ceilingfire-sprinklerrepairsprinkler-system
I found this sprinkler head in the ceiling of my new place with a cover cap missing – later I found the cap in a drawer.
How would I go about replacing this? Is there a way to attach the original cap? Do I need to get a professional to take a look? I'm afraid of accidentally setting the sprinkler off.
If the pipe itself is in good enough shape, you could just use a compression cleanout plug (sometimes called a mechanical cleanout plug) or a pressure testing plug:
There are quite a few different styles of these, but almost all of them work by compressing and expanding a rubber gasket inside the pipe. 2.5" might be hard to find at the big box stores, but they are available numerous places online. The main consideration would be finding one that would seat deep enough into the pipe that it would be beyond the notches for the old cap.
Given the bracket itself is threaded, assuming the screws hold firmly (eg: are correctly sized for the threads in the bracket, and the bracket is not stripped), I would install it without the nuts.
The screws are transferring the shear load from the white cover to the metal bracket, which in turn is held to the ceiling box, which in turn (as other have pointed out) is hopefully firmly attached to structure. Even if you could somehow get those nuts on, they would not be supporting any load -- they would just be preventing the screws from pulling out.
From your pictures, the bracket looks of sufficient depth that there a couple complete threads, and the screws extend quite far past the bracket (meaning it would take several full rotations to get them out, which is not something that will happen to a light just hanging from the ceiling).
So:
Best Answer
I was in total agreement with Ecnerwal, that those bumps were to hold it in wall board. Finding this picture tells a different story: (notice the bumps and the threaded insert)
My best guess is that up in the ceiling somewhere, is the rest of the part; those bumps thread into it, or you can just push (see, 4.a). Be gentle with it, those are 50 bucks.
Again, I agree with E, don't touch it unless you own the building. Even then, you might want to call in whomever has the service contract for it.