British Standard Pipe Taper, BSPT, R external and Rc internal (ISO 7-1)

R 3/8 R 3/8 (19 TPI, 55 degree, taper 1:16)

3/8 in trade size. Tapered thread, sealed on the thread, tapered thread, 55 degree flank angle. Basic major diameter 16.66 mm / 0.656 in.

Identify a fitting you are holding All BSPT (R) sizes

Measure across the crests

16.3 mm to 17.1 mm

Basic 16.66 mm / 0.656 in

The visual tell

What the end of the fitting looks like

Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it. Thread shown: R 3/8 (19 TPI, 55 degree, taper 1:16).

No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting.

Where it seals: The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread.

Dimensions

Thread size, pitch and diameters for R 3/8.
Designation	R 3/8
Thread	R 3/8 (19 TPI, 55 degree, taper 1:16)
Threads per inch / pitch	19 TPI (1.337 mm pitch)
Basic major diameter (male), at the gauge plane	16.66 mm / 0.656 in
Measured across the male crests	16.3 mm to 17.1 mm
Basic minor diameter (mating female)	14.95 mm / 0.589 in - computed from the 55 degree thread profile, not measured
Thread form	55 degree flank angle, tapered, taper 1 in 16
Seat	Tapered thread, sealed on the thread
How the size is named	Sized by trade size, same nominal series as BSPP.

The three charts that measure this size print 16.3 mm, 16.6 mm and 17.1 mm, and none of them is wrong: a 1 in 16 taper means the diameter depends on where along the thread the caliper sits. The first reads the first full thread at the entering end, the second reads near the gauge plane, the third reads the largest point up by the hex. The basic figure above is the gauge-plane diameter from ISO 7-1 Table 1, which is the one a gauge checks.

Working pressure

No working pressure is published for this connection. Ratings for it vary by manufacturer, body material, port form and seal, and no two independent sources in the register give the same figure under conditions they both state. A generic number here would be a decoration, not a rating. Read the pressure from the catalogue of the fitting you are actually installing.

A fitting with two different ends is only ever as strong as the weaker end, and any rating assumes the fitting is undamaged, correctly assembled and within its temperature range. Pressure ratings are safety-relevant: confirm them against the manufacturer specification before the system is charged.

What it mates with

A female Rc port of the same trade size, or a BSPP parallel port where the standard permits the taper-into-parallel combination. Sealant or PTFE tape is required.

A tapered thread seals by wedging, so it strips the port if it is over-tightened chasing a leak. Use sealant or PTFE tape, and stop at the published turns past finger tight rather than at a feel.

Do not mix these up

Fittings that measure like a R 3/8

Every size below sits within 0.9 mm of this one across the male thread crests, which is inside what a worn or plated fitting and a hurried measurement produce together. Those marked same thread will screw together with this fitting.

Fittings in other standards whose male thread diameter is close to R 3/8.
Fitting	Thread	Basic major Ø	Seat	Will it thread on?
G 3/8 · BSPP (G)	G 3/8 (19 TPI, 55 degree)	16.66 mm	Parallel thread, sealed on a face gasket	Same thread - it will screw together and the seal will be wrong
-06 JIS 30 (G 3/8) · JIS 30 degree flare (BSPP)	G 3/8 (19 TPI, 55 degree)	16.66 mm	30 degree flare cone or seat	Same thread - it will screw together and the seal will be wrong
3/8 NPT · NPT / NPTF	3/8-18 NPT	17.15 mm	Tapered thread, sealed on the thread	No - 0.49 mm apart on diameter, and a different pitch
10L (M16 x 1.5) · Metric DIN 24 degree	M16 x 1.5	16 mm	24 degree cone (DIN)	No - 0.66 mm apart on diameter, and a different pitch
8S (M16 x 1.5) · Metric DIN 24 degree	M16 x 1.5	16 mm	24 degree cone (DIN)	No - 0.66 mm apart on diameter, and a different pitch
3/8 in SAE 45 · SAE 45 degree flare	5/8-18 UNF	15.88 mm	45 degree flare cone	No - 0.78 mm apart on diameter, and a different pitch

This list is computed from the published diameters in the database rather than kept as a hand-written list, so a correction to a table can never leave a stale warning behind.

Answers

Common questions about R 3/8

What thread is a R 3/8 fitting?

R 3/8 runs a R 3/8 (19 TPI, 55 degree, taper 1:16) thread: 19 TPI (1.337 mm pitch), basic major diameter 16.66 mm / 0.656 in. Published identification charts read 16.3 mm to 17.1 mm across the male crests, which is the figure a caliper produces.

How do I tell a R 3/8 apart from a fitting that measures the same?

Look at the sealing surface. No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting. The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread. The thread is tapered and the flank angle is 55 degrees.

What does a R 3/8 screw into?

A female Rc port of the same trade size, or a BSPP parallel port where the standard permits the taper-into-parallel combination. Sealant or PTFE tape is required.

What pressure is a R 3/8 rated for?

No working pressure is published here for this connection. Ratings vary by manufacturer, body material and seal, and no two independent sources in the register give the same figure under stated conditions for it. Read the rating from the catalogue of the fitting you are installing.

BSPT (R)

Other sizes in this standard

Where these figures come from

Dimensions are as defined by ISO 7-1, BS 21. FittingKey is an independent reference and is not affiliated with, endorsed by or approved by any standards body or fitting manufacturer.

Every value on this page was checked against 5 independent published sources:

ISO 7-1, Pipe threads where pressure-tight joints are made on the threads, Table 1 - ISO, ISO 7-1:1994(E), third edition (standard, read directly, read 2026-08-27)
Hydraulic coupling identification chart E2/50101 - Gates, Gates Europe, 2004 (manufacturer, read 2026-08-27)
Thread and connector identification guide - Gerrard Hydraulics (RYCO), AS 1722.1, AS 1722.2, BS 21, ISO 7, ISO 228, ANSI/ASME B1.20.1 (distributor, read 2026-08-27)
How to identify British threads and fittings - Adaptall (manufacturer, read 2026-08-27)
BSP thread charts, BSPP and BSPT - Machining Doctor, ISO 228-1, ISO 7-1 (reference table, read 2026-08-27)

Which published source supplied each column, and what it gave.
Column	Sources	What they gave
Basic major diameter	iso-7-1, machiningdoctor-bsp	Read directly from ISO 7-1:1994 Table 1, where the major diameter is defined AT THE GAUGE PLANE, and matched by an independent reproduction. ISO 7-1 lists no 5/8 and no 7/8 size.
Measured male thread diameter	gerrard-id, adaptall-british, gates-idchart	Three charts measuring at three planes on a taper, differing by up to 0.8 mm. Every size page publishes the whole span and says why it is wide.

Sources disagree: BSPT male thread diameter, every size.

12.8 mm at R 1/4, at the first full thread from the end (gerrard-id)
13.1 mm at R 1/4, near the gauge plane (adaptall-british)
13.6 mm at R 1/4, at the largest point (gates-idchart)

All three are correct measurements of the same fitting. A 1 in 16 taper loses 1.59 mm of diameter over 25.4 mm of length, so where the caliper sits decides the reading. The size page publishes the whole span, and the basic column carries the gauge-plane diameter from ISO 7-1 Table 1, which is the figure a thread gauge checks.