Cast Iron Stack Leaking

Hi guys. My name is Adam. I am doing a bathroom remodel. I was in the process of removing a lead pipe from the vanity to the cast iron stack. I have noticed a leak. It's in a spot that will probably require removal of part of the stack. I'm looking for some options on what I should do here and how I should do it. Thank you for any information you can provide.

I have attached a photo. I have drawn a red line where I have found the crack. The crack is at least 2 inches long.

Wow, look at that beast of a fitting!

This project is above a DIYer's (and probably most Jr. Plumber's) skill level. That entire section shown will have to be cut out and rebuilt, as well as support installed for the roof stack...lest it come crashing down and tear a hole in your roof.

Yes, you may want to bring in the storm troopers on this.

I remember the first time I cut a 4" cast iron pipe going up through the roof, and for some reason, having worked so much plastic, I forget that a ten foot section of 4" cast weighed 110 pounds.

If you do intend to cut the cast, first measure the outside dimension. New cast is 4-3/8" and much of the old stuff is 4-1/8", which is considered a copper size.
A no hub band for that would be a copper by NH cast.
The copper side going over the old cast, and the NH side would work for either new cast or plastic.

Thanks for the quick response! I measured the outside dimension of the pipe to be 4 3/8. Thanks for the heads up on the old vs. new dimensions. If I were to rent a snap cutter and cut at the red lines shown would it be feasible to replace that section with PVC? If this is no good where would you suggest I cut? I would install a stack clamp to support the piece above. Is one stack clamp sufficient or more?

Thanks again,
Adam

Technically, probably. But, some places may not allow it since if the CI from above is not properly supported, it could crush the pvc in the middle. You, or whoever does this, must ensure the upper section is properly supported BEFORE you cut anything, as it's heavy enough to kill you and do serious damage to the structure if it falls just right. Also note that depending on the condition of the CI, a snap cutter may shatter the pipe rather than making a nice cut. Verticals are probably in better condition than horizontals, since they rarely have standing water in them, but it's no guarantee.

I suppose if using stack clamps on the upper part is too risky I could cut at the bottom and replace the entire upper with PVC all the way through the roof. I'm in a one story and the length of pipe is not that long. Maybe 10 feet or so. I'd rather not do this. I didn't even want to touch the stack at all.

A stack clamp on top of the plate would be fine. Anywhere you attach that can be supported.
4-3/8" works with standard no-hub, though they do make a cast iron (4-3/8" x Plastic) too.
A ratchet snap cutter will do the job. You can rent them.