Pros and cons of coupling vs. threaded when transitioning from copper to PVC

Question

I'm replacing a copper DWV pipe with PVC since it's rotting out. It looks like this:

There's the cast iron on the bottom going into the foundation, followed by a brass cleanout, a brass tee above that, and then finally the copper that I'll be replacing.

Most sites I've been to recommend leaving a bit of copper and using a coupling to make the transition. But a few site recommend using a threaded connection with the understanding that the copper half should be the female half and the PVC should be male.

On this site I found the following with regards to using a coupling:

Q: Is this any better then soldering a copper female onto DWV and using a male PVC/ABS to connect to it?

A: yes it is better. Torque the band down to 60 inch-pounds and you'll not have to worry about it.

Okay... WHY is it better? What are the pros and cons of each method?

Definition: DWV - Thin copper pipe for drain, waste, vent usage.

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Edit:

On the page linked above, the one of the posters shows the following two couplings:

No-Hub (corrugated):

Pro-Flex (smooth)

I went and purchased a coupling with a smooth metal band around it that was "for connecting no hub pipes of similar or dissimilar materials." The PVC would barely fit in it and the copper was too thin and I could easily pull it off. Since the sizes were so different the metal band bent badly. I saw in the linked post that the rubber was supposed to be thicker at the copper end. I guess I just didn't understand how much thicker.

Follow-up questions:

What is the use of the corrugated coupling? What other features should I look for in an appropriate coupling?

Answer 1

I agree with lowercaset's answer: No-hub coupling is the way to go. To answer your followup question:

While different pipe materials will have the same nominal pipe size, they will have different actual outside diameters. When selecting a no-hub coupling to join dissimilar pipes, you need to select one with the correct ends for the two pipes that you are connecting.

When you look at the website for the por-flex couplings, you'll see a list of models and what their connection combinations are. You need to pick one that matches what your connecting.

As for the corrugated coupling in your post, that looks like a typical coupling used for joining cast iron.

The other option that you have is to remove all of the copper/brass right back to the hub of the cast iron. Then you can connect the PVC to the cast iron hub by the use of a rubber donut.

Answer 2

Even a properly installed threaded copper x pvc connection will eventually have problems due to the differing rates or expansion/contraction + plastic embrittlement as it ages. That is not to mention that rarely is a plastic x metal threaded joint properly done to manufacturers specs. The plastic usually gets over tightened into the metal, leading to an even faster failure of the joint.

None of those are a problem if you use a properly rated no-hub coupling. No-hub couplings are simpler to use, faster and less prone to failure.