As designed, the impeller drive on the Model 59 blower has a key and a 5/16" bolt to secure it to the drive shaft stub from the gearbox which then transmits power to the auger drive.
Unfortunately with this arrangement, the bolt will break, leaving only the key to secure the hub to the shaft. If debris does make its way through the augers, and binds the impeller in any way, the hub will crack in the key way. Sometimes you can salvage the hub by removing the key and using the bolt. In the Model 47 blower on my 420, though the hub was so badly damaged, that it had to be replaced, at a cost of $275.
Because of these problems in the past, I have removed the key relying on the bolt to secure the hub. Unfortunately, if you load the nose of the blower into a snowbank, the torsional load on the impeller will break the bolt. I have been using a Grade 5 bolt, but am wondering whether a Grade 8 bolt would hold up better.