Overview
Specifies dimensions, tolerances, and tolerance classes for parallel (straight) pipe threads designated "G" — the BSPP (British Standard Pipe Parallel) family — in sizes G 1/16 through G 6. The threads provide mechanical retention only; the pressure-tight seal is always achieved by a separate sealing element (O-ring, bonded washer, or machined seating face). This is the dominant parallel pipe thread standard in Europe, Asia-Pacific, and on hydraulic and pneumatic equipment worldwide.
Scope & Applicability
Applies To Industries
Covers — parallel (straight) pipe threads in the BSPP "G" family, where the sealing element is always separate from the threads:
| Designation | Thread Type | Tolerance Class | Typical Use |
|---|---|---|---|
| G ½ A | External (male) | Class A — tighter | Hydraulic fittings, instrument ports |
| G ½ B | External (male) | Class B — looser | General plumbing, compressed air |
| G ½ | Internal (female) | Single class | All G-thread applications |
Size range — G 1/16 through G 6 (the same nominal sizes as ISO 7-1; thread dimensions are identical but the form is parallel/straight rather than tapered).
Sealing methods covered by this standard — ISO 228-1 specifies the thread geometry only. The sealing element is defined in the fitting or port standard:
- O-ring face seal → ISO 6149, DIN 3852, SAE J1926
- Bonded/Dowty seal washer → fitting manufacturer specification
- Cone seat or machined face → the applicable equipment standard
Does NOT cover (exclusions) — the following are governed elsewhere:
- Tapered BSPT threads where sealing is on the threads → ISO 7-1 (R/Rc/Rp)
- North American taper pipe threads → ASME B1.20.1 (NPT) and B1.20.3 (NPTF)
- Metric O-ring port threads → ISO 6149 / DIN 3852
- The sealing element (O-ring, bonded washer) itself → the applicable fitting/seal standard
- Tube-to-port fittings using G thread as the port connection → ISO 8434 series
G threads and BSPT (R/Rc) threads share the same 55° Whitworth form and pitch — they will physically engage but the parallel/taper geometry mismatch means the joint cannot make a proper seal. Never use G × Rc for a pressure-tight connection.
Key Requirements
ISO 228-1 establishes the following minimum requirements for G parallel pipe threads:
Thread Geometry
- Thread angle — 55° included angle, Whitworth form with rounded crests and roots (same profile as ISO 7-1 BSPT, but the pitch cylinder is parallel — not tapered).
- Thread form — parallel (straight); major, pitch, and minor diameters are constant along the thread length. No taper — no wedging action — no self-sealing.
Thread Data by Size
| Size | TPI | Major Dia. (mm) | Pitch (mm) |
|---|---|---|---|
| G 1/8 | 28 | 9.728 | 0.907 |
| G 1/4 | 19 | 13.157 | 1.337 |
| G 3/8 | 19 | 16.662 | 1.337 |
| G 1/2 | 14 | 20.955 | 1.814 |
| G 3/4 | 14 | 26.441 | 1.814 |
| G 1 | 11 | 33.249 | 2.309 |
| G 1¼ | 11 | 41.910 | 2.309 |
| G 1½ | 11 | 47.803 | 2.309 |
| G 2 | 11 | 59.614 | 2.309 |
| G 2½ | 11 | 75.184 | 2.309 |
| G 3 | 11 | 87.884 | 2.309 |
| G 4 | 11 | 113.030 | 2.309 |
| G 6 | 11 | 163.830 | 2.309 |
Thread dimensions are identical to ISO 7-1 (BSPT) — the only difference is that G threads have no taper. This is why a G fitting will physically screw into an Rc port, but will not seal properly.
Tolerance Classes
| Class | Applied To | Fit Characteristic | Typical Application |
|---|---|---|---|
| A | External thread only | Tighter — less clearance | Hydraulic O-ring and cone-seat ports |
| B | External thread only | Looser — more clearance | General plumbing, compressed air |
| (none) | Internal thread | Single tolerance | All G applications (no class suffix) |
Sealing — What the Standard Requires
ISO 228-1 specifies thread geometry only. It does not specify a sealing method. The pressure-tight joint is made by one of:
- O-ring in a groove machined into the port face (defined by ISO 6149, DIN 3852, or SAE J1926)
- Bonded (Dowty) seal washer — a metal washer with bonded elastomeric inner ring
- Elastomeric or soft-metal seating face — a conical or flat surface on the fitting nose
- PTFE tape — not defined by the standard, but widely used in low-pressure plumbing practice
Designation Format
- External thread, Class A: G ½ A or G 1/2 A
- External thread, Class B: G ½ B
- Internal thread: G ½ (no class suffix)
- Old British designation still found on legacy equipment: ½ BSP or ½ BSPP
Technical Details
How G-Thread Connections Work
ISO 228-1 defines the parallel "G" thread — the standard pipe retention thread throughout Europe, Asia-Pacific, and on hydraulic/pneumatic equipment worldwide. Unlike tapered threads (NPT, BSPT), a G thread has no wedge action and makes no contact with the mating thread at the crests or flanks under sealing stress. The thread's job is purely mechanical: hold the fitting in position so the actual sealing element can be compressed.
