I have two outlets in my pool, one at the bottom and one at the top, which I use for vacuuming. I was going to shut off a plug I bought that size, assuming my pool's bottom outlet has a 1.5" thread. It turns out it's actually slightly larger, as the 1.5" threaded connector slides in and out fairly easily. With this plug I was able to squeeze the hole just enough to close the hole securely and now my vacuum works very well.