Problem: the motor binds after replacing the rotor

Solution: Oversized Rotor or improper rotor gap

If the fan is hard to turn when the motor end cover is installed, but easy to turn

when loosened, the rotor should be removed and lightly sanded. Remove the

rotor and lightly sand by placing rotor in the palm of hand and sanding on a flat

surface making figure eight rotations. (Use the finest grade of sandpaper available).

this text taken from Desa's Tech Tips
Set the Rotor Gap:
  1. Remove rotor, check motor shaft for up and down movement. If present, replace motor.
  2. Replace rotor and vanes.
  3. Slightly loosen the 2 Gap Adjusting Screws. This allows a slight up/down movement of the pump body.
  4. Insert feeler, slide pump body down to obtain the proper gap.
  5. Re-tighten the 2 Gap Adjusting Screws, remove feeler.
  6. Manually spin fan blade to insure there is no friction.
  7. If you have decent hearing, slowly rotate the fan. At each 90 degree interval you should hear a slight "click" as each blade falls against the pump body. If you don't hear all 4 blades click, disassemble and find the fault.

Hint: a small piece of paper the correct thickness works great to measure the gap!