This fundamental difference means three things:
- A G thread can be fully tightened with no seal whatsoever if the sealing element is absent.
- Leakage from a G connection is almost always a sealing element problem, not a thread problem — more torque will not fix it.
- Re-assembly requires inspection and replacement of the sealing element before re-use.
Sealing Methods for G-Thread Connections
| Sealing Method | Defined In | Best For | Notes |
|---|---|---|---|
| O-ring face seal | ISO 6149, DIN 3852, SAE J1926 | Hydraulics, high pressure | Re-usable O-ring if undamaged |
| Bonded (Dowty) seal washer | Fitting manufacturer spec | General hydraulics, fuel | Single use only — always replace |
| Soft metal / elastomeric cone | Equipment standard | Instrument ports | Inspect cone condition on re-make |
| PTFE tape (low-pressure only) | Not in standard; field practice | Water, compressed air | Not suitable for high-pressure hydraulic |
G-Thread Tolerance Classes
The standard defines two external tolerance classes and a single internal class:
| Thread | Class | Clearance | Use |
|---|---|---|---|
| External (male) | A | Low | Precision hydraulic and O-ring face-seal ports |
| External (male) | B | Higher | General plumbing; looser fit acceptable |
| Internal (female) | (none) | Single | Mates with both A and B external |
Class A is always specified when an O-ring groove port or cone seat relies on accurate concentricity. For domestic water or compressed air where the tolerance margin is large, Class B is sufficient.
G Thread vs. Related Standards — Comparison
| Property | G (ISO 228-1) | R/Rc (ISO 7-1) | NPT (ASME B1.20.1) |
|---|---|---|---|
| Thread angle | 55° Whitworth | 55° Whitworth | 60° V-form |
| Form | Parallel | Tapered 1:16 | Tapered ¾ in/ft |
| TPI at ½″ | 14 | 14 | 14 |
| Major dia. G/R ½ | 20.955 mm | 20.955 mm | 21.336 mm |
| Sealing method | Separate element | Thread flanks + sealant | Thread flanks + sealant |
| Self-sealing? | No | Yes (with sealant) | Yes (with sealant) |
| G × Rc assembly | Physically possible | — | — |
| G × Rc leak-tight? | No | — | — |
National Equivalents
| Standard | Country/Region | Notes |
|---|---|---|
| ISO 228-1 | International | Base document |
| EN ISO 228-1 | European Union | Direct adoption |
| BS 2779 | United Kingdom | Predecessor; now superseded by ISO 228-1 |
| JIS B 0202 | Japan | Equivalent geometry; used on Japanese equipment |
| AS 1722.2 | Australia | National adoption of ISO 228-1 |
Legacy fittings marked "BSP" or "BSPP" (no ISO number) follow BS 2779, which is dimensionally identical — the threads are interchangeable with ISO 228-1 G threads.
Engineering Notes
A G thread alone will never make a pressure-tight joint — always verify the sealing element.
- G is a retention thread, not a sealing thread — tightening a G connection harder will not stop a leak if the sealing element (O-ring, bonded washer, or seating face) is absent, damaged, or incorrectly fitted. The force goes into thread stretch, not sealing load.
- Do not mate G with R/Rc (BSPT) — G external × Rc internal threads share the same 55° Whitworth form and pitch, so they will engage, but the parallel G thread cannot wedge into the tapered Rc port. The result is false engagement: the thread shoulders out before proper sealing depth is reached, and the joint will leak.
- Do not mate G with NPT — though both are pipe-size numbered, NPT has a 60° angle and different pitch; cross-mating will cross-thread or produce an unreliable assembly.
- Bonded seal (Dowty) washers are single-use — the elastomeric inner ring permanently deforms on first pressurisation; always replace with a new washer on disassembly. Re-using a compressed Dowty seal is a chronic source of leaks in hydraulic maintenance.
- O-ring groove condition — for O-ring face seal connections, the groove surface finish and depth are critical. Scratches across the groove are leak paths; inspect and replace the port body if the groove is damaged. O-rings can be re-used only if undamaged.
- PTFE tape on G threads (low-pressure water/air) — in practice, PTFE tape is commonly applied to G threads in building services for domestic water and compressed air. This technically misapplies the standard (the tape acts as a sealant on the thread flanks, not a proper separate element), but is widely accepted at low pressures. Do not use PTFE tape on G threads for high-pressure hydraulic or instrument duty.
- Tolerance class matters for hydraulic ports — Class A (tighter) is required for O-ring face-seal and cone-seat hydraulic port applications; Class B (looser) is acceptable for general plumbing where the seal margin is large.
Source, Edition & Status
- Publisher
- ISO (International Organization for Standardization)
- Edition Year
- 2000
- Industries
- General Industry, Building Services, HVAC, Offshore, Marine, Skid Manufacturing
- Last Reviewed
- 2026-07-01
- Last Updated
- 2026-07-01
- Verified By
- Engineering content reviewer
- Content Status
- reviewed
- Official Link
- www.iso.org