How it connects
The threaded shank passes through a hole in the bottom of the toilet tank. A rubber shank washer seals the tank floor from the inside, while an external locknut threads onto the shank from underneath to clamp the valve securely in place.
Male and female halves
Male externally threaded straight pipe/shank; mates with a female threaded plastic or brass ballcock locknut (and accepts a female 7/8" ballcock supply line connector at its tip).
Usually connects to
- Matching tank-floor shank hole, commonly about 1 in, with rubber/conical shank gasket
- Matching fill-valve shank locknut
- Matching 7/8 in female ballcock supply-line nut
Common sizes
- North American shanks commonly use nominal 7/8 in ballcock / 15/16-14 straight thread, about 0.90–0.94 in (23–24 mm) OD, with some descriptions calling it 7/8-14.
- Typical tank holes about 15/16–1 in; threaded shank length often 1–2 in and varies by valve.
- UK/European versions may use 1/2 BSP (about 21 mm OD) or 3/8 BSP; these are different fits.
Where it is used
residential plumbingcommercial plumbingtoilet tanks and cisternsGravity-flush toilets
Variations
- Integral plastic shank
- Brass/bronze insert or shank, including Fluidmaster 400A-036
- Standard 2–2.5 in or extra-long thick/antique-tank shank
- G3/4 BSP non-North-American variant
How to recognise it
Recognized as the threaded cylindrical stem protruding from the bottom of a toilet fill valve, featuring coarse straight external threads with a wide rubber sealing flange/washer above and a large winged locknut beneath.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| NPT pipe thread | NPT has tapered threads designed for interference sealing, whereas fill valve shanks are straight mechanical threads sealed by a rubber washer on the flange and a flat/beveled washer on the supply nut. Ballcock is straight 15/16 in. x 14 TPI; NPT is tapered at 14 TPI with ~1.05 in. OD; G3/4 is ~1.04 in. OD at 14 TPI parallel. |
| Toilet water supply connection (7/8" ballcock coupling) | Both share the same external thread on the valve shank: the upper portion is clamped against the tank by the locknut, while the lower protruding tip mates with the supply coupling. |
| Toilet flush valve shank | Flush valve spuds/shanks are much larger (typically 2" or 3" nominal diameter) compared to the ~15/16" fill valve shank. |
| Standard pipe thread (NPT/NPSM) | Fill valve shanks use specialized standard 15/16"-14 ballcock straight threads, not tapered NPT pipe threads. |
| 1/2 in. FIP / 3/8 compression supply end | Those are the wall-valve ends of the supply line; the ballcock end is the larger 7/8 nut. |
Also called
ballcock shank thread7/8-inch ballcock shankToilet fill valve mounting shank7/8 in. ballcock thread7/8 BC15/16-14 UNS shank
Good to know
In North American plumbing, the external thread on a toilet fill valve shank serves two mating functions: mechanical bulkhead mounting to the tank wall via a locknut and gasket, and fluid supply connection at the tip via a 7/8" ballcock coupling nut.
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