Well, the bearing plate for which I've been waiting finally arrived, so I was able to to complete the rebuild that began over one month ago.  This photo reveals why the rebuild was necessary.  Notice the vents around the periphery of the motor housing.  It is through these vents that air flows.  Some of them were completely covered over with dirt. 

The most important components -- the armature and motor field.  Without them, nothing happens!

Motor field assembled inside the motor housing.

I'm not sure what these vents are for.  I suppose if you want to bench test the motor, you can observe what's going on.  However, you can't access the commutator in order to seat motor brushes. 

Motor housing vent closed.

I'll try to move this along.  This bearing is actually an upgrade from the one originally used.  By the time the D4 came along, Rainbow started using a double sealed bearing on the bottom.  Since it fits, I did the upgrade. 

And here's the bearing plate that was so hard to get!!!  Notice how the plastic shell surrounds the motor housing.  That's because the fans draw air through the vents on the bottom of the motor, around the motor itself, and finally through the holes in the periphery of the bearing plate. 

This bushing/grommet, which is missing in the previous photo, is important to prevent air leaks!

Now, the fan assembly begins, starting with the "air guide" which goes on top of the bearing plate.  There are three "stages" to this fan assembly.  Each fan stage has its own housing air guide and spacers. 

And here it is, all assembled and installed in the lower housing!  Before assembling all the fans, I recommend testing the motor.  If something isn't right, you won't have to go through the head ache of taking everything apart again!  Mine tested out just fine. 

During the rebuild process, I messed up the end of the armature shaft while trying to loosen a stubborn nut.  So I had to do some filing in order to knock off the high spots so the separator nut would fit.  One of the things I find interesting is that the motor support ring has a lip that also serves as a secondary gasket for the water basin.  In case someone is wondering, I did install the rest of the screws in the flange!  LOL!  I mention that, because I often forget to do something! 

And the final step -- installing the motor gasket on the flange. 

And here she is!  All dressed up and ready to go! 

Servicing the D3 motor is a much more labor intensive job than the D4.  This is the second one I've done and I still haven't found a more efficient way of doing things. 

@ Kenny -- the motor setup in your D2 is very similar to this one.  However, there is ONE major difference.  Air flows through the motor instead of around the motor.

@ Joel -- You've noticed the MAJOR difference in this motor from the D4 and E2 designs.  With the newer models, the fans sit directly on the water basin.  However with the D3 and earlier models, everything is flipped.  The fans are on top and the motor sits directly on the water basin. 

No, the fans on top are the suction fans.  Take a look at the above photos.  In the photo of the bottom bearing and bearing boot -- that is actually the bottom of the motor assembly where the spider and separator attaches.  Notice all the slots around the the periphery.  Air is sucked by the fans (on top) through those holes.  The air is then vented around the sides of the motor before passing through the holes around the periphery of the bearing plate and finally into the fan assembly itself.  The fan assembly sits on top of that bearing plate.  Notice too the plastic shroud that surrounds the motor.  It keeps the air flow concentrated and directed from the slots on the bottom of the motor (water pan side) and up through the periphery of the bearing plate and finally through the suction fans themselves.  With the D4, Rainbow went to a completely different (and more efficient and compact) motor/fan design with the motor itself on top and the fan assembly on the bottom where the spider and separator are attached.  The D3 motor does look awkward because it looks top heavy.  But the fan assembly on top is actually very light (being made of aluminum, I think).  Once the motor is attached to its support ring and the support ring is attached to the lower vacuum housing, it is very stable.  Additional support and stability is provided by the plastic baffle inside the top housing of the vacuum.  You can see the stripped baffle through the opaque plastic on top.  By the way, there is an internal cooling fan on the armature too.  You can see that in the second photo.  The attached photo is a more clear view of the top of the bearing plate where the fan assembly sits.  Again, the fans draw air through those holes around the periphery.  (I took this photo while "bench testing" the motor before I assembled the fans on top of it)