For the Video related to this part go to YouTube and Search for: Hot Tub Spa Part 2" Heater Split Nut Information How To
This part is a 2" which measures about 3" ID Heater Split Nut Kit which includes the split nut , (2) Screws and a gasket for use with a 2" Heater Union that would measure about 3" Edge to Edge on the threaded side. 
The Heater Flange Split Nut mounts to the heater tube using 2 screws to hold it in place and screws onto the heater threaded union with a gasket between the union and tube. 
740016010067,Split, Nut, 2", heater split nut, Heater, Union, gasket, Split Nut, 2"bt, Heaters, Unions,2,inch,89-238-1044, 89-238-1040,400-5161 , 400-5421 , 50086 , 610266 , 806105081858 , 5039-050 ,compression, balboa, gecko, acc, hydroquip, therm products, hydro-quip
If your plumbing is 2" and your heater is 2" this part is correct.
This version is the most popular version. 1 1/2 x 1 1/2 is 2nd most popular and 1 1/2" X 2 is 3rd most popular.This part is used in hot tubs to plumb a heater tube to a heater union. Note: The Heater tube in pictures is NOT included!!
  The Lower 48 states is Free. I Ship International Through the Global Shipping Program Only!! Note: The Heater tube in pictures is NOT included!!