Quick Answer / Summary: The bore is 4 mm for standard plugs and 2 mm for miniature ones. The mounting thread varies by manufacturer, commonly M8 or a 5/16 inch equivalent for panel jacks. Depth behind the panel must clear the wiring and any nut. Measure all three before replacing a socket, because none can be inferred from the others.
Jack sizes are described by three unrelated figures: the bore that accepts the plug, the thread that fixes it to the panel, and the depth behind the panel. A jack can match on one and fail on the others.
The Three Independent Dimensions
Bore, thread and depth are chosen independently by the manufacturer, so two jacks with the same bore can be completely different parts. Bore determines which plugs fit; thread determines whether it will mount in an existing hole; depth determines whether it fits inside a shallow enclosure.
Bore and Grip
A 4 mm bore may be a plain cylinder, a split spring or a sprung insert. Two jacks measuring 4 mm can hold a plug very differently, which is why bore alone does not predict grip.
Thread and Panel
A jack with an M8 thread will not fit an existing 5/16 inch hole cleanly, and forcing it damages both. Where the panel already has a hole, the thread is the constraint that decides the replacement.
Measuring a Socket Before Ordering
Measure the bore with calipers where accessible, or with a known plug: a standard plug that fits snugly identifies a 4 mm bore. Measure the thread across its outside diameter and count the pitch if possible. Measure depth from the panel face to the rearmost part.
When the Thread Is Unmarked
Compare the thread against a known bolt: an M8 nut will run on an M8 thread and not on a 5/16 inch one. This test takes seconds and prevents a return.
Depth Behind the Panel
A deep jack in a shallow box forces the wiring into a tight bend, and the bend load transfers to the solder or clamp joint. Check depth against the enclosure’s internal clearance, not just its external size.
Thread Pitch and Nut Fit
Beyond the outside diameter, the thread pitch decides whether an existing nut will run on. An M8 coarse nut will start on an M8 fine thread and then bind, which feels like a damaged thread rather than a mismatch.
Jack Size Reference
| Dimension | Options | Decides |
|---|---|---|
| Bore | 4 mm, 2 mm | Which plug fits |
| Mounting thread | M8, 5/16 inch | Panel hole compatibility |
| Depth | 15-30 mm typical | Fits the enclosure |
| Grip type | Plain, split, sprung | Retention force |
| Insulation | Panel bush or none | Metal panel safety |
Practical Applications
Replacing a worn panel socket, specifying jacks for a new enclosure, matching a jack to an existing drilled hole, and checking depth in a shallow project box.
Frequently Asked Questions
What sizes do banana jacks come in?
The bore is 4 mm standard or 2 mm miniature. The mounting thread and depth vary by manufacturer, commonly M8 or 5/16 inch for panel jacks.
Will any 4 mm jack fit my existing hole?
No. The bore says which plug fits, not which hole the jack fits. Check the mounting thread against the existing hole before ordering.
How do I measure a banana jack bore?
Use calipers inside the bore, or test with a known 4 mm plug. A plug that seats snugly with no wobble identifies a standard 4 mm bore.