Female Adapter 1/4" Tube x 3/8" BSP Coned - DMfit

Female adapter 1/4" tube × 3/8" BSP(P)

Straight fitting combining a 1/4" push-fit connection on the tube side with a 3/8" BSP parallel female thread on the thread side, with a coned internal seat.

On the tube side the principle is standard: the tube pushes fully home, the collet locks it, the O-ring seals it. It is the thread side that deserves attention.

BSP(P): the thread does not seal

BSP(P), British Standard Pipe Parallel, ISO 228, is a cylindrical thread. Unlike a tapered thread such as BSPT or NPTF, its threads do not wedge into each other: they cannot seal. Their only role is to clamp the two parts together.

Sealing therefore happens elsewhere: on a seat inside the fitting. And that is where the seat type becomes decisive.

Coned seat: what it implies

This fitting has a coned internal seat. The male thread screwed into it must have a matching coned end: the two cones press against each other as the joint is tightened, and that contact is what makes the connection tight.

Direct consequence: no flat gasket to fit. The cone does the job on its own. A flat gasket inserted into a coned seat does not compress correctly and even prevents the cones from bearing, a common assembly mistake that ends in a leak.

Do not confuse it with the flat-seat version. There, sealing takes place on a flat face, with a mandatory flat or fibre gasket. Both versions share the same BSP(P) thread and screw together just as easily, but they are not interchangeable: a flat-faced male in a coned seat bears on an edge only, and leaks.

So always check the geometry of the male before ordering, not just its thread size.

The 1/4" → 3/8" step

The fitting also works as a reducer: it connects a 1/4" tubing run to a 3/8" threaded port, a common size on valves, filter heads and pump bodies. One part where an adapter plus a fitting would otherwise be needed.

Where to use it

On reverse osmosis systems and water treatment installations using quick-connect fittings, anywhere a 1/4" line has to screw onto a 3/8" BSP coned male: valves, solenoid valves, pressure gauges, pump bodies, filtration heads.

Fitting

Thread side: screw in by hand until the cones make contact, then tighten a quarter to a half turn with a spanner. No more, on a polymer body, over-tightening deforms the coned seat and creates the very leak you were trying to avoid.

Tube side: cut square with a tube cutter, push fully home, pull lightly to check engagement.