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I am working on a diy project. It calls for a specific pump (SU fuel pump fwiw) with what’s sold as 1/4”Bsp female threads.
Bought the pump and a 1/4”bsp male thread to 6mm ID hosetail. 
clearly the two items, whilst sold as 1/4bsp are different sizes.
no problem, pop off to local toolstation who have a 1/2”to1/4” bsp converter. 1/2” way to large but confusingly the 1/4” is the same as the pump diameter.
Ok so the hose spigot I was originally sold was sent was wrong. As a check I asked if they had a 1/4bsp fitting I can check, and that was the size of the original spigot I brought in.
so I have two items claiming to have 1/4bsp female threads being the same size and similarly two items with 1/4bsp male threads being the same size, but smaller diameter than the matching female threads.
confused? I certainly am! what on earth is going on?
what do I need to buy to fit my pump?
Could be 3/8”
3\8ths? About 15mm ID
The brass hosetail looks like 1/4 BSPP (parralel) is the pump 1/4 BSP tapered?
Might be a NPT thread or some obscure American thread too!
The hose tail(s) are 1/8”bsp
1/4”bsp has a major diameter of 13mm ish like your pump.
whoever you’re buying your fittings from is thick!
when I did the fuel system on my bus
I'm guessing that's something more interesting than the STW standard issue T6 California @PK13?
Well it's a vw. It did have full on fuel injecton but I've gone back to carbs less stress less fittings.
Sometimes simple is best.
@sui everything is back in a big box in my garage thankfully vw engine wiring is all at one end so it was just unplugging the pin plug. When I get time I will revisit it definitely unless electric becomes viable in a full camper.
I took the engine out to finish some panel work and just laziness made me put the carbs ect on I can be stubborn for all the wrong reasons lol.