Measurement between thru bolts on this wet end: 3 5/8 inches ( please see: Determining whether your motor is a 48 or 56 Frame Motor.)
Frame Size:  Motors are made in 2 Frame Sizes on Thru Bolt Motors. Thru Bolt spacing on a 48 Frame motor will be less than 4 inches.  A 56 Frame Motor bolt's will be spaced more than 4 inches. Sundance Exception: On some Sundance Spas, they used some Emerson Motors that was a 56 Frame but had a thru bolt spacing of 4 inches if that is the case you would need the 56 Frame 6.1 inch diameter(used between 1997-1998 on their Theramax II Pump see note below*). 
Determining whether your motor is a 48 or 56 Frame Motor.

The spa and pool industry use various "Frame" sizes to mount their "wet end" or pump to. A bracket mount motor has a welded base for mounting. These motors can be a 48 or 56 Frame. 

Other types of motors such as the "C" Frame or Round Face and the Square Flange are all 56 Frame.

There are several ways to figure out the Frame size. One is to look at the spec sheet where it says Frame (FR) it might have some letters with it but will have 48 or 56 as the frame size.

The other way to determine is to measure the thru bolts (the 4 bolts that run thru the length of the motor and screw onto the wet end or volute of the pump). Measure the distance between the thru bolts,
 DO NOT measure diagonally but from one bolt to the next bolt.

48 FRAME distance between the 2 bolts is 3 5/8 inch (Less than 4 inches)

56 FRAME distance between the 2 bolts is 4 1/8 inch*(Over 4 inches)
So if your bolt pattern is under 4 inches you have a 48 Frame and if your bolt pattern is over 4 inches you have a 56 Frame.

But see the Exception below to the above measurements:

*PLEASE NOTE: AQUA-FLO FMXP FRAME Wet ends, have two different sizes on their 56 Frame Motors. They used 2 different motor manufactures, Emerson and Magnetek which can be different sizes.
Sundance Exception: On some Sundance Spas, they used some Emerson Motors that was a 56 Frame but had a thru bolt spacing of 4 inches (used between 1997-1998 on their Theramax II Pump). These pumps are the 6.1 diameter 56 Frame Wet Ends that we reference.
You must measure the distance between the thru bolts to determine the correct pump wet end for your spa.

How to Determine Spa Plumbing Size

Pump plumbing sizes are nominal, and not actual outside diameter (OD) of the threaded intake and/or discharge. Use a tape measure and check your old pump thread size against the sizing table

Plumbing Size      Pump Thread OD (approx.)
1 1/2 Inch                 2 3/8 Inch
2 Inch                      3 Inch
2 1/2 Inch                 3 5/8 Inch