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Why won't my US hose fitting screw onto a UK tap?

A US garden hose thread and a UK tap thread are different standards, so a fitting from one country will cross-thread or refuse on the other.

Two different worlds of thread

Garden hose in the United States and Canada uses GHT, the Garden Hose Thread, also called NH. It is a 3/4 inch-11.5 straight thread. It is a hose thread, meaning it is parallel and seals on a washer rather than on the thread itself.

In the UK and much of Europe, outdoor taps are cut with BSP threads, usually 1/2 inch or 3/4 inch BSP, and most gardeners then clip on a proprietary quick-connect system such as Hozelock or Gardena. GHT and BSP are not interchangeable.

Why they feel close but bind

GHT and BSP can look similar at a glance because their diameters are in the same neighbourhood, but the thread pitch and the exact diameters differ. Start a GHT fitting on a BSP tap and it may catch a thread or two before it jams, which fools people into forcing it and damaging both parts.

The naming also misleads. A 3/4 inch BSP thread is not 3/4 inch across; the 3/4 inch refers to a nominal bore, and the actual thread outer diameter of 3/4 inch BSP is around 26.4mm.

Bridging the two

To use US hose kit on a UK tap, you fit a GHT-to-BSP adapter, matching the BSP size of your tap (commonly 1/2 inch or 3/4 inch BSP) to the GHT side of the hose. Because both are washer-sealed hose or parallel threads, you rely on a good washer rather than tape.

Many UK users skip threaded hose ends entirely and adapt the tap to a Hozelock or Gardena click fitting, then run quick-connects from there.

Identifying what you have

If the kit came from North America, assume GHT. If it came from a UK or European DIY shop, assume BSP plus a proprietary clip. When in doubt, measure the thread outer diameter and count threads per inch, then match to the standard rather than forcing a near-fit.

Related

Sources: ISO 228 (BSP) thread standard · Hozelock