See pic below.
Smelled a lil musty and it turned out the plumbers who installed the checkvalve also reused one of the old cleanout caps, which was easily removed by hand. I removed that and put a new one in.
As well, I decided to open up the checkvalve cover as I never saw the mechanisim inside.
It was tight on screws, but I'm going to presume you want it tight tight so that the gasket is fully seated --- so that in the event of water, the water is contained from the checkvalve door as well as no water leaking out of that cap.
Are there supposed to be washers under these nutscrews in order to properly spread the torque load on the cover ?
Not sure if I should go pick some up and put them there.
It is a metal cover and it was probably "smoothed" around the bolt heads, so whether you add washers or not is up to you.
Licensed residential and commercial plumber