Garden hose thread (GHT 3/4")

You want to split a tap outlet in two, or drop an adapter between the hose and a spray nozzle from a different brand — but the part you buy is never quite the one you need. Printing it is tempting: it's a cylinder with a thread and a flat face. Except this thread has two quirks an ordinary bottle cap doesn't, and either one ruins the part if you ignore it: it carries water under pressure, and it lives alongside at least three similar-looking standards that are easy to confuse. This article explains how to print a GHT adapter that screws onto the tap and doesn't blow apart.

What GHT is, and why size is what matters

GHT (Garden Hose Thread) is the standard for domestic taps and hoses across North America. Its full designation is 3/4"-11.5NH: a hose thread, straight, eleven and a half threads per inch. The domestic size — the one on almost every outdoor tap and every garden hose — is 3/4 of an inch. When someone says "hose thread" and nothing more, this is what they mean.

The usual split is simple: the tap ends in a male thread, and the end of the hose carries a rotating female nut that screws onto it. A printed adapter typically has both — female on one side to grab the tap, and whatever you need on the other — so you'll be modelling both types (male and female) of the same thread and giving them different clearances. That asymmetry is half the job.

Straight, not tapered: the dimensions that count

The first thing to be clear about is that GHT is a straight thread, constant in diameter along its whole length. It is not an NPT. NPT — the pipe thread you'll see in plumbing, not in the garden — is tapered: it seals by forcing metal against metal as it narrows. Model a GHT as if it were tapered and it won't screw onto anything; try to seal a GHT by cranking the thread down like an NPT and it still won't, because that isn't where it seals.

:::spec{title="GHT 3/4" (3/4"-11.5NH), nominal dimensions"}

Dimension	Value
Thread type	straight, not tapered
Major diameter	~27.0 mm (1-1/16")
Pitch diameter	~26.5 mm
Threads per inch	11.5 TPI
Pitch	~2.2 mm (25.4 / 11.5)
Profile	NH (~60°, truncated crest and root)
Seal	rubber washer on the face, not on the thread
:::

Look at the last row, because it's the one most people miss. GHT does not seal on the thread. The watertight seal comes from a rubber washer seated in the bottom of the female nut, which is squashed against the flat face of the male end as you tighten. The thread only supplies the force that compresses that gasket. This frees you up: you don't need the flanks to fit with an interference press or to plumber's precision. You need them to screw together smoothly, and you need the face that meets the washer to come out flat and clean. Design that face as a genuine sealing surface — a flat seat, perpendicular to the axis, with no seam steps — and let the rubber do its job.

How it threads in FDM

The 2.2 mm pitch is good news. At a 0.2 mm layer, each turn of the helix climbs eleven layers, plenty to draw crest, flank and root without smearing them together. GHT is a coarse thread; the resolution problem that kills fine threads simply doesn't show up here. Why pitch is the deciding factor is covered in Modelling threads.

What you do have to solve is the clearance, and it runs the usual way: the printed male comes out fattened because the bead bulges the outline outward, and the female comes out tight because the bead bites the cavity inward. Model both to exact size, and the flanks interfere before they seat — the thread seizes on the first turn. The fix is to leave flank clearance, and how you share it out depends on what your part screws against.

Here's the detail almost everyone skips: most of your adapters don't screw printed part against printed part, but printed part against metal thread — the tap, a hose nut, a fitting. The metal can't be touched: it comes out at its exact factory size. So all the clearance has to go into the printed part, whichever type it is. If your part is the female that hugs a male metal tap, you open up the female. But if your part is the male that a metal nut screws onto, don't leave it at nominal size: it comes out fattened and the nut won't start, so you have to slim down the printed male. And because you can't split the gap between two parts, threading against metal usually needs more clearance — on the order of 0.20–0.40 mm — than between two printed parts, where 0.15–0.20 mm shares out nicely.

