BSPP threads
Identifying parallel pipe threads and specifying the separate sealing interface needed for a complete connection.
Tags: parts-and-design, specifications, threads, fluid-connections
On this page
Identity and standard
BSPP identifies parallel pipe threads. Parker uses G followed by nominal size, such as G1/8. Parker fitting-thread guide
Swagelok shows multiple end connections using ISO 228/1 threads: a 30° mating-seat arrangement, gasket compression in a counterbore, and a shoulder compressing a gasket against a flat surface. Their common thread family does not make these sealing geometries interchangeable. The guide also distinguishes the 55° BSP family from 60° NPT threads. Swagelok, printed pages 22–28 and 36–40
The standard checked here is ISO 228-1:2000, edition 4, published September 2000 and confirmed in 2022. Its scope covers parallel internal and external fastening threads, sizes 1/16 through 6. These threads mechanically assemble components; pressure sealing requires an appropriate interface outside the threads. ISO directs thread-dimension/form verification and recommended gauging systems to ISO 228-2. ISO 228-1 record and scope
Keep three questions separate: which threads mate, which surfaces seal, and which complete assembly meets the service requirements. A matching thread designation answers only part of the first question.
Recognizing the thread
Swagelok's MS-13-77 revision H guide describes ISO 228/1 as straight, with a 55° flank angle and rounded roots and crests. Its identification procedure compares diameters along the thread, measures diameter and pitch, then identifies the thread standard and end connection. Its nominal 1/4 BSPP entry has 19 threads per inch. These identification observations do not replace limit gauging. Swagelok identification guide, printed pages 7, 28 and 36
A useful receiving record preserves the measured diameter, pitch and measurement locations alongside the proposed designation. Record whether the feature is internal or external and include a photograph or drawing of the sealing interface. Do not turn an approximate caliper reading into an undocumented manufacturing tolerance.
Thread compatibility and sealing compatibility
ISO 228-1 explicitly separates its fastening threads from ISO 7-1 joints that seal on the threads. Specify the actual standard rather than accepting the word “pipe” or a shared nominal size as sufficient identification. ISO scope
For a specific manufacturer example, Parker's Catalog 4260, October 2025, describes a Form A connection using a bonded washer centered at the hex and a Form B arrangement using a metal gasket between mating faces. These are catalog-specific arrangements. Parker also notes that BSPP-end pressure ratings depend on the sealing washer used; a rating shown for a taper-threaded end must not silently become the rating of its parallel-thread counterpart. Parker catalog, printed pages 27 and 30
Obtain the selected fitting and port drawings together. Resolve seat geometry, spotface/counterbore, seal material and supplied hardware before ordering. Use the manufacturer's instructions for that actual assembly when choosing tightening procedures; a thread name cannot determine installation torque.
Original pitch calculation
For a single-start thread with 19 TPI, the ideal axial pitch is:
pitch = 25.4 mm/in ÷ 19 threads/in = 1.33684 mm/thread.
Three complete turns would correspond to 4.01053 mm of ideal relative axial travel while the threads remain engaged and unconstrained. This arithmetic is not a required engagement length, a tightening instruction or a measure of seal compression. A real shoulder or seat can stop travel, and the number of turns available depends on the actual parts.
The example explains why pitch matters when identifying two visually similar connections. It supplies no major diameter, thread tolerance, load capacity or pressure rating. Obtain those requirements from the specified standard and component documentation.
Handoff and acceptance
Use the following as proposed procurement and inspection fields:
| Field | Information to preserve |
|---|---|
| Thread | Standard/edition, full size designation, internal/external, required tolerance and engagement |
| Sealing interface | Fitting/port drawing, seat or face geometry, seal part and material |
| Operating conditions | Fluid, temperature, pressure and component-specific qualification |
| Manufacture and receipt | Required gauges, inspection criteria, coating and condition of sealing surfaces |
| Installation | Exact component instructions, replacement hardware and assembly acceptance method |
If a supplier only confirms “BSPP,” ask for the missing interface definition before substituting parts. If the threads fit but the seal arrangement differs, stop the substitution until the engineering requirement is resolved. Thread inspection, sealing-surface inspection and assembly performance address different aspects of acceptance.
Source scope
ISO's public abstract establishes the standard's scope; this page does not reproduce normative dimensional tables. Swagelok's cited document is MS-13-77 revision H, February 2025. Parker's catalog is explicitly October 2025; its examples describe that product family rather than every BSPP connection. Calculations and the handoff fields are original explanatory guidance.