Cast Iron Main Vent Stack

19 years ago

I am replacing galvanized steel waste and vent pipes with pvc (at least I'm tring to). I've opened the wall that houses all the pipes in my bathroom. When I cut the vent pipes open they were nearly completely clogged with a very hard to penetrate substance - I guess this explains the my slow/non running drains. When attempting to unscrew the vent pipe that goes directly into the cast iron main vent stack, it broke off leaving a significant amount of rusty build up on the inside threads (of the main vent stack). My question is, since the wall is now open, should I replace the entire main vent stack with PVS/ABS or just cut in a new tap? From reading numerous posts on this site I realize changing the entire stack is a huge project but since all other vent pipes in this bathroom were nearly closed shut with build up, I'm assuming the main vent stack might also be. However, there is no apparant damage to the stack. I'd hate to leave it as is, close the wall then realize that it too is clogged.

Any suggestions/help is greatly appreciated.

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Comments (3)

brickeyee
19 years ago
How old is the stack?
Cast iron remains the quietest drain system.
The sheer weight and stiffness make the system nearly silent.
It is usually not very hard to remove even a corroded galvanize ring from inside a female thread, it just takes a little time.
Even if the threads get damaged some, there are a number of sealing compounds that can be used to restore the threads and seal a new pipe in the firring.
thibs
Original Author
19 years ago
The stack is 55 years old. I would rather not replace it if not necessary.

My original post may not have been too clear. The pipe that I broke off while attempting to unscrew from the main vent stack was a 1.5" galvanized steel pipe (originating from sink) which tapped in (horizontally) to the MVS.

Are you saying that the threads are part of a galvanized ring inserted into the cast iron pipe?
Are the sealing compounds you refer to available in HD or Lowes? Can you give me a name of one?

Thanks for the help.
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brickeyee
19 years ago
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