Hi Rossco,
On the Hobart 20qt mixer, the clearance can be increased by turning clockwise the set screw which is tighten upside down on the bowl support inside the pedestal (a little shield located under the bowl support might be removed in order to make this screw reachable). The mixer should be unplugged for this operation.
I hope that your Thunderbird mixer is designed like the Hobart one, and that this description will help you. Regards, holorim