I'm glad you got it running but that's crazy.
First- best engineering practice would be to NOT have the worm gear pressed directly onto the motor shaft. If the motor fails you'll have to find an exact replacement. There should be a U-joint, etc., between the motor shaft and worm gear. (Look at American Models or S-Helper Service for example.)
Second- why didn't Dean advise you (or offer himself) to press the worm gear into its proper place on the motor shaft? Then the original screws could be fully tightened and there would be no need for the washer!