1/2″ BSP – 14 TPI, used the world over for plumbing threads, though quite often with rather loose tolerances.

The way I did mine was to buy one of those braided flexi-hoses for connecting kitchen taps with a small (1/8 BSP I think) male thread on one end and a female 1/2 BSP on the other to connect to the test pump. I then made a small adaptor to screw into a convenient bush on the boiler with a 1/8 BSP female thread to attach to the existing hose end.

The advantage of doing this is that the existing hose is short and rather inflexible, and it’s just a real nuisance to attach to the boiler. You can get 500mm long flexis and they make it all much more manageable. And you can still use it for testing your plumbing.

By the way, the pressure gauge on mine was reasonably consistent, but read about 20 psi low.

Note those tap-connectors shrouded with braided, very fine stainless-steel wire are not really designed for frequent flexing, contact over edges and so forth, and consequent broken strands are very hard to see but very effective thorns.

How do I know…?

Ouch!

(You could add an outer sleeve of non-adhesive heat-shrink sleeving but I’d still heed Jason’s point about their pressure rating first.)