One of the trickiest parts is putting the cores in. I bought 2 gasket sets and I dab a little RTV on in a few places to hold the core gasket on.
That particular core was only in use for about one winter, so I didn't think it needed cleaned. The orange RTV was from last time.
The little clips that hold them in only go one way-I think this is the reason some struggle with this part:
I start the top screws, then reach in and hold the core tight to the housing and attemp to start the bottom screws. These are the tough ones, but after a few tries, it's not to bad. On one of them, I had to use a longer screw to line up the hole. Don't run it in to far or it may hit the core.
Now, the fan motors. I had 3 originals and 3 repops. After testing them all, I decided that the originals run wayyy faster and stronger than the repops. I picked the best 2 and cleaned and painted them. Remember the fiberglass nubs for the screws? These WILL strip out easy if your not careful!
Remember to insure a good ground on the ground wire.