Allows our drain plug lights to be installed in SOME masterctaft and MB Sports drains. Some mastercrafts already have 1/2" npt (normal) drains. Some have other designs. If you want to make sure, email us a photo of your drain plug on your particular mastercraft. Typically you can tell if you need this adapter by looking at your drain flange. If it has 4 screws on the garboard flange and your plug has a rubber O-ring on it, this is the adapter you need.