This is an 1.5" Inch Flow-thru heater housing split nut union. It is made to replace heater flange nuts. The overall diameter of the 1.5" split nut union is 2-3/4" & the flange side opening is 1-7/8". It has a large flange for stainless steel housings, and this split nut is for heaters only. This split nut will fit; Allied Innovations, Brett Aqualine, Balboa, Cal Spas, Gecko, Gulf Coast, and Hydro-Quip heater assemblies along with many others, with metal 1.5" heater assemblies.
Note: The 1.5" is not the actual measurement but a representation of the inner diameter of the plumbing that is used.