ISO Parallel Tube Fittings: ISO 228-1 (G) Straight-Thread End Connections in Stainless & Alloy
ISO parallel tube fittings are instrumentation tube fittings that come with an ISO parallel (straight, non-tapered) thread end connection per ISO 228-1, designation letter G. The seal forms on a bonded washer, O-ring, or captive gasket at the port face. Not on the thread flanks. Crestflo builds these connectors, adapters, and male/female fittings with an ISO parallel thread on the port side and a compression (single- or double-ferrule) connection on the tube side, available in 316/316L, 904L, and high-nickel corrosion-resistant alloys, in both imperial and metric. Each one is dimensionally interchangeable and qualified to ASTM F1387. Heat-number traceability comes standard, backed by EN 10204 material certification.
Ready stock for standard items · made-to-order for specials · 6-8 week delivery.
Here, ISO parallel means one specific thing: a fluid-system straight thread, ISO 228-1 G, sealing on a gasket or O-ring. Not a tapered plumbing pipe thread that seals on the flanks. Not an electrical ferrule.
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Also Known As: ISO 228 Fittings, G-Thread & Parallel Pipe Thread Fittings
Specifiers use several different names for this end connection, though they all point to the same ISO 228-1 straight thread. You might see it called an ISO parallel thread fitting, an ISO 228 fitting, or ISO 228-1 thread. Others label it a G thread fitting or G-thread tube fitting, a straight thread fitting, or a parallel thread tube fitting. When the seal is made of elastomer, catalogs describe it as a bonded-seal (Dowty/USIT) fitting, an O-ring seal fitting, or a straight-thread O-ring fitting.
In imperial sizes, this same thread trades widely as BSPP (British Standard Pipe Parallel); Crestflo's BSPP (G-thread) tube fittings page covers that naming in detail. Metric parallel ports, meanwhile, show up as ISO 9974 or ISO 6149 M-series threads. Size call-outs you'll run into most often include G1/8, G1/4, G3/8, G1/2, G3/4, and G1. Some catalogs even tag a gasket-sealed male ISO parallel thread with the industry code RS.
What Is an ISO Parallel (ISO 228-1 G) Thread?
An ISO parallel thread is straight, not tapered. ISO 228-1, part of the ISO 228 series, defines it and marks it with the letter G. Its major and minor diameters stay constant along the full length of the thread. Because it doesn't wedge, this thread can't seal through thread interference the way a tapered thread does. The joint holds the parts together; sealing comes separately, from a washer, O-ring, or gasket. That's the reverse of a tapered NPT (Crestflo's ASME B1.20.1) or BSPT connection, where the flanks themselves seal, often with the aid of sealant or tape. ISO 228-1 states plainly that it covers pipe threads "where pressure-tight joints aren't made on the threads." Values follow the applicable standard and can be adjusted to suit the requirement.
How an ISO Parallel Connection Seals: Bonded Washer, O-Ring & Gasket
ISO parallel (G) connections seal against a dedicated element at the port face or shoulder, never against the thread itself. Three arrangements are standard:
- Bonded seal washer (Dowty / USIT), a metal ring with a captive elastomer inner lip, squeezed under the fitting hex against a flat spotface. This falls under DIN 3852 Form A.
- O-ring on a straight spotface (soft-seal), here the O-ring sits on a machined shoulder and gets energised the moment the shoulder lands. DIN 3852 Form E covers this one.
- Metric O-ring boss, an O-ring captured in a chamfered boss per ISO 6149, common on European instrument ports.
The seal is an elastomer, so the O-ring or washer compound sets the temperature limit for the connection, not the metal fitting. Pick a compound that matches your service conditions with Crestflo's seal-material temperature selection guidance.
ISO / DIN Parallel Port Standards: ISO 228-1, ISO 1179, ISO 9974, ISO 6149 & DIN 3852
ISO parallel ends fall under a family of parallel-thread port standards. The table below sets out the published scope for each one. Figures follow whichever standard applies, and they can be adjusted to suit requirements.
| Standard | Designation | Thread form | Seal method | Typical use |
|---|---|---|---|---|
| ISO 228-1 | G (e.g. G1/4) | Parallel, 55° Whitworth | External gasket / O-ring | General fastening thread; imperial parallel ports |
| ISO 1179 | G | Parallel | Bonded seal or O-ring at spotface | Fluid-power & instrument ports (former DIN 3852) |
| ISO 9974 | G and M | Parallel | Elastomeric / bonded seal | Ports for connections with elastomeric sealing |
| ISO 6149 | M | Metric parallel | O-ring in chamfered boss | Metric O-ring boss ports, hydraulics & instrumentation |
| DIN 3852 | G and M | Parallel | Form A bonded washer / Form E O-ring | Defines the port spotface & seal form |
ISO Parallel Seal (Port) Forms: DIN 3852 Form A, Form E & ISO 6149 Boss
The port form sets where the seal sits and which element gets used. Pick the form that matches how your equipment port is machined.
