Cleanout cap

[gianmatt]

I need a new cleanout cap for an old pipe. The opening is 3.5", no threads. I can't figure out what to get. The plumbing store gave me one that says "3.5 inches", but it's actually more like 4. Is this like carpentry where a 2x4 isn't really 2x4?

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[Geofd]

If there are no threads on the fitting no threaded fitting will work , the are plastic caps called spin tights you can buy them at home Depot or a plumbing supply house