This capacitor has been measured The air gap of plates fully centered was measured at approx.   ....... All plates are not exactly the same distance apart but they can be adjusted. No plates touch each other.. The shaft is 1/4 inch The following below is ARRL data on air gap versus voltage. Per page 265 out of 15th edition ARRL Radio handbook, Figure 26. Usual breakdown ratings of common plate spacings air gap in inches/ peak voltage breakdown GAP   Voltage .030 1,000 .050 2,000 .070 3,000 .100 4,000 .125 4,500 .150 5,200 .170 6,000 .200 7,500 .250 9,000 .350 11,000 .500 15,000 .700 20,000