Repairing a hydraulic machine and not sure whether the thread is BSP or NPT? Although they can look the same, differences in diameter, pitch and profile can prevent a correct assembly and cause oil leaks. This guide explains how to identify BSPP, BSPT and NPT threads with a caliper and a thread pitch gauge.
The three most often confused
| BSP parallel (BSPP) | BSP taper (BSPT) | NPT | |
|---|---|---|---|
| Standard | ISO 228/1 | ISO 7/1 | ASME B1.20.1 |
| Form | parallel (straight) | tapered, 1° 47' | tapered, 1° 47' |
| Thread angle | 55° | 55° | 60° |
| Where it seals | outside the thread: on a washer, a sealing ring or a cone (see "How each one seals") | on the thread itself, with a suitable sealant | on the thread itself, with a suitable sealant |
The designations G, R, Rc and Rp
In hydraulic catalogues BSP threads are often shown with these letters:
| Designation | Type | Standard |
|---|---|---|
| G | Parallel, male or female | ISO 228-1 |
| R | Taper, male | ISO 7-1 |
| Rc | Taper, female | ISO 7-1 |
| Rp | Parallel, female, sealing on the thread with an R male | ISO 7-1 |
A BSPT male (R) can be fitted into a taper female (Rc) or a parallel female (Rp), according to ISO 7-1. An Rp female is not exactly the same as a G female, even though both are parallel: Rp belongs to ISO 7-1 and seals on the thread; G belongs to ISO 228-1 and seals outside the thread.
Step by step
- Parallel or tapered? On a male thread, measure the outside diameter near the end, in the middle and near the base of the thread. If the diameter stays the same, the thread is parallel. If it increases towards the base, it is tapered. On female threads identification is harder and may need suitable gauges.
- Measure the diameter. Use a caliper to measure the outside diameter of the male. On tapered threads the value changes along the length, so use it only as a first reference.
- Identify the pitch. Use a thread pitch gauge to count the threads per inch (TPI).
- Compare with the table. Also check the type of connection and the surface where it seals. For JIC, DIN metric, ORFS and SAE, see the hydraulic threads table in HO CALC.
Identification table
Basic reference diameters in millimetres, and threads per inch (TPI).
| Size | BSPP (G) | BSPT (R) | NPT | TPI BSP / NPT |
|---|---|---|---|---|
| 1/8" | 9.73 | 9.73 | 10.29 | 28 / 27 |
| 1/4" | 13.16 | 13.16 | 13.72 | 19 / 18 |
| 3/8" | 16.66 | 16.66 | 17.15 | 19 / 18 |
| 1/2" | 20.96 | 20.96 | 21.34 | 14 / 14 |
| 3/4" | 26.44 | 26.44 | 26.67 | 14 / 14 |
| 1" | 33.25 | 33.25 | 33.40 | 11 / 11.5 |
| 1.1/4" | 41.91 | 41.91 | 42.16 | 11 / 11.5 |
| 1.1/2" | 47.80 | 47.80 | 48.26 | 11 / 11.5 |
| 2" | 59.61 | 59.61 | 60.33 | 11 / 11.5 |
These are nominal reference values, not necessarily the diameter found at any measuring point. On tapered threads the table diameter is the one at the gauge plane, and the measurement depends on the position along the thread.
Note: inch sizes are nominal thread designations, not the outside diameter. For example, a 1/2" BSP thread is about 21 mm in outside diameter, not 12.7 mm.
Watch out for 1/2" and 3/4": since BSPT and NPT have the same number of threads per inch and very close diameters, identification may require checking the thread profile (55° on BSPT and 60° on NPT), using an optical comparator or suitable gauges if needed.
How each one seals
- Tapered (BSPT and NPT): in conventional connections the seal is made on the thread itself, normally with a suitable sealant such as PTFE tape or a liquid sealant compatible with the fluid and the service conditions. Some variants seal differently, such as NPT with O-Seal, which seals on an O-ring against the face. There is no standard tightening torque.
- In hydraulic systems, keep fragments of PTFE tape out of the circuit, where they can contaminate valves and other components.
- Parallel (BSPP): the threads do not seal; the seal is made outside the thread. There are two different cases:
- BSPP adaptor in a hydraulic port: the seal is made at the shoulder, with metal washers, bonded washers ("Usit" type), sealing rings or other specific geometries. On DIN 3852 connections, form A uses a sealing washer, form B a metal cutting edge and form E an elastomeric ring.
- BSP hose fitting on an adaptor: the seal is made between the 60° cone surfaces. The 60° cone is not a feature of the BSPP thread, but of this type of connection.
Do not fit NPT into BSP. They have a different thread angle and, in most sizes, a different number of threads. Even when they seem to go in, they do not seal well: use a BSP-NPT adaptor.
In our shop
- BSP adaptors: unions, elbows, tees, plugs and reducers, including the BSP-BSPT male elbow and the BSP-NPT bush.
- BSP crimp fittings (60° cone, and BSPT male) and BSP flat face.
- JIC adaptors, with JIC-BSP and JIC-BSPT versions.
- Metric DIN 2353: GE tube-to-thread connectors in BSP and NPT, WE elbows in BSPT and NPT.
- Sealing washers: Usit BSP and copper BSP washers.
Can't identify the thread? Send us a photo of the connection and, if possible, a caliper measurement. The Hidro Orfão technical team will help you identify it and find the right adaptor or hose fitting.
Sources
- Swagelok, Thread and End Connection Identification Guide (MS-13-77).
- REMS, thread tables (ISO 7-1, ISO 228-1, ASME B1.20.1).
- ISO 7-1, scope.
- Hydro Holding / Tieffe, HH catalogue, technical data; Vitillo, DIN adaptors catalogue.
- Hidro Orfão field practice.