A number of Tektronix knobs deteriorate and crack. The same goes for the plastic gears. They were made by over-molding plastic on metal hubs. The plastic continues to shrink and eventually cracks.
The pen turret drive gear in a Tektronix 4662 plotter and the thumb wheel on a Tektronix W plugin are two such examples shown here. The plastic pieces are intact but no longer fits around the hub.
I simply turn down the diameter of the hub on a lathe to fit the smaller diameter of the plastic gear or knob and then glue it all together with epoxy. It works quite well.