No I don't know the exact size I used. I have bins full of screws. Just find a screw that is small so you don't weaken the strength of the tank. I drilled my hole slightly smaller, not much, to have material to cut threads into. Then I got 2 machined thread screws, both the same tread. One was self tapping the other wasn't. I tapped the threads with the self tapping screw then put the other one in after I shortened it. I didn't want a screw sticking out in my tank. The one Mike at notcig is making uses a larger plastic or nylon screw. You might want to go that way. There are a few tanks people made with filler ports though. That's just how I did it. I drilled it by hand, no press.