6 mm Tube Fittings: Instrumentation-Grade Stainless & Alloy Compression Fittings
A 6 mm tube fitting is an instrumentation-grade compression, or ferrule, fitting that grips 6 mm outside-diameter tubing to form a leak-tight, gas-tight joint between tube and component in stainless steel and high-nickel or corrosion-resistant alloys. On this page, "6 mm tube fittings" refers specifically to instrumentation stainless-steel and alloy compression fittings for 6 mm OD tubing. It doesn't cover pneumatic push-to-connect, plumbing, or copper compression fittings.
Crestflo builds 6 mm fittings for the US market, engineered as drop-in replacements and backed by ASTM F1387 performance qualification. Materials span 316 and 316L up through Hastelloy. Standard items ship from stock, specials are made to order, and delivery runs 6-8 weeks. Request a quote or request a sample to get a US project moving.
Also Known As: 6mm Compression Fitting, Double-Ferrule & Metric Tube Fitting Designations
The same component goes by several names, depending on the buyer or the drawing. Most people call it a 6mm compression fitting, a 6mm compression tube fitting, a 6mm double-ferrule or twin-ferrule fitting, a 6mm single-ferrule fitting, a 6mm instrumentation fitting, or a 6mm ferrule fitting. Stainless callouts describe it differently: a 6mm stainless steel tube fitting, a 6mm stainless steel compression fitting, a 6mm SS compression fitting, or a 6mm 316 tube fitting. When the focus is on its role at the joint, it's written as a 6 mm tube connector or a 6mm tube union. On metric drawings you'll sometimes find it labeled 6mm metric tube fitting, 6mm OD tube fitting, or Ø6 fitting. Interchange requests, meanwhile, often ask for a 6mm Swagelok fitting or a 6mm Swagelok equivalent. Whatever the label, all of these point to the same instrumentation ferrule fitting detailed below.
What a 6 mm Tube Fitting Is: Working Principle & Single vs Double Ferrule
Crestflo builds 6 mm compression tube fittings that seal by mechanical force alone. Tightening the nut drives one or two ferrules onto the 6 mm OD tube. This action collets the tube and creates a leak-tight, gas-tight joint, with no heat, no flux, and no elastomers anywhere in the primary connection. Buyers can choose single-ferrule or double-ferrule (twin-ferrule) designs, and ferrules come hardened or non-hardened depending on the application.
The double-ferrule design splits sealing and gripping between two separate ferrules, and that separation is what lets instrument lines under vibration get made up and remade consistently. The body itself is metal-to-metal, so the fitting carries no elastomeric temperature ceiling. What actually sets the rating? The alloy and the tube. For more on ferrule design, see the tube fitting ferrules, single-ferrule, and nut-and-ferrule pages.
6 mm Tube Fitting Dimensions: Metric OD & the 6 mm vs 1/4 Inch Distinction
6 mm is a metric-series nominal OD. Convert it and you land at roughly 0.236 in, or 0.2362 in for the precise figure. That's close to 1/4 inch tubing at 6.35 mm, but it isn't identical. The two outside diameters differ by about 0.35 mm, so 6 mm and 1/4 in fittings can't be swapped across the OD. Bridge them with a reducer instead of forcing a mismatched tube. The figures come from standard metric/imperial OD tables and can be adjusted to requirement.
| Nominal tube OD | OD (inch, ref) | Nearest imperial size | Interchangeable? | Series reference |
|---|---|---|---|---|
| 6 mm | 0.236 in | 1/4 in (6.35 mm) | No, different OD; use a reducer | Metric series (ISO 8434-1 ref) |
For the full metric and imperial range, check the tube fitting size chart and the metric tube fittings hub. Need something bigger or smaller? Step up to 8 mm, 10 mm or 12 mm. If you're unsure how metric and imperial sizes relate, the metric vs imperial tube fittings explainer covers it.
6 mm Tube Fitting Materials: 316/316L Stainless, Duplex, 6Mo & Nickel Alloys
The 6 mm line runs from austenitic stainless through duplex, super-austenitic and high-nickel alloys. 316/316L stands as the default instrumentation grade. For sour, chloride, or high-temperature service, exotic corrosion-resistant alloys are on offer. There's no brass on the US line. Compositions and mechanical properties follow the relevant ASTM/UNS standards, and they can be tailored to whatever the job requires.
| Family | Grades | Typical driver | Grade page |
|---|---|---|---|
| Austenitic stainless | 304/304L, 316/316L, 904L | General instrumentation, chloride resistance | 316/316L, 304/304L, 904L |
| Duplex / super-duplex | 2205, Super Duplex 2507 | Strength + chloride SCC resistance | Duplex 2205, Super Duplex 2507 |
| Super-austenitic (6Mo) | 254 SMO, AL-6XN, Alloy 20 | Seawater, acid service | 254 SMO, AL-6XN, Alloy 20 |
| Nickel alloys | Inconel 625/825, Monel 400, Hastelloy C-276 | High temperature, sour/aggressive media | Inconel 625, Inconel 825, Monel 400, Hastelloy C-276 |
Sour service calls for alloys rated to NACE MR0175 / ISO 15156, and those are available on the 6 mm line. Browse the full range from the instrumentation fitting materials hub.
