Screw caps get most of the attention in lamp-cap gauging because E27 and E14 are everywhere. But a surprising share of the caps on a production floor are bayonet or pin types, and the gauges for them answer questions the thread ring never asked. Treat them as a different family, not as a screw cap with the thread ignored.
B22 bayonet: slots, pins and contact
A B22 cap locks by pushing in and twisting, so gauging has to check the pin height, the slot geometry that guides the bayonet, and the position of the contact surfaces, not a thread at all. The gauges look different for a reason: a "go" check for a bayonet cap confirms the pins enter and seat at the right depth, and a check on pin height confirms the contact will actually meet the lamp-holder contact. A cap that passes a screw-style thread gauge is meaningless here because there is no thread to gauge.
| Cap type | Main checks | What a screw gauge cannot see |
|---|---|---|
| B22 bayonet | Pin height, slot, contact position | The twist-lock seating |
| GU10 pin | Pin diameter, length, twist lock | The 240° rotation engagement |
| G13 fluorescent pin | Pin diameter, length, spacing | The end-stop and insertion depth |
GU10: pins and the twist lock
A GU10 cap has two pins that enter straight and then rotate to lock. The gauges check the pin diameter and length and the geometry that lets the cap rotate the required amount and stay put. Because the engagement is a rotation, a pin that is slightly short or thin can still enter but fail to lock in the holder, which a simple "does it fit" check will miss. Gauge the pins, not just the body.
G13 and other pin caps
Fluorescent and similar tubes use pin caps where the important dimensions are pin diameter, length and the spacing that lets the tube seat in the socket end-stop. The pins take the wear and the bending, so gauge them on a schedule and keep a reference cap for comparison. A bent or thin pin reads fine on a visual check and fails in the luminaire.
Common mistakes
The biggest one is reaching for the E27 ring out of habit and writing "cap OK" for a bayonet part. The second is checking pin diameter but ignoring pin length, which is what actually determines seating. Keep the bayonet and pin gauges physically separate from the screw gauges, label each check with the cap type, and record the gauge identity the same way you would for a thread gauge. A bayonet cap judged only on its body dimensions is not a gauged cap.
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