BSP pipe fittings. Very confused!

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!

1/4”bsp has a major diameter of 13mm ish like your pump.

+1

BSP fittings are sized based on the ID pipe they were attached to.  So modern 15mm (OD) copper pipe has 1/2″  fittings, whereas 8mm has 1/4″ fittings.