BSPP vs BSPT — How to Tell G from R

Same 55° thread form, same TPI, same nominal size. One is tapered.

The whole difference BSPP is parallel. BSPT is tapered. Everything else about the two threads is essentially identical

Identify one in ten seconds

Run a caliper across the first thread and across the last thread of the male fitting. If the two readings match, it is BSPP (G). If the reading grows along the thread, it is BSPT (R) — the taper is 1 in 16, so over a 1/2″ fitting you should see roughly half a millimetre of difference. That is small but perfectly measurable with cheap calipers.

The second tell is the seal. A BSPT joint has nothing but thread. A BSPP joint has a bonded washer, an O-ring, or a machined cone seat, because it cannot seal without one.

Designations you will see on drawings

Mark	Standard	Means
G	ISO 228-1	Parallel, male or female. Needs a separate seal.
R	ISO 7-1	Taper male.
Rc	ISO 7-1	Taper female.
Rp	ISO 7-1	Parallel female, intended to take a taper male.

Rp and G have the same basic profile. They differ only in tolerance class. This is the detail that makes catalogues contradict each other, and it is why an Rp fitting will happily accept a G male even though the two are specified in different standards for different jobs.

Size by size

Note the last column. At every shared size the thread counts are identical, so counting threads will never tell these two apart.

Size	BSPP major mm	BSPT major mm	BSPP TPI	BSPT TPI	Thread count
1/8″	9.728	9.73	28	28	same
1/4″	13.157	13.16	19	19	same
3/8″	16.662	16.66	19	19	same
1/2″	20.955	20.95	14	14	same
3/4″	26.441	26.44	14	14	same
1″	33.249	33.25	11	11	same
1-1/4″	41.910	41.91	11	11	same
1-1/2″	47.803	47.80	11	11	same
2″	59.614	59.61	11	11	same
2-1/2″	75.184	75.18	11	11	same
3″	87.884	87.88	11	11	same
4″	113.030	113.28	11	11	same

The BSPT major diameter is quoted at the gauge plane. Because the thread is tapered it keeps growing past that point, so a direct comparison of the two numbers only holds at one place along the thread.

What happens if you mix them

A BSPT male into a BSPP female will thread in, wedge, and stop early. It may hold low pressure. It is also steadily splitting the female fitting, and the failure when it comes is usually the fitting cracking rather than the joint weeping.

A BSPP male into a BSPT female goes in easily and never tightens — the female is getting narrower while the male stays the same, so contact happens at one point instead of along the thread. There is nothing to seal against.

The pairing that is correct and standard is R male into Rp female, which ISO 7-1 explicitly permits, sealed with compound on the thread.

BSPP chart → BSPT chart → NPT vs BSP → JIS PT and PF →