TES-LOK Hex Coupling Tube Fittings — female internal threads of the same size on both ends, short precision-machined hex body. The standard inline connector for joining two male-threaded pipes, nipples, or studs of the same thread size in instrument and process assemblies. SS 316 · Brass · Monel · Inconel · NPT · BSP · BSPT · BSPP · 1/8"–2" · Up to 6,000 PSI · ISO 9001:2015 certified.
Equal Hex Coupling
Reducing Hex Coupling (different sizes)
A hex coupling is a short inline fitting with female internal threads on both ends of the same size, machined from a single hexagonal body. It is used to join two male-threaded pipes, nipples, or studs of the same thread size in a straight inline run. The hexagonal body cross-section provides wrench flats that allow the coupling to be gripped by an open-end or combination wrench for installation and removal without slipping, without marring the body surface, and without requiring a pipe wrench that could damage the fitting.
In instrument and process tubing assemblies, hex couplings are the standard method for connecting a hex nipple to an equipment port, joining two nipple extensions together, connecting a threaded instrument stub to a threaded valve outlet, or closing two male-threaded sections of an assembly that need to be field-joined. Because the coupling body is machined from solid bar stock, the internal bore is smooth with no sharp steps, maintaining clean flow and minimising pressure drop through the connection point.
| Fitting | End 1 | End 2 | When to Use |
|---|---|---|---|
| Hex Coupling (Equal) | Female thread (same size) | Female thread (same size) | Joining two male-threaded components of the same thread size inline — pipes, nipples, studs |
| Reducing Hex Coupling | Female thread (large) | Female thread (small) | Joining two male-threaded components of different sizes inline — inline size change |
| Hex Nipple (Equal) | Male thread (same size) | Male thread (same size) | Extending or bridging two female-threaded ports of the same size |
| Reducing Hex Nipple | Male thread (large) | Male thread (small) | Inline size reduction between two female-threaded ports of different sizes |
| Female Adapter | Compression tube | Female thread | Transitioning from a compression tube run to a male-threaded stud on equipment |
| Union Connector | Compression tube | Compression tube | Joining two tube runs of the same OD inline — no threads involved |
| Thread Form | Standard | Seal Method | Common Application |
|---|---|---|---|
| NPT (Female) | ASME B1.20.1 | PTFE tape on mating male threads | US-standard instrument equipment; oil & gas; petrochemical; American process plant |
| BSPT (Female) | BS EN 10226 | PTFE tape on mating male threads | UK/European standard tapered BSP ports; hydraulic and pneumatic systems |
| BSPP (Female) | BS EN ISO 228-1 | Dowty bonded seal on mating male stud shoulder | Hydraulic and pneumatic equipment with parallel BSP ports; European process plant |
| Thread Form | Female NPT (ASME B1.20.1); Female BSPT (BS EN 10226); Female BSPP (BS EN ISO 228-1) |
| Thread Size Range | 1/8" to 2" (both ends same size for equal coupling) |
| Body Form | Hexagonal — full hex body length for maximum wrench engagement |
| Working Pressure | Up to 6,000 PSI (SS 316, ambient); 3,000 PSI (Brass) |
| Standard Material | SS 316 (ASTM A276 / A479) |
| Optional Materials | SS 316L, SS 304, Brass (CW617N), Monel 400, Inconel 600, Hastelloy C-276 |
| Surface Finish | Bright annealed or passivated (SS); natural (Brass); electroless nickel plating on request |
| Temperature Range | -196°C to +450°C (SS 316); -10°C to +120°C (Brass) |
| Thread Gauging | Each coupling gauged with GO/NO-GO thread gauges per applicable standard before despatch |
| Quality Certification | ISO 9001:2015; EN 10204 3.1 MTCs available; PMI testing on request |
| Compatibility | Compatible with all standard NPT/BSP male-threaded fittings, pipes, and nipples |
| Thread Size (Both Ends) | NPT Available | BSPT Available | BSPP Available | Typical Use |
|---|---|---|---|---|
| 1/8" | ✓ | ✓ | ✓ | Small gauge, vent, and drain connections; miniature instrument ports |
| 1/4" | ✓ | ✓ | ✓ | Standard instrument impulse lines; pressure gauge connections; analyser sample ports |
| 3/8" | ✓ | ✓ | ✓ | Medium-bore instrument lines; flow meter connections; hydraulic signal lines |
| 1/2" | ✓ | ✓ | ✓ | Process instrument connections; manifold block ports; valve body outlets |
| 3/4" | ✓ | ✓ | ✓ | Process pipe connections; pump outlet nipples; larger instrument manifold ports |
| 1" | ✓ | ✓ | ✓ | Main process line connections; larger valve body outlets; industrial instrument headers |
| 1-1/4" | ✓ | ✓ | ✓ | Industrial pipeline branches; hydraulic return lines; large-bore manifolds |
| 1-1/2" | ✓ | ✓ | ✓ | Large-bore process pipe connections; tank outlet coupling; industrial valve connections |
| 2" | ✓ | ✓ | ✓ | Main process line full-bore connections; large industrial valve body; pump connections |
Thread sealant is applied to the male-threaded component being inserted into the coupling — never inside the coupling socket itself. The correct sealant depends on the thread form of the male component.
