How it connects
Straight mechanical threads pull the swivel nut tight against the fill valve shank, axially compressing an internal elastomeric washer (often cone or flat shaped) against the flat face/inner bevel of the plastic or brass shank to seal.
Male and female halves
Female swivel nut (plastic winged nut or nickel/brass hex nut) with captive washer, which threads onto the male threaded shank of a toilet fill valve.
Usually connects to
- Matching 7/8 in nominal (15/16-14 UNS) male fill-valve/ballcock shank
- Matching male-thread bidet T-adapter or toilet-supply plug
Common sizes
- North American toilet connection: 7/8 in trade size, described as actual 15/16-14 straight thread; some listings call it 7/8-14 and give about 22–24 mm diameter. Confirm the fit designation.
- Supply lines commonly 9–20 in long; opposite ends can be 3/8 or 1/2 in compression, 1/2 in FIP or 1/2 in push-to-connect.
Where it is used
residential plumbingcommercial plumbingsanitary fixturestoilet water supplybidet seat installationtoilet supply lines
Variations
- Winged plastic hand-tight nut
- Plated brass/nickel/chrome metal hex nut
- Integral flat washer or cone washer
- Loose repair nut and rubber washer
How to recognise it
Recognizable by an oversized plastic nut (often with hand-tightening wings/ribs) or hex metal nut at one end of a toilet supply hose, internally threaded with straight machine threads (~24mm / 15/16" major diameter) and fitted with a captive black rubber or white elastomeric washer recessed at the bottom.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| NPT pipe thread | 1/2" FIP is tapered with 14 TPI and smaller outer thread diameter (~0.84" vs ~0.938" / 15/16" for ballcock), sealing on threads rather than a face washer. NPT is tapered and seals on the threads; the ballcock nut is larger (~24 mm vs ~21 mm), parallel, and seals on a washer. |
| 3/4" Garden Hose Thread (GHT) | Garden hose has a coarser pitch (11.5 TPI) and larger diameter (1-1/16" OD) compared to the 14 TPI of the 7/8" ballcock nut. GHT is ~26.5 mm with 11.5 TPI and used on hose bibs; ballcock is 7/8 in. and only on toilet fill valves. |
| Fill valve locknut (tank locknut) | The locknut is a large, thin plastic nut that clamps the fill valve to the tank and doesn't seal. The supply nut is smaller and sits below it on the end of the shank. |
| Faucet supply nut (3/8 in compression or 1/2 in IPS) | Faucet connectors use 1/2 in IPS or 3/8 in compression nuts. The 7/8 in ballcock nut is clearly bigger and only fits toilet fill valves. |
Also called
7/8 in ballcock nutToilet Ballcock ConnectorToilet Fill Valve Nut15/16-14 UNS toilet nutballcock coupling nuttoilet supply nut7/8 in BC female; 7/8 in. BC7/8-14 ballcock thread
Good to know
Although universally referred to in trade terminology as '7/8" Ballcock' or '7/8" BC', the thread is physically 15/16"-14 UNS straight mechanical thread. The seal relies entirely on the elastomeric washer pressed against the shank end, requiring no thread sealant or PTFE tape.
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