| Port / seal form | Seal element | Where it seals | Notes |
|---|---|---|---|
| DIN 3852 Form A | Bonded seal washer (USIT/Dowty) | Flat spotface under the hex | Most common gasket-sealed G thread |
| DIN 3852 Form E | O-ring | Straight machined shoulder | Soft-seal, reusable; "RS"-code male threads |
| ISO 6149 boss | O-ring | Chamfered metric boss | Metric M-series O-ring port |
| DIN 3852 Form B | Metal-to-metal / tapered seat | Cone seat | Tapered, not a parallel gasket seal |
ISO Parallel Thread Sizes & Designations: G1/8 to G1 and Metric M-Series
Standard ISO 228 parallel threads run G1/8, G1/4, G3/8, G1/2, G3/4, and G1, with larger sizes available too. Metric parallel port threads follow the ISO 9974 / ISO 6149 M-series. The G1/4 fitting is the most-requested size for gauge and instrument hook-ups. The dimensions below are the published ISO 228-1 nominal values, not estimates. Crestflo can provide the full size and pairing list on request; just request the ISO parallel size guide. For a quick reference, there's also the tube-fitting size chart.
| ISO 228 designation | Threads per inch | Metric equivalent context | Common tube-OD pairing |
|---|---|---|---|
| G1/8 | 28 | — | 1/8", 3 mm, 6 mm |
| G1/4 | 19 | — | 1/4", 6 mm, 8 mm |
| G3/8 | 19 | — | 3/8", 10 mm |
| G1/2 | 14 | — | 1/2", 12 mm |
| G3/4 | 14 | — | 3/4", 18 mm |
| G1 | 11 | — | 1", 25 mm |
| M-series | per ISO 9974 / 6149 | M10×1, M12×1.5, M14×1.5, M16×1.5 | metric tube ends |
Values per applicable standard; customizable to requirement.
ISO Parallel Fitting Configurations: Male Connector, Female Connector, Adapter, Elbow, Tee & Union
Crestflo supplies the ISO parallel (G) thread end on the full range of instrumentation body forms, each mated to a compression tube end on the other side. Available configurations include the ISO parallel male connector, female connector, adapter, elbow, tee, and union. Tube-side compression ends run up to 2" (50 mm) OD, in single- and double-ferrule form, imperial and metric, with hardened or non-hardened ferrules to requirement. Other thread ends are available on the same bodies: NPT, NPTF, BSPT, BSPP, and metric.
ISO Parallel Fitting Materials: 316/316L, 904L, Inconel & Sour-Service Alloys
ISO parallel ends come in the full stainless and high-nickel corrosion-resistant alloy range, chosen for the service conditions, not the thread. Stainless options are 304/304L, 316/316L, and 904L. Duplex and super-austenitic grades include Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, and Alloy 20. Nickel alloys include Inconel 625, Inconel 600/601/718/825, Incoloy 800/800H/825, Monel 400/K500, Hastelloy C-276/C-22/B-2/B-3/X, and Nickel 200/201. Titanium Gr2 is available too. For sour service, fittings ship compliant with NACE MR0175 / ISO 15156 in the appropriate alloys, so be sure to specify sour service on your RFQ. These pages are here to help with grade selection; Crestflo supplies these alloys only in instrumentation fitting, valve, and tubing forms.
Standards, Approvals & Traceability
Crestflo holds the standard approvals you'd expect for instrumentation tube fittings. Compression performance is qualified to ASTM F1387, the benchmark that backs up drop-in replacement claims. Parts for sour service meet NACE MR0175 / ISO 15156. Each part is stamped with a heat/lot number. EN 10204 MTC 3.1 comes standard, and 3.2 is available on request; details are covered under material test certificate traceability. Crestflo also carries ISO 9001 / 14001 / 45001 and PED certification. On the testing side, parts go through hydraulic proof, burst and impulse, pneumatic, pull-out, vacuum, and thermal cycling, plus PMI and in-house NDT. Destructive testing happens at labs accredited to ISO/IEC 17025, and third-party inspection can be arranged on request. Datasheets and CAD go out to customers who ask for them.
Specifying & Ordering ISO Parallel Tube Fittings
Specifying an ISO parallel fitting takes six pieces of information: body form (male connector, adapter, elbow, and so on), tube OD (imperial or metric), ISO parallel thread size (G1/8, G1 or metric M), seal/port form (DIN 3852 Form A, Form E, or ISO 6149 boss), material, and ferrule type. Ordering-code schemes are Crestflo-proprietary, so request the ordering-code guide from our engineering team, and we'll return the exact part callout for your configuration. Need to match a competitor thread code or part? Request your Swagelok® or Parker® cross-reference: we interchange by dimension and thread standard, backed by ASTM F1387, and we don't publish invented competitor part numbers. For scope, see the full master interchange.
