The people want to extend the height of their PVC clean out plug.
I see the male adapter that screws into the clean out plug threads.
But the other end has no threading for the cap to screw on.

How was the old cap secured, or more to the point, why did they buy an adapter that wouldn't take a cap? I've seen plenty of outdoor clean out caps that are fastened with gold seal clamps.

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The adapter that has threads for a screw in cap is easily available. Where is this clean out located?

The clean out cap is outside the bathroom window that goes to the septic tank.
The pvc caps just screws on. It is level with the ground. The wanted to extend the height a bit, yet they want to be able to remove the extension if they needed to snake the line. They haven't purchased anything yet.

Install a male adapter and then install a female adapter (also called an end clean out) into the male adapter...add screw in plug to female adapter. You can purchase a STREET female adapter as pictured below (requires no pipe) and extend the cleanout a few inches, or if more is needed, purchase a length of pipe and a regular female adapter.

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Wow I see it. That will solve the problem. I wonder if HD will cut the pipe.
A 10 foot length is too much. 12 inches is more than enough

HD doesn't cut PVC where I am, but they have a section full of short lengths.

I echo Mark's advice. Just add square plug and you are set

Milo

I like Mark's idea, good luck.

John