Plumbing Questions? Ask a Plumbing Expert.
Can you post a picture of the part you're talking about? You can post a picture here by clicking on ADD FILES next to the SEND button
The only screw type device on a Fluidmaster fill valve is the adjustment screw that adjusts the float height which controls the water level in the tank. On some Fluidmaster models the fitting the refill tube connects to has several flow options and a replacement fill valve will come with several options. I don't think it's likely that this part would just fall out and if it did the refill tube would no longer be connected to the body of the fill valve.
I just gave you a detailed answer
You're welcome. Do you have it back together? is the toilet functioning?