G9 is a push-in cap with two wire loops that engage sprung contacts in a ceramic holder. There is no screw, no bayonet and no locking action, so retention depends entirely on the geometry of the loops and the spring force in the holder. A cap that is a tenth of a millimetre out can still light, and then fall out when the fitting warms up.
Wire diameter, loop spacing, loop height above the cap body, and the free length of the loop where it engages the contact. The first three are straightforward plate or slot gauges; the fourth is checked with a functional holder, because free length is what determines whether the loop deflects enough to be gripped.
G9 holders are usually ceramic, and ceramic holds its dimensions well but has no compliance. Unlike a metal holder, which can deform slightly to accept a marginal cap, a ceramic holder either accepts the cap or does not. That means the gauge limits should be treated as hard limits, and a batch that is marginal against the gauge should be assumed to fail in the field.
G9 lamps run hot at the cap, and the holder contacts lose spring force with thermal cycling. A cap that is correctly gauged still depends on the holder; where a fitting shows repeated lamp loss, check the spring force of the holder before rejecting the lamps.
