This pump is a rated 4HP Viper 2 1/2" x 2 1/2" 56 Frame Spa Pump... The measurement from the edge of the thread to the edge of the thread on the Suction and Pressure side is 3 11/16"..  It is an unusual pump not found in most hot tubs.. It is in some brands like Coast and some others and found in many Swim Spas.. It has a Viper wet end designed for 56-Frame motors along with two double-shielded ball bearings with moisture-resistant grease for quieter operation and long life. The dual speed motors have insulated windings for increased efficiency and large, highly versatile 6-1/2" diameter wet end for higher head pressure. A split capacitor (start and run) allows for less energy consumption. 

Spa Pump Features:

• Viper wet end designed specifically for 56-frame motors
• Two double shielded ball bearings with moisture resistant grease for quieter operation and longer life
• Two speed motors have four sets of insulated windings for increased efficiency
• 56-frame motor for added strength, cooler operating temperature, and longer motor life
• Large 2-1/2" intake for improved flow performance and reduced noise
• Large 8" diameter wet end for increased volume capacity
• Split capacitor, (start and run) for less energy consumption
• Thermally protected to extend life expectancy of motor
• Highly versatile wet end can be rotated every 90 degrees
• Dimensions (H x W x L): 9.08" x 8.05" x 15.72"
  
  
• California Title 20 Compliant