This Kit features a 1 1/2" Threaded Split Nut that goes over the flange of a 1 1/2" Heater Flow Through style Tube... And a 1 1/2" Threaded x 1 1/2" Slip PVC Heater unions as well as a gasket that fits between the union and the tube flange... This kit is for one side of a heater tube you will need two kits for both sides of the tube.. see the size chart below and look at the pictures with measurements to be sure this is the correct part for you... 

 Plumbing and Related Parts

This is a chart for the outside Diameter of your plumbing pipes or Threaded Fittings so you can figure out what size parts to order. Note: The actual size a part is called is not the actual size of the hole in the part it is really the Inside Diameter of the pipe that would glue into that part which doesn’t make sense, since you cannot see inside the pipe. Therefore I am giving you the measurements of the outside of the pipe so you know what size part to order.  For example a part that ½” plumbing would glue in to, would actually measure ¾” inside the hole of the part and on the outside of the pipe that glues into the part.
Pipe Outside Diameter chart:
½” Outside Diameter is ¾”
¾” Outside Diameter is 1”
1” Outside Diameter is 1 ¼”
I have never seen 1 ¼” in Hot Tubs
1 ½” Outside Diameter is 1 ⅞”
2” Outside Diameter is 2 ⅜”
2 ½” Outside Diameter is 2 ⅞”
Threaded Parts Outside Diameter Chart:
1” Pump Thread Outside Diameter is 2”
1” Pump Union Thread Inside Diameter is 1 13/16”
1 1/2” Heater Union or Pump Thread Outside Diameter is 2 7/16”
1 1/2” Heater Nut or Pump Union Inside Diameter is 2 5/16”
2” Heater Union or Pump Thread Outside Diameter is 3 1/16”
2” Heater Nut or Pump Union Inside Diameter is 2 15/16”
2 1/2” Heater Union or Pump Thread Outside Diameter is 3 11/16”
2 1/2” Heater Nut or Pump Union Inside Diameter is 3 9/16”