This Balboa HydroAir extended wall fitting is well-suited to thick-walled spas (up to 50 mm), such as wooden hot tubs.
The nut is not necessary for thicker walls, as the threaded adapter will screw right up to the spa wall, and effectively act as a nut.
For shells that are less than 12 mm thick, use short-threaded wall fitting instead.
You should install it with a gasket and also use silicone between the wall fitting and the interior wall of the spa.
Hole width to drill (external thread diameter) : 60 mm
Maximum wall thickness: 50 mm
External diameter of the face of the fitting: 85 mm
Inside diameter of the face of the fitting: 45,5 mm
Balboa HydroAir : 10-3803 or 30-3803