The standard bubble cap size will be labeled a "25mm cap" which means
it will fit onto a 25mm (total width) quartz banger, but doesn't mean
that caps will always 25mm wide. A cap labeled as a "25mm cap" is merely
stating that will best work on a 25mm quartz banger.
We suggest to look for a 25mm* bubble cap and pair it with a 25mm* beveled edge quartz banger for the best combination.
*Some users may prefer to use a 30mm quartz banger, which will need a
30mm cap. Other users may prefer a 20mm quartz banger, which can use a
25mm cap or a 20mm cap.