Please find these plumbing parts...

Please correct me if I have not understood correctly.

You have a 3.5" threaded connection because you removed the cleanout plug. That would make this a cleanout adapter mounted in a 4" cast iron hub. That is the only thing 3.5" that I have ever seen.

A PVC chevron plug would solve your clearance problem and the plastic might thread where the metal won't but you want to adapt and install a 1.5" drain line here.

The normal action here would be to remove the adapter body, clean the hub, use a 4" donut rubber adapter and either install a new CO or convert to pipe. That may not be possible for you because of your floor and getting to it. If it were mine this is the route I would go. Half a day and a few drill bits later it would be out of there.

Searching the web for a 3.5" male adapter the only thing I could find was one made by Carlton for conduit. So you might find that at an electrical supply house. I don't think the grey PVC meets plumbing code but then this install does not either. If it works, it works.

I suggest you call a couple of real supply houses both in the plumbing and electrical fields. This is not something you are going to find at the BORG.

If you want to I will call my local supply house. They will at least know if the part is made in the plumbing field. They carry a lot of weird stuff.

Replying in pieces...

re: "You have a 3.5" threaded connection because you removed the cleanout plug. That would make this a cleanout adapter mounted in a 4" cast iron hub. That is the only thing 3.5" that I have ever seen.'

OK...that's possible. I thought it was all one piece, but you could be right.

re: "A PVC chevron plug would solve your clearance problem"

I searched 3 on-line plumbing supply sites and the keyword "chevron" was not found at any of the sites.

This is the only direct hit on a Google search for "chevron plug" and I'm pretty sure this isn't what I need:

re: "remove the adapter body, clean the hub, use a 4" donut rubber adapter."

If what I have is what you described, this might be the way to go. I'll have to take a closer look and see if there is a removable CO adapter.

At the present time, the female portion of the CO is about 85 - 90% above the slab, but the bottom portion is slightly "below grade" with the slap contoured downward in the last 1/2" of so. That's why the 3

re: "I suggest you call a couple of real supply houses ..."

I went to one of the oldest and best known plumbing supply houses in the area. That is where I bought the 3 1/2" to 3" cast adapter (with the large rim) and the 3" to 1 1/2" cast adapter that I'm using now. I also believe that this same supply house is where I bought the rimless 3 1/2" to 1 1/2" cast adapter 5 or so years ago when I added the sink to the basement bathroom. I brought in what I had cut out yesterday and they said they didn't carry it and that parts for 3 1/2" cast iron are getting harder to find.

re: "If you want to I will call my local supply house.

Thanks for the offer. Can I email you a photo of the adapter that I cut out? If I remove the -sblock- portion of your addy will that work? Or can you email me and I'll reply?

Let me know, and thanks again.

The part you are searching for is called a Double Tapped Bushing. In your case you are looking for a 3 1/2 x 1 1/2 Double Tapped Bushing.

They are still made and used in the petroleum and irrigation businesses. Here is a link to one manufacturers line:

Would any of these work? What I was actually looking for was a rubber freeze out plug, but in a larger size.