| Coupling Thread Form | Sealant on Mating Male Thread | Application Notes |
|---|---|---|
| Female NPT Coupling | PTFE tape (2–3 wraps) or anaerobic thread sealant on male NPT thread | Seal by taper — thread deforms as it advances into the female socket. Do not over-tighten; follow torque values by size. |
| Female BSPT Coupling | PTFE tape (2–3 wraps) on male BSPT thread | Same taper-seal principle as NPT. BSPT and NPT are NOT interchangeable — different pitch and angle. Do not mix. |
| Female BSPP Coupling | Dowty bonded seal on male BSPP stud shoulder | Parallel threads — no taper seal. Dowty washer compresses between male shoulder and coupling face to seal. No PTFE tape. |
The full-length hexagonal body provides maximum wrench-flat contact area, allowing high installation torques without slipping or rounding. Unlike pipe couplings that require a pipe wrench biting into the OD (which damages the surface and makes re-use difficult), hex coupling flats engage cleanly with an open-end wrench and can be assembled and disassembled multiple times without body damage.
The short hex body keeps the connection length minimal, which matters in dense instrument panel layouts and manifold assemblies where every millimetre of space is critical. The hex coupling adds only the coupling body length to the overall assembly — there is no additional length from a pipe wrench clearance requirement as with standard pipe couplings.
Standard SS 316 construction resists chloride attack, sour gas (H2S), boiler condensate chemistry, and offshore atmospheric corrosion. SS 316 hex couplings do not require periodic re-coating or galvanic isolation when assembled with SS 316 nipples — the all-SS assembly is fully compatible and eliminates dissimilar metal corrosion risks common with carbon steel couplings in stainless assemblies.
Every TES-LOK hex coupling is thread-gauged with calibrated GO/NO-GO gauges to the applicable standard (ASME B1.20.1, BS EN 10226, or BS EN ISO 228-1) before despatch. This ensures consistent thread engagement, predictable make-up torque, and leak-free joints from the first assembly — no field thread chasing or re-cutting required.
Hex couplings from 1/8" to 2" in NPT, BSPT, and BSPP, across six materials (SS 316, SS 304, Brass, Monel, Inconel, Hastelloy), are available from Tesco Steel & Engineering under a single quality system. Mixed-material assemblies (e.g. SS 316 coupling with Monel nipples) can be sourced and certified from one supplier, simplifying procurement and MTC documentation for offshore and critical service projects.
EN 10204 3.1 material test certificates are available for all SS 316 and exotic alloy hex couplings. PMI (positive material identification) testing available on request. Full heat and lot traceability supports documentation requirements for oil & gas, petrochemical, offshore, and pharmaceutical instrument piping installations.
▶ What is a hex coupling tube fitting?
A hex coupling is a short inline fitting with female internal threads of the same size on both ends, machined from a hexagonal body. It joins two male-threaded pipes, nipples, or studs of the same thread size. The hex body provides wrench flats for installation with an open-end wrench without damaging the fitting body.
▶ What is the difference between a hex coupling and a hex nipple?
A hex coupling has female (internal) threads on both ends — it receives two male-threaded components. A hex nipple has male (external) threads on both ends — it is inserted into two female-threaded ports. They are complementary: a hex coupling joins two male studs; a hex nipple bridges two female sockets.
▶ What is the difference between a hex coupling and a reducing hex coupling?
A hex (equal) coupling has the same thread size on both ends. A reducing hex coupling has different sizes on each end, joining two male-threaded components of different sizes with an inline size change. Specify the equal coupling when both components are the same thread size; specify the reducing coupling when they differ.
▶ Can I use PTFE tape with a BSPP hex coupling?
No. Never apply PTFE tape to a male stud entering a BSPP (parallel BSP) coupling. Parallel threads do not seal by taper — a Dowty bonded seal on the male stud shoulder is required. PTFE tape on BSPP produces a false torque feel without sealing. Only NPT and BSPT (tapered) thread assemblies use PTFE tape.
▶ Can NPT and BSPT male threads be mixed in the same coupling?
No. NPT and BSPT are not interchangeable — they have different thread pitches and flank angles (NPT: 60°; BSPT: 55°). Mixing them causes cross-threading and false engagement that appears tight but leaks. Both ends of the coupling must match the same thread standard as the mating male components.
▶ How do I install a hex coupling correctly?
Apply sealant to each male component (PTFE tape for NPT/BSPT; Dowty seal for BSPP). Thread the first male component into one end and tighten using an open-end wrench on the coupling hex flats. Apply sealant to the second component, thread it in from the other end, and tighten using a back-up wrench on the first component to prevent rotation. Never use a pipe wrench on hex flats. Pressure-test after assembly.
▶ What sizes are available?
1/8" to 2" in NPT, BSPT, and BSPP — all standard instrument and process pipe sizes are available from stock. Non-standard sizes manufactured on request.
▶ What is the pressure rating?
Up to 6,000 PSI in SS 316 at ambient temperature. Brass versions rated to 3,000 PSI. Rating is governed by thread engagement depth and the smaller thread size in the assembly. EN 10204 3.1 MTCs and PMI available on request.
SS 316 · Brass · Monel · Inconel · NPT · BSPT · BSPP · 1/8"–2" · Equal & Reducing · ISO 9001:2015 · EN 10204 3.1 MTCs available
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