It's a little hard to measure with a pipe sticking out of it, but it looks like it's about 3 & 1/4 inches. Maybe
this picture of the filler grommet will help? I got it from
this page at American Classic, our main sponsor.
If I were you, I'd buy the grommet, then measure its diameter (inside and out) so you can cut the hole properly. You're going to need the grommet anyway.
