The magnets are polarized and yes you can install them wrong if you don't pay attention.
The shimming I mention is the magnet to magnet edge spacing. Without armature in place they will pull to the metal shell easily but they can also pull sideways and the factory spacing will disappear. Once installed and shimmed the epoxy should be allowed to cure overnight before assembly of the rest of the starter. You also should note the top and bottom spacing of these magnets as both should be installed the same depth.
Since there was only two, I installed them just by guessing.
Don't laugh at me, OK? But I used Gorilla hot glue. After looking at the armature and seeing some of the copper wires broken, It's going in the scrap pile anyways. I saved the keeper, the gear and the washers. But the rest of it, is junk. Sooooo, I thought what the hell, just glue to see if I can get the spacing right.
What every they use at the factor, to attach these magnets, is tough stuff. I couldn't get it all off the casing. Seriously, I had a wire wheel, that fit pretty good inside the casing. And I went through to fully charged batteries on my cordless, before I stopped. And there was still some adhesive on it. (and see just how good this glue is. (Probably no better than the rest)
I have another decent starter with one lose magnet that I'll get serious with. It has 4 magnets, and