The threads are most likely directly into the cast iron. When you break a pipe off it usually leaves a ring of the broken pipe inside the female fitting that needs to be removed. This rung can be cut with a fine tooth hacksaw (32 TPI) and then removed. A real plumbing supply house will have powdered sealers that mix with water. They are a very fine grain version of cement. This is applied to the remaining threads in the cast iron and a new nipple screwed into place (or even a NPT to slip PVC fitting). The sealer will conform to the outside of the new threads and form a complete seal. You could even unscrew the fitting and examine the 'molded in place' threads if you wanted (pipe dope will then be needed when you put it back together). I do this type of stuff enough to own the IPT taps needed to actually chase the threads, but even that sometimes fails to make a joint that will come up tight. The 1.5 IPT tap runs about $80

Good Morning Jeremy, I received your drawing via email and have reviewed it thoroughly. Before we begin, let me take a moment to commend you on both your code research and the proper use of fitting nomenclature. This is the type of description I would expect to receive from a 2nd year Apprentice; you have done your homework well. You are correct in your assessment that the original installers made this project far too complex. Not only can you combine the lavatory and shower or tub drains as you suggest as we go deeper into your project you will see that there is a distinct advantage in doing it in that manner. If I am understanding your description correctly on your main CI stack you have a 4" CI Double Tee (Sanitary cross) with a 4" closet bend to the closet flange on either side of the stack, then directly above that you have a second 4x4x2x1-1/2 Double Tee for the shower, tub and lavatory drains. We can simplify this but I would be hesitant to remove any fitting from that 4" vertical stack because cutting into a CI stack can be a very dangerous undertaking. CI stacks are extremely heavy and in most cases they were originally installed to be self-supporting or at best a couple strap-hangers that have by now rusted through or perhaps a couple pieces of wood blocking, which was crudely fastened in place. If you cut a section out you have the risk of the remaining stack above the cut falling down, causing serious structural damage as it rips waste arms out of the walls, possibly ripping a large opening in your roof and not to mention the risk that it would fall on you, causing serious personal injury or even death. I would begin at the 4" sani-cross where the closet bends are attached. Using a CI snap cutter cut the CI about 3" from the face of the hub leaving enough so that you can attach a rubber 4x3 reducing Fernco coupling NOTE: when your house was built the WCs had a 3.5 or 5gpm flush rate and required a 4" drain. When 1.6gpf WCÂs were first introduced they had serious problems with clogging. It was later determined that a 1.6gpf WC should be on a 3" line to reduce clogging. From the 4x3 Fernco you can connect an ABS "3x3x2 side inlet closet bend to a 3" closet flange, then run a 2" line to both the tub and the shower. The reason for enlarging the tub drain will be apparent in a moment. (Note: Code standard for a tub drain is 1-1/2" however there is an exception in the code which allows us to increase the size of a trap or drain line by one nominal trade size of pipe.) Next install an 1-1/2" riser in the wall behind the lavatory, with an 1-1/2" Tee for the lavatory waste arm to the lavatory trap and connecting the top of the riser to the existing steel vent with an 1-1/2" Fernco rubber coupling in the manner that you described. At the base of the riser you can install an 1-1/2" 1/4bend, however, given that the connection will be in the stud bay in the basement, and in an accessible location, I would prefer to install an 1-1/2" Wye & 1/8bend or a Combo, with the side opening connected to the riser. On the upstream end of the Wye or Combo I would then install an 1-1/2" female thread adapter and an 1-1/2" screw in plug to form a cleanout for the horizontal drain line. For each lavatory drain I would then run a 1-1/2 line straight from the base of the riser to the point where it will intersect with the tub or shower drain line at a right angle. Connect the lavatory drain to the tub or shower drain line by means of a 2x2x1-1/2 Wye & 1-1/2 1/8 bend or a 2x2x1-1/2 combo. (If you canÂt find a reducing Wye or Reducing Combo you may use a straight 2"Wye & 2" 1/8bend with a 2"x1/2" reducer but be careful here, a Tee would be prohibited.) In this configuration the tub and shower drains are wet vented from the lavatory drain thus all the tub and shower drain line on the downstream side of the Wye & 1/8bend is defined as a "vented branch" and the remaining short run on the upstream side of that Wye or Combo to the tub or shower trap is well within the 5Â maximum length limit for a 2" waste arm. The reason that we increased the tub drain line from 1-1/2" to 2" is because under the UPC an 1-1/2" horizontal branch line is limited to 1dfu, which is not sufficient for a tub drain, which is 3dfuÂs, however by increasing to 2" the line is rated for a maximum of 4dfuÂs, which is enough for both the tub(3dfu) and the lavatory (1dfu). If you have the second sanitary cross on the main stack as was mentioned before you can now cut the lines on the side inlets about 3" from the hubs and cap the lines with rubber Fernco caps. Note: for cutting the CI I would strongly recommend you rent a "CI snap cutter". Most tool rentals will rent a snap cutter for about $15 a day, and with the snap cutter you can make a good cut in about a minute. CI can be cut with a reciprocating saw but in my experience it will take about 20 minutes per cut and you will burn up so many blades that it will end up costing more than the rental cost of the snap cutter. CI can also be cut by means of a circular saw or side grinder with a metal cutting abrasive wheel, but using an abrasive wheel is never a good idea. Keep in mind that sewer gas is highly flammable and potentially explosive, not to mention the shower of sparks and debris you have from the wheel, and here again, if you figure in the labor time and cost of the cutting wheels it is still more economical to rent a snap cutter.

The sink/tub drains must be above the WC drains. Code prohibits any fixture arms below the WC fixture arms.

I have examined your photo in detail and find a number of issues that would be code prohibited today, however given that we have not installed oakum & lead joint cast iron in the last 40years it must be assumed that it is all original to the structure and that it did pass code at the time of installation therefore you are not obligated to upgrade it unless you make modifications to the fixtures or piping in the downstairs bathroom. On the other hand, since the downstairs bathroom is venting off that stack you would not be permitted to discharge a watercloset into that stack on the second floor unless you first establish an auxiliary vent for the first floor bathroom. The problem here is that the line to the watercloset is a 4" line therefore it would require a 2" auxiliary vent.