G13 caps carry two pins at a fixed spacing, and the holder is a sprung contact pair that expects the pins to arrive parallel and at the right distance apart. In practice the diameter is well controlled by the pin supplier, while the spacing and the squareness of the pins relative to the cap body are where production drifts.
The go gauge is a pair of holes at the maximum material condition of the pin and the minimum spacing; both pins must enter together. The no-go gauge uses the opposite limits. Because the pins are long and slender, any bending shows up as a failure of the pair even when each pin individually fits, which is why the simultaneous test is essential rather than convenient.
A pin that is parallel to its neighbour but tilted relative to the cap face will enter the gauge and then sit at an angle in the holder, reducing contact area and raising contact temperature. Simple spacing gauges do not catch this. A seating check, where the cap face must sit flush against a reference plane with the pins engaged, does.
On a 1.2 m tube the cap is at the end of a long lever, so any handling force applied to the tube rather than the cap bends the pins. Gauging should happen with the tube supported near the cap, and a cap that fails after transport should be re-gauged before the lot is rejected, because the damage may be packaging rather than manufacture.
