What is this test cap-like thing?

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Whatsthat

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It's in the basement floor but lacks the rubber "meat cleat" of a test cap. The bolt head is out of sight, and I'm afraid to completely take off the wing nut without getting creative for keeping the bolt from falling into, presumably, the cast iron sewer line under it.

When I unscrew it to peek in, there is a metal plate stuck inside what looks like a cast iron flange. It cannot be pried up.

So, I don't know what it is, what purpose it serves, and how to get it out. Could you help me figure it out, please? TIA!!

Pictures are at:
https://i.imgsafe.org/d4dfd2764c.jpg
https://i.imgsafe.org/d4e01568a8.jpg
https://i.imgsafe.org/d4e06737ea.jpg

ps- I needed to get under it to learn more about a complete blockage of the 40 year old rough-in shower stub three feet to its left (which I discovered was completely blocked with ??? when trying to learn about why, for the first time, the washing machine discharge drain three feet to its left is draining too slowly for the same washing machine that has been there for 9 years. (I snaked it to no avail.)
 

Reach4

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It could be somebody did that to stop a backflowing drain.

If you tap the screw downward, without removing the wing nut, that may let the plug de-expand enough to lift the plug out.
 

hj

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It is an expansion test plug and you do NOT remove the bolt. You loosen the bolt and then lift it out of the pipe, although it sounds like your pipe is undersized and it is jammed in, but it will still pry out.
 

Whatsthat

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It is an expansion test plug and you do NOT remove the bolt. You loosen the bolt and then lift it out of the pipe, although it sounds like your pipe is undersized and it is jammed in, but it will still pry out.

Thanks for the quick reply. The test plugs I am familiar with all look different. Following a sandwich analogy, the one I have in my floor is two sandwich bread slices with nothing in between (see pics #2 & #3), whereas the rubber test plugs have a top slice and a rubber filling with no bottom slice. My mystery plug has no rubber between the two metal plates, and the bottom plate is entirely inside the lip of the cast iron pipe. Moving it left and right does not expose a gap in the lower metal plate.
Any more info is appreciated!!
 

Kreemoweet

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Looks sort of like the concrete was poured with that "plug" in place. The gods only know what might
be under there; time to bring out the demo hammer and expose that nonsense so it can be fixed.
 

Reach4

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It looks to me as if you could grab the thread under the top plate, remove the top plate, and screw the nut onto the thread. I don't know if that helps you if the bottom plate won't come out. A 12 in. Locking Clamp Pliers would seem to be suitable.
 

Whatsthat

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That "thing" between the two plates SHOULD be the rubber gasket seal.

Yep!
After spending a bit of time with a mallet and screwdriver, I was able to separate the fused rubber from the pipe opening. Apparently not being opened for a couple of decades makes a difference!
Thanks for your help.
 
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