6 mm Tube Fitting Types: Unions, Connectors, Elbows, Tees & Bulkhead Fittings
Every fitting shape in the catalog comes in 6 mm OD. That means a 6 mm tube union, a 6 mm tube connector, an elbow, or a tee can all be built to match your line layout. You'll find dimensional detail on each shape's own page.
| Shape | Description | Product page |
|---|---|---|
| Union | Joins two 6 mm tubes inline | Union |
| Connector | Tube-to-thread connector | Connector |
| Male / female connector | Tube × male or female thread | Male connector, Female connector |
| Elbow / tee / cross | Directional and branch fittings | Elbow, Tee, Cross |
| Reducer / adapter | Bridge to another OD or end | Reducer, Adapter |
| Bulkhead union / port connector | Panel and port mounting | Bulkhead union, Port connector |
| Cap / plug / weld connector | Line termination and weld ends | Cap, Plug, Weld connector |
6 mm Tube Fitting End & Thread Connections: NPT, BSPT, BSPP, JIC & SAE-ORB
6 mm fittings aren't limited to tube-by-tube ends. They're also available with the standard instrumentation thread and weld terminations: NPT (per ASME B1.20.1) and NPTF, BSPT and BSPP, ISO parallel, JIC 37° (SAE J514), SAE-ORB, socket-weld and butt-weld. Thread dimensions follow the applicable standards, though they can be customized as needed. Full details live on the dedicated NPT, NPTF, BSPT, BSPP, ISO parallel, JIC 37°, SAE-ORB, socket-weld and butt-weld end pages.
6 mm Tube Fitting Pressure Rating, Temperature Limits & Performance
A 6 mm fitting doesn't carry one fixed working pressure. Wall thickness and alloy set that number, following standard tube working-pressure tables, and the fitting itself is rated to match whatever tubing it grips. Temperature capability works the same way: it hinges on the alloy, since the metal-to-metal body has no elastomer in the primary seal. Values follow the applicable standard, though they can be adjusted to what you need. Want a certified rating for your exact 6 mm wall and alloy? Request the rating on your RFQ.
Crestflo compression fittings meet ASTM F1387, the performance standard for compression tube fittings. That standard covers hydraulic proof, burst and impulse testing, pneumatic testing, flexure fatigue, rotary flex, vibration, thermal cycling, pull-out and vacuum testing. Across the range, these fittings come in end connections up to 2 in. (50 mm) OD.
6 mm Tube Fitting Installation, Make-Up & Torque
A 6 mm compression fitting comes together by turns, not by torque. Seat the 6 mm tube squarely against the fitting body, snug the nut down finger-tight, then turn it a set amount further to swage the ferrule or ferrules onto the tube's outer diameter. Over-torquing is the field error most technicians make. Do that, and the ferrule distorts, or the tube wall does. Either way, the seal fails. Tighten to the specified number of make-up turns, not to a torque reading. Check that you've pulled up enough on the first installation with a gap-inspection gauge, not a torque wrench. Make-up turns and gauge dimensions follow the applicable fitting standard, and both can be adjusted to your requirements. Ask our engineering team for the 6 mm make-up and gauging guide when you submit your RFQ.
6 mm Swagelok, Parker & Hy-Lok Interchange: F1387 Drop-In Replacement
Crestflo's 6 mm fittings are built for a true drop-in swap with the major instrumentation brands. That claim rests on ASTM F1387 performance qualification, not on copying anyone's look. Interchange means equivalence, nothing more. It doesn't mean affiliation. Swagelok® is a trademark of Swagelok Company. Parker®, Hy-Lok®, Hoke® and Fitok® belong to their respective owners, and Crestflo has no affiliation with, and no endorsement from, any of them.
We won't hand you a fixed part-number table. Instead, our engineering team checks the exact 6 mm equivalents against your current brand and series, one line at a time. Request your Swagelok®/Parker® cross-reference and we'll map it out for you, item by item. Curious about the specifics first? The interchange write-ups cover it: Swagelok cross-reference, Swagelok alternative, Parker A-Lok cross-reference, Hy-Lok cross-reference, Hoke cross-reference, and Fitok cross-reference.
