Motors for pumps come in more variety that most people are aware of. In choosing a motor for your pool or spa pump you need to know quite a bit of information. You need the know at least the voltage, the horse power, speed(s), and frame size.
On bracket mounted motors you can determine the frame size of the motor by measuring the distance between the thru bolts on the motor where it attaches to the wet end. If the distance is 3 5/8” it is a 48 frame. A 56 frame will measure 4 1/8” between the bolts.
In choosing a complete pump for your spa or pool you not only need to know all the information on the motor (i.e. voltage, horse power, etc.) but you need to know about the wet end of the pump. Minimum, you need to know the form (side or center discharge) and the plumbing size.
You can determine the size of the plumbing on a pump by measuring the O.D. of the wet end threads. With the pump unions removed measure the overall outside diameter of the threads.
If threads measure about 3”, the plumbing is 2.0”. 1 1/2” plumbing measures about 2 3/8” and 2 1/2” plumbing would be about 3 5/8”. Don’t forget to make sure all of the plumbing is the same size.