The housing material on the lowest part of a 9" isn't really thick enough for a good pipe thread. The best way to add a drain is to weld on a bung. That said, I did it the easy way. I drilled and tapped for a 1/4" pipe plug, but I was really careful not to run the tap too deep. If yours leaks, you can try some \Teflon tape. If that doesn't fix it, drill & tap to a 3/8 pipe plug, or better yet, weld on a bung.