BSP Threads: BSPT vs BSPP and How to Identify a Fitting Size
Most site problems on threaded lines start from one simple misconception: that all BSP threads are the same. In fact there are two different families, and they do not have the same purpose — one is made to seal, the other to hold a mechanical connection. This page explains the difference, which threads TUPY Brazil fittings carry, and how to work out the real size of a part in your hand.
BSPT versus BSPP: two standards, two purposes
Feature
BSPT — taper thread
BSPP — parallel thread
Reference standard
ISO 7-1
ISO 228-1
Male thread geometry
Taper
Parallel
Purpose
Sealing on the thread
Mechanical connection
Sealing method
Metal-to-metal engagement with a sealing compound
Separate washer or seat
Inspection method
Thread gauge to ISO 7-2
Go/no-go system to ISO 228-2
Usual designation
R for male, Rc for taper female
G for male and female
TUPY Brazil BSP fittings have internal parallel pressure-tight threads (Rp) and external taper pressure-tight threads (R), produced to ABNT NBR NM ISO 7-1. Socket threads are to ABNT NBR 8133 and ISO 228-1. Other thread types can be produced to order.
The practical consequence is simple: a part with a parallel thread cannot be made tight just by pulling it up harder. If a joint is leaking and the part has a parallel thread, the problem is not torque — it is the missing washer or seat.
Identifying a fitting or pipe size from the measured diameter
The nominal size of a threaded fitting matches no physical dimension of the part: "half inch" is neither the inside diameter nor the outside diameter. To identify an unmarked part, measure the thread diameter with a caliper and compare it against the table below:
Dp — outside diameter of the male thread (on the pipe).
Db — inside diameter of the female thread (inside the fitting).
C — useful thread length.
Nominal size (inch)
Dp (mm)
Db (mm)
C — useful thread length (mm)
¼
12.8
11.5
9.7
⅜
16.3
15.0
10.1
½
20.4
18.6
13.2
¾
25.9
24.1
14.5
1
32.6
30.3
16.8
1¼
41.1
39.0
19.1
1½
47.0
44.9
19.1
2
58.6
56.7
23.4
2½
74.1
72.2
26.7
3
86.6
84.9
29.8
4
111.4
110.1
35.8
5
136.7
135.1
40.1
6
162.0
160.9
40.1
Thread inspection: gauges and the gauge plane
A pressure-tight taper thread is checked with a gauge, not with a caliper. The measuring reference is the "gauge plane" (PC) — the section of the taper thread at which the diameter equals the nominal value.
The internal (female) thread is checked with a plug gauge and must fall between the minimum and maximum positions of the gauge plane.
The external (male) thread is checked with a ring gauge, on the same minimum, nominal and maximum logic.
Parallel threads (fastening threads) are checked with the go/no-go system to ISO 228-2: the "go" side must run fully home and the "no-go" side must not advance beyond the permitted limit.
The effective thread length can be checked with a caliper against column C in the table above.
Pressure-tight threads on TUPY BSP fittings are inspected to ISO 7-2 using number 1 and number 3 gauges.
Practical notes
A taper male thread into a parallel female thread is the standard, accepted combination in the BSP system; the seal comes from the taper progressively engaging the parallel thread.
Over-tightening does not seal the thread, it puts the body under stress. The permitted amount of wrench tightening for each size is given in the installation guide.
Before ordering a replacement part, identify the thread type on the existing part; parallel and taper threads look alike from the outside but behave completely differently when sealing.
On high-temperature and high-pressure lines the thread engagement length (column C) matters directly; a pipe cut shorter than the useful thread length becomes the weak point of the joint.
