I needed a new motor for my roof fan, ordered it on Thursday, and
picked it up 40 miles from here on the same day. Friday, I went to the
motor store in town to retrieve the old motor.

Same model number, on the box and on the name plate. I compared them
and they seemed the same.

Installed the new one this morning at 9. Attic a little warm, but with
my shirt off, not uncomfortable.

Always tricky but harder than I remember to get the blade and the motor
past the 3 brackets, even though they are disconnected from each other
and dangling. I wish I could put the blade on after putting the blade
and motor in place separately, but that would not work. Not a lot
of clearance. Motor has to be vertical, and if the blade were higher,
would hit the cover of the fan. Which says Nautilus on it, btw. I
pushed it and not brittle yet, afaict.

Get all three brackets bolted to each other, two of them tightened down,
but the third is tight already and still 3/8" between the two brackets.

That's when it dawns on me that the new one has a layer of smooth black
plastic around the top 3rd. Maybe 2 or 3/32th" thick. (The motor is
about 3" high and 5.5" in diameter.)

Get out of the attic and check the old motor. No, it doesn't have that
plastic. I wonder how hard it woudl have been to rip it off.

And now I wonder how long the bracket will hold with no lock washer
(does that matter in this case?) and the two pieces of sheet metal not
touching.

I'll call the fan company asaic.