possible to use epoxy to stop a leak from a 4" horizontal cast iron lead joint?

If yes, can you recommend a product and installation tips?

Cutting, removing and replacing the leaking 4" wye would be a major project and involve work on two separate floors of this home.

Epoxy will work if you clean all oil and debris from the joint AND the joint is dry

srloren

if you can get your hands on some hard roofing tar or sometimes called tar bitumen, you can melt it into the cracks on the lead to make a long lasting seal.
have you tried simply tapping the lead with a caulking iron?
roofers have the tar in big blocks.
in the old days we would pack the oakum into a cast iron joint and then add in some chunks of the hard tar and pour the hot lead.

it seems that the trick is to compress or dig out the lead then seal it with something.

there are various epoxy products on the market and I have done a bunch of stuff with epoxy products from Abatron, System 3, Rot Doctor etc...those are specialized epoxy for wood, fiberglass and metal repairs. Most of these products you can get it in liquid (poured application), paste (fill in cracks although in overhead or vertical applications may cause slumps), and putty form. They are not cheap, the latest metal repair epoxy I got was SilverTip Metal Weld from System Three and it's like $60 a quart but they are strong and fast curing.

The reason they have different epoxy products for different materials (wood vs fiberglass vs metal) is because the epoxys are formulated to approximate the best thermal expansion and contraction of the materials you are trying to bond. The wood epoxy will thermally expand the closest with wood so when temperature changes and wood moves the epoxy moves with it instead of tearing a crack. You want the metal epoxy to bond metal. I have used them and they are indeed strong and waterproof. However you need a gap to apply them. So you need to dig out old loose lead or tap them in so give yourself some room to patch those in. If the surfaces have oil, grease, rust and dust, it will not work.

I have also tried the $5 big box "magic" epoxy sticks that are two part play dough type for pipe repairs and those do not work at all for me, I tried to repair a hole in a PVC irrigation pipe that's strangled by tree roots and it leaked. May be OK for drain, not sure.

It's a big job to do it right. In a pinch and if you can get the area really dry(I mean dry!!!!) get a tube of 100% silicone from home depot(Dap Silicone Plus 10.8 oz. Clear Premium) put on a rubber glove and wipe a whole hand full of the silicone into the area that's dripping. That will buy you some time. I've come across patches I did 10 years ago and there still holding strong.

If it's the actual lead joint that is leaking, a hammer and chisel should stop the leak, by "recaulking" the joint ( pounding the lead in further). If the cast iron is corroded through, I don't think epoxy will work. In that case, cut it out and replace with PVC.

Thank you for all the thoughtful replies. Restores my faith in mankind! lol good group of guys on this forum.

What I actually did: There was a street 4" CI wye leaded into a pipe hub. I cut and removed the hub from the street fitting. This was an hour job. I drilled out and removed the lead and oakum. Used my grinder and a cut off wheel to remove the hub. Use a carbide blade to cut the existing 4" horizontal CI pipe.

I used a 4-band no hub coupling and a 4" CI x Pl shielded coupling (because this is what I had in my garage this morning). Installed a short section of 4" ABS pipe. Tested. Done deal. Another happy homeowner. I did get something in my eye but just flushed it out and good to go. This pipe is 90 years old and the reason why this section rotted out was that it was flat in the area. Water sitting there over 90 years caused a 1" hole.

Location was in a crawl space and just getting there was a job. 3.5 hours total job. Plus I'm going to bill for some job deprep. Even though I covered the sewage soaked area with a tarp I need to sanitize my things.