problems with sewer line

Hi All,
Had a problem with the sewer system on the second floor.
Ended up opening up the wall on the first floor and had to cut out a three-foot section of 4" cast iron sewer line. Sealed up the system by installing a 3-foot section of 4" PVC (sch-40) using two 4" Fernco rubbers.
I can't get the Ferncos to seal. The top Fernco weeps just enough to show a moisture ring at the top. The bottom fernco has significant leakage at the bottom. I tightened the clamps in stages with a screwdriver and then switched to a ratchet with 1/4" bit. The Fernco rubbers are simply not sealing on the cast iron.
Ugh. I'm at a loss. How can I seal this beast and still be able to get access in the future???
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How rough is the surface of the cast iron pipe? Also, maybe you are just being too weak in your tightening.
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On 9/14/12 2:12 PM, JTeller wrote:

Could you put some type of caulking or silicone sealant under the Ferncos? Would a couple wraps of the stretchable electrical tape under the Ferncos do any good?
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get

Fill the whole connection with silicon paste, then tighten the clamp.
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My guess is that your corrosion crack is extending past the point that you cut.
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On 9/14/2012 2:12 PM, JTeller wrote:

I suspect that you used a flexible coupling like one of these: http://www.fernco.com/plumbing/flexible-couplings these make no attempt to keep the piping lined up. I also suspect that your new pipe is not a good fit in the old pipe run.
Change out the couplings to one of the shielded type: http://www.fernco.com/plumbing/shielded-couplings "No Hub" clamps work well on cast iron/PVC connections. These hold the pipe intersection in line. The new pipe should have an 1/8 to 1/4 gap at each coupling.
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JTeller wrote:

One person wrote,
"My guess is that your corrosion crack is extending past the point that you cut."
I think that may be a possibility, especially since you wrote, "The top Fernco weeps just enough to show a moisture ring at the top". Seeing that on the top connection seems unusual to me.
But, if it is not a cast iron pipe that is still cracked, the other thing that I would try is to sand/grind the outside of the cast iron to make a smoother surface for the Fernco to seal with, and also add 100% silicone caulk in the connection between the Fernco and the cast iron to help it seal better.
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