Threads query (gas): In a 3/8ths thread, exactly *what* part measures 9.625mm?

[Tom]

I have a propane gas appliance that claims to use 3/8 inch fittings but I can't seem to measure any occurence of a 9.525mm dimension (3/8th inch) anywhere in the hardware.

My measurements reveal the following data:

Minor diameter (male): 15.6mm Major diameter (male): 16.3mm Therefore, thread depth: 0.35mm Thread pitch: 1.3333mm (= 19 tpi) Left-hand thread

I've been trawling many popular Google hits on the subject but without success.

I'm just looking for an adapter for this thread so I can mate it with a commonly available 15mm copper pipe termination.

Tim

[Roger]

The message from Tom contains these words:

Should be 3/8" BSP but some of your measurements don't quite fit:

Pitch 19 tpi Major diameter 16.662 mm Minor diameter 14.950 mm

[Bob Minchin]

There is no part of a 3/8 BSP thread that measures 3/8". Historically it is the thread that would be cut on thick wall steel pipe with 3/8" bore Be aware BSP male threads can either be tapered or parallel. Left hand threads are traditionally used for fuel gases and should have notches cut in the hexagonal 'corners' to indicate which way to turn them I would not expect adaptors to be readily available to dissuade improvised fitting.

Find someone with a lathe who can make you an adaptor up from some brass to solder onto your 15mm pipe.

Bob

[Dave Baker]

[nightjar]

3/8 inch British Standard Pipe Thread is the thread that could be cut on the outside of a wrought iron pipe with a bore of 3/8 inch. The major diameter is 0.656 inches and the thread is 19tpi.

Colin Bignell

[RobertL]

AIUI, in the original standard the 3/8" was the internal diameter of the pipe (the bore) but sometime after the original standard was defined they increased the standard bore so nothing on a 3/8" pipe measures 3/8" any more.

Robert

[nightjar]

What happened was that wrought iron (up to 2.5 inches) and cast iron (3 inches and above) were replaced with steel, which could have a thinner wall for the same bursting strength. As the OD had to remain the same, the bore changed. Today, Class E uPVC pipe is about the same wall thickness as the original materials, at least in larger sizes - I don't usually deal with anything smaller than 1 inch bore.

Colin Bignell

[RobertL]

out of interest, does anyone know when this change took place?

Robert

[Tom]

Thanks Bob, and others, all is much clearer now. I wondered about the small notches on the facets of the fastening nut but they are exactly as you state, indicating the left-hand thread. The threads in my case are parallel and the seal facilitated by a conical end section mating with a convex part withing the female connector.

A one-piece stock/bespoke adapter would work but I think will opt to do it with a short and suitably terminated length of hose - One end

3/8 BSP (left-handed) hose connector (when I can find one) with the other end using a domestic water plumbing thread.

If I do stumble upon a one-piece brass adapter with left/right-hand thread combination then I am still interested in that as a solution. It wil be intriguing to to tighten anyway!

I tend to use BES for these items, does anyone have any other online stockist suggestions that might offer the pipe or one-piece adapter?

[nightjar]

I would expect that it largely happened during the second half of the 19th century, once the Bessemer process allowed steel to be produced cheaply.

Colin Bignell