You know, Brian, I just redid my gauges last winter and don't remember the size. Can't check right now 'cause I'm at work.
HOWEVER, Cletis says in a thread under "instrument panel gauges", posted last August, to replace them with 1/4 20 knurled brass nuts. Thataway, you can tighten and loosen them with your fingers.
Also, if your '54 has a radio grill, the atttaching hardware is the same size.
