There are two screws on the bottom of a fogger, one to fill it and one in the 510, which are you t5alking about
On the 510 the O ring is used to insulate the positive post, the screw, from the negative, outside and most of the atty, and it is devinately required.
On the fill screw it is used to seal the
tank and prevent leaks. If you do not mind leaks it is optional.
I can't see how either of these O rings would fall off while tightening. Sometimes when I take out the fill screw the ring stays on the screw and some times it stays in the atty but I have never had a problem wiht it coming off when tightened. Is your O ring broken?
I have a floating pin on the
mods I use with it so I have never touched the screw in the 510.