Certifications, Testing & Traceability for 6 mm Fittings
Crestflo holds ISO 9001, 14001, and 45001 certification, along with PED certification. Its 6 mm compression fittings carry the standard approvals expected for this product type: ASTM F1387 performance qualification, plus NACE MR0175 / ISO 15156 for sour service. Testing covers the full F1387 suite, PMI, in-house NDT, and destructive testing, all carried out at ISO/IEC 17025-accredited labs. Third-party inspection is available on request.
Every part carries a heat/lot number, so it can be traced from start to finish. EN 10204 3.1 material test certificates come as standard, and 3.2 certificates are available on request. Origin is never a mystery: material comes from India, though US or European melt can be arranged on request, with full traceability maintained either way. Certificates sit in the quality area, where you'll find details on material traceability and US origin & compliance. Datasheets and CAD files go out to customers whenever they ask.
Why Crestflo for 6 mm Instrumentation Tube Fittings
Crestflo runs the instrumentation arm of a four-decade-old, Government-of-India-recognized export house with a solid delivery record across the Middle East, and today it builds instrumentation fittings, valves, and flow-control components for buyers around the world. Take 6 mm fittings: they come with ASTM F1387-qualified drop-in interchange, a full SS-to-nickel alloy lineup, heat-number traceability backed by MTC 3.1 (3.2 if you need it), low MOQs with private-label options on the table, and ready stock on standard items, made-to-order runs for specials, and 6-8 week delivery. US buyers can reach the team directly at sales@crestflousa.com · +1 346 594 9005 (Houston, TX). Request a quote, request a sample, or contact the engineering team.
Frequently Asked Questions: 6 mm Tube Fittings
What is a 6 mm tube fitting?
A 6 mm tube fitting is a compression, or ferrule, fitting. It grips 6 mm OD tubing and forms a joint that's both leak-tight and gas-tight, built for instrumentation work. Manufacturers offer it in single- or double-ferrule versions, made from stainless steel, high-nickel alloys, or other corrosion-resistant materials. Don't confuse it with a pneumatic push-to-connect fitting or an ordinary plumbing compression fitting; it's neither.
Is a 6 mm tube fitting the same as 1/4 inch?
No. A 6 mm tube runs about 0.236 in OD; 1/4 in tubing measures 6.35 mm OD. Roughly 0.35 mm separates the two outside diameters. Not a trivial gap. So 6 mm and 1/4 in fittings won't swap directly on the OD. Bridge the two sizes with a reducer instead of forcing a tube that doesn't fit.
What materials are 6 mm fittings available in?
The 6 mm line runs 304/304L, 316/316L (the default) and 904L stainless, plus Duplex 2205 and Super Duplex 2507. It also covers 254 SMO, AL-6XN and Alloy 20, along with nickel alloys such as Inconel 625/825, Monel 400 and Hastelloy C-276. Brass isn't part of the US line. Compositions follow the applicable ASTM/UNS standards.
Are Crestflo 6 mm fittings interchangeable with Swagelok?
Crestflo's 6 mm fittings are built to drop straight in for Swagelok and other major brands, matching their dimensions exactly. That claim carries real weight: it's backed by ASTM F1387 performance qualification. Swagelok® is a trademark of Swagelok Company, and Crestflo isn't affiliated with it or endorsed by it. Send in your cross-reference request. Our engineering team will map it out, line by line.
What is the pressure rating of a 6 mm tube fitting?
Wall thickness and alloy set the rating, drawn straight from standard tube working-pressure tables, and each fitting is rated to match the tubing it grips. Values follow the applicable standard, though they aren't fixed; you can tailor them to your requirement. Every fitting meets ASTM F1387-qualified standards, tested for hydraulic proof, burst and impulse performance. Request a certified rating for your specific wall and alloy under ASTM F1387 on the RFQ.
How do you install a 6 mm tube fitting, by torque or by turns?
A 6 mm tube fitting goes together by turns, not by torque. Bottom the tube in the fitting, snug the nut down finger-tight, then bring it the specified amount further to swage the ferrule onto the tube. Don't over-torque it. On the first make-up, check pull-up with a gap-inspection gauge instead of a torque wrench. Turns and gauge values come from the applicable standard and can be adjusted to fit the requirement.
Do you offer single-ferrule and double-ferrule 6 mm fittings?
Yes. We offer both single-ferrule and double-ferrule (twin-ferrule) designs at 6 mm. Ferrules come hardened or non-hardened, whichever the application calls for.
Are 6 mm fittings available for sour (NACE) service?
Yes. NACE MR0175 / ISO 15156-compliant alloys are available at 6 mm, suited to sour and aