For a couple of years, I’ve been reading about the problems with the side gears on the Bachmann Spectrum HO scale Shay locomotives. I started having the same problems with mine; cracked and loose gears. A couple of weeks ago, I ordered their repair kit. It comes with the right side truck gears as an assembly, and screws. It took me all of 15 minutes to remove the old sides and install the new ones. Once in place, I weathered them to match the other sides, and it was complete. While I had the trucks off, I slightly enlarged the holes where they attach to the locomotive. This allowed them to rock a little side to side and front to rear. One of the advantages of the Shay prototype was its ability to run on less than ideal track. Doing this helps the model to emulate the prototype.
The part number from Bachman is H81912, and it’s listed as " Geared drive line". Hope this helps some of you that had the same problems and turned your Shay into a static display.