Which replacement sewer cleanout cover should I order

[anonyx]

I need a cap for an old sewer cleanout. The house was built in 1954, and I figure the sewer cleanout is probably from then.

I'm trying to figure out what exactly I should order. I tried to measure it. The inner circle diameter (Marked with a red line in this photo) looked about 3.25" The outer rim of the inner circle (yellow line) was about 4.5". The diameter of the whole thing (blue line) is about 6.5". I'm thinking maybe I mis-measured the inner ring and maybe this 3.5" brass plug on Amazon is what I need? https://www.amazon.com/Oatey-42373-...==&sprefix=sewer+cleanout+plug,aps,267&sr=8-5

Does anybody have extensive experience with old sewer cleanout standard sizes? Any constructive comments welcome. Thank you.

 

[Twowaxhack]

Probably 3.5” brass plug is what fits.

 

[anonyx]

Probably 3.5” brass plug is what fits.

Thank you! I'd pretty much come to the same conclusion, but I wanted to hear it from someone else, because I know nothing about plumbing. I looked at 3.5" plugs in other materials, PVC, Lead, etc, but think I will order the brass.

 

[anonyx]

Update: The 3.5" brass plug finally arrived in the mail today. It's too big. I notice that the diameter of it is actually more than 3.5". I guess I try the 3" next?

 

[anonyx]

Follow up: I received the 3" cap and it ended up working fairly well. (See attached photo.) I was afraid of breaking the rusty inner threads and I didn't want the cap to get stuck and be impossible to remove, so I've only screwed it in a little bit. Since the diameter is about 3.25", I would not have guessed that a so-called 3" cap is the size that I needed. Live and learn.

 

[Rickyman]

Coat the threads with pipe dope, I like Megaloc but you can use any brand you like

 

[anonyx]

Thank you Rickyman! I didn't even know it existed.