:::spec{title="Starting clearances for a printed GHT 3/4" (PLA/PETG, 0.4 mm nozzle, 0.2 mm layer)"}

Parameter	Starting value	Why
Flank clearance (printed against printed)	0.15–0.20 mm total, on the pitch diameter	absorbs the fattened male and the tightened female
Flank clearance (printed against metal)	0.2–0.4 mm, all in the printed part	the metal side can't be touched; you can't split the gap
Where the clearance goes	always in the printed part	if the metal is the female, slim the printed male; if it's the male, open the female
Orientation	thread axis vertical	crest and root come from clean stacked layers
Sealing face	flat, perpendicular to the axis, no seam	it's what the rubber washer presses on
:::

On orientation there's no room for opinion: print with the thread axis vertical, perpendicular to the bed. That way the helix climbs layer by layer and each turn rests on the one below it. Lay it down, and half the thread hangs in the air on its underside, droops, and the fit you measured disappears. If the adapter has a thread at both ends, orient it axis-vertical all the same: the two threads share an axis, so a single orientation saves both. Why the lower flank is still a gentle overhang — and why that eats into part of your clearance — is in Modelling threads.

Water pressure breaks it a different way

A bottle cap holds back nothing; a hose adapter takes mains pressure, which in a home sits around 3–6 bar and can spike higher via water hammer when you shut a valve abruptly. That pressure hunts for the weakest point of the part, and it's worth understanding exactly where it's going to pull, because intuition misleads you here.

Pressurised water pushes the walls of the cylinder outward, and it generates two unequal stresses at once. The hoop stress — the one that opens the tube lengthwise, like a ring being stretched — is p·r/t, and it is twice the axial stress — the one that would separate the tube into two slices — which is p·r/2t. Hoop wins: whatever the orientation, it's what breaks the part first.

Here the axis-vertical orientation works in your favour again, for the same reason it did on the thread. Printed upright, the layer lines stack along the axis, and the bead of each layer runs around the circumference continuously. In other words: the dominant hoop stress travels along the bead — the strong direction, continuous material — while the bond between layers, the weakest thing in an FDM part, only ever sees the axial stress, which is half. Laid down, it would be the reverse: the hoop stress would cross the layer joins and the part would split there the first time. Upright, the thread and the pressure ask for the same thing.

That doesn't mean upright is invulnerable; it means the failure moves. With the hoop stress pulling within the plane of a layer, the weak point becomes the vertical print seam and the boundary between adjacent perimeters: if the wall is thin, the part cracks along a longitudinal split — vertical, following the seam — due to a lack of adhesion between neighbouring beads. And there's a second, subtler failure that needs no crack at all: FDM laminate, even printed solid, retains micropores between beads and between perimeters, and pressurised water weeps through them without anything breaking. A part can hold the pressure without cracking and still sweat through its walls.

PETG usually behaves better than PLA here, thanks to its toughness and its better tolerance of moisture and sun; PLA is easier to print, but it turns brittle outdoors and in the heat of a patio in summer. Whichever you pick, geometry rules: thick, solid walls around the water, not a shell with infill.

Confirm it's GHT 3/4" before you print

Before you model anything, make sure the thread in front of you really is a GHT 3/4". The name "hose thread" covers several standards that are not interchangeable:

• North American GHT, which is what this article is about.
• European hose threads, often derived from BSP (British Standard Pipe thread, with a different profile and pitch), common on taps and fittings outside North America.
• Quick-connect push-fit systems (click-on, no thread), which don't screw at all and snap together under pressure.

They look alike enough that a GHT will almost start threading into a BSP tap and then jam, leaving you wondering whether it's your clearance or your standard. Don't guess. Measure the major diameter of the male with callipers and count the threads per inch: over the crests, a genuine GHT 3/4" gives ~27 mm and 11.5 threads per inch. If your tap or hose shows different figures, your thread isn't the one in this article and the dimensions in the table won't serve you. There's no clean conversion between them: each one demands its own diameter and its own pitch.

What you can print from here

With GHT 3/4" under your belt, the useful adapters follow naturally: a GHT-to-another-diameter adapter to marry a hose to a nozzle from a different brand, a spout or diffuser that screws straight onto the tap, a drip-irrigation connector that steps down from the hose to fine tubing, a simple splitter with one GHT outlet and one cap thread. They all share the same recipe: a coarse, forgiving thread, a tenth or two of flank clearance (more if the other side is metal), a vertical axis, and solid walls where the water passes.

For the thread profile itself — how to model flanks that print clean and hold without stripping — carry on to Modelling threads and Printable threads: buttress and trapezoidal profiles. And if your adapter is also going to carry a press fit or a sliding interface alongside the thread, Real printed clearances gives you the concrete tenths for that other part.