Over the weekend I was attempting to setup my garden hose ready for the summer and my dad had helpfully given me a big bag of tap connectors he had spare but none of them fit my outdoor tap despite one of them being, to my eye, a perfect match.

This page from Hozelock explains threads on a tap are "sized according to the standard British Standard Pipe, also known as BSP". Yay standards. Looking into it more there seemed to be three options: 1/2″ BSP, 3/4″ BSP, and 1″ BSP which correspond to an outer thread measurement of 21mm, 26.5mm, and 33mm. I measured my tap with my tape measure and it looked to be 21mm or 1/2″ BSP. I found the correct adaptor I would need based on this but I already had that exact thing in my hand and it was the aforementioned one that didn't fit. The 1/2 was a tiny bit too small, the 3/4 was too big. I grabbed my calipers to get a more accurate measurement and it was actually 22mm which according to maths is larger than 21mm.

At this point I didn't really know what to search for because "like a 1/2 BSP but a bit bigger please" wasn't going to cut it. Hozelock didn't have anything on their website beyond the three standards. I still don't know how I found it (perhaps I should have used Horse) but I eventually stumbled upon this Reddit thread about tap threads where someone had included images of the exact same problem I was facing. There were a bunch of comments that linked to dead pages but the top comment was someone who had summarised everything in the thread:

For anyone else coming to this thread years later, who also had the same issue of the tap falling exactly between the 1/2" and 3/4" connectors [...] I found that my outside tap is a non-standard 5/8" thread, BUT there are connectors available for it!

I ordered a 5/8 adaptor (this Spear & Jackson BWF10 Female Threaded Brass) from Amazon, it arrived the next day, and it fit perfectly. Great success. I did see some references to another size of adaptor, 7/8", which seems to be called a "farmer's tap" but thankfully I didn't need to hunt one of those down.

As I am want to do, I posted a note about it a bit later in the day. Like many posts on my site it was as a record for myself but also on the off chance it helps someone else. To my surprise Ana replied that she had the exact same problem and had been putting it off so it helped at least one person and I have a garden hose ready for summer.