Why Crestflo for ISO Parallel Tube Fittings
Crestflo is the instrumentation arm of an export house with four decades behind it, recognized by the Government of India, with a proven delivery record across the Middle East and now active in the US market. Its ISO parallel line pairs the ISO 228-1 G thread with real advantages. Chief among them: ASTM F1387-qualified drop-in interchange with Swagelok, Parker, Hoke, Hy-Lok, and Fitok. Lead time runs 6-8 weeks, versus the 20-plus weeks typical of the majors. Every fitting carries both imperial and metric markings. The full stainless steel and high-nickel/CRA alloy range is on offer. MOQs stay small, and private-label is available on request. Traceability isn't an afterthought either: heat numbers are recorded, with certificates issued under EN 10204 MTC 3.1/3.2. Origin is Indian, though US or European melt can be arranged on request, backed by full material traceability. For details, see our US-origin & TAA compliance note. Standard items ship from stock. Specials are made to order. Either way, delivery runs 6-8 weeks. We Engineer Confidence.
Reach Crestflo's US sales team directly: email sales@crestflousa.com or call +1 346 594 9005. We're based in Houston, TX.
Frequently Asked Questions: ISO Parallel & G-Thread Tube Fittings
What is an ISO parallel thread?
An ISO parallel thread is straight, not tapered. ISO 228-1, part of the ISO 228 series, defines this fastening thread and marks it with the letter G. Its diameter stays constant along the full length, which means the seal doesn't form on the threads themselves. Instead, it happens at the port face, on a bonded washer, an O-ring, or a gasket.
Does an ISO parallel fitting seal on the thread?
No. An ISO parallel (G) connection doesn't seal the way a tapered NPT or BSPT thread does. It needs a separate seal: a bonded or USIT washer under the hex, to DIN 3852 Form A, or an O-ring seated on a straight shoulder, covered by Form E or ISO 6149. The thread itself just holds the joint together, nothing more.
Is ISO parallel the same as BSPP?
Dimensionally, ISO 228-1 "G" and BSPP describe the same 55° parallel thread. Same geometry, different names. BSPP is the older British trade name; ISO 228 designates it as "ISO parallel," or G, in the ISO system. Crestflo's BSPP tube fittings page walks through that naming in more detail.
Are ISO parallel and NPT threads the same?
No. NPT (ASME B1.20.1) is a tapered US thread; it seals on the flanks using sealant or tape. ISO parallel, or G, thread is straight instead, and it seals against a gasket or O-ring. The two have different forms and different angles, so they don't interchange. Crestflo covers this in more depth here: ISO/G thread vs NPT.
What is ISO 7-1 (Rp/Rc) and how does it relate to a G thread?
ISO 7-1 covers pipe threads where the joint itself has to hold pressure, right at the thread. Rp is a parallel internal thread; Rc is a tapered internal thread, and both mate with an R tapered external thread. The ISO 228-1 G thread on this page works differently. It's a straight fastening thread, and it seals on a separate gasket or O-ring rather than on the thread itself.
What are the key differences between ISO and ANSI/ASME threads?
ISO parallel and metric threads follow ISO thread forms, marked with G and M designations. ANSI/ASME threads work differently: NPT and UN rely on inch-based forms, and NPT carries a taper on top of that. The sealing method isn't the same, nor is the thread angle, nor the pitch. Don't cross-thread ISO and ASME fittings together.
What thread sizes do you offer for ISO parallel fittings?
Standard ISO 228 sizes G1/8 through G1 cover the base range, with larger sizes and metric parallel (M-series) port threads available on request. Dimensions follow the published ISO 228-1 and ISO 9974/6149 values, and they can be customized to requirement.
Which alloys are available with ISO parallel ends?
We work with 316/316L and 304/304L stainless steels, along with 904L, Duplex 2205, and Super Duplex 2507. Add 254 SMO, AL-6XN, and Alloy 20 to that list. Nickel alloys are covered too: Inconel, Incoloy, Monel, and Hastelloy. Round it out with Nickel 200/201 and Titanium Gr2.
Are ISO parallel fittings NACE-compliant for sour service?
Yes. Crestflo supplies ISO parallel fittings that meet NACE MR0175 / ISO 15156, made in suitable corrosion-resistant alloys. Note sour service on your RFQ so the correct grade and hardness get selected.
Do you provide material certificates for ISO parallel fittings?
Every part gets marked with a heat/lot number. EN 10204 MTC 3.1 comes as standard, and 3.2 is available if you request it. That gives complete material traceability, from the mill right through to the finished fitting.
Where are ISO parallel fittings made and how fast can I get them?
Crestflo serves customers across the US. Made-to-order specials run 6-8 weeks for delivery, while standard items ship from ready stock. Standard product comes from India, though US and European melt is available on request. Origin gets stated plainly on our US-origin & TAA compliance page.
Can I match a Swagelok or Parker ISO parallel thread code?
Yes. We interchange by dimension and thread standard, backed by ASTM F1387. We won't publish invented competitor numbers. Instead, we give you your equivalent directly, request your cross-reference, and our engineering team will match it.
Related Tube Fittings & Resources
- Tube fittings (parent range), every thread-end and body-form option Crestflo offers
- BSPP (G-thread) tube fittings, this is the ISO 228-1 thread, just under its trade name
- NPT and metric tube fittings, the remaining thread-end choices
- Thread identification guide, spot an ISO parallel thread with confidence
- BSPP vs BSPT and NPT vs BSP comparisons
- Material test certificate & traceability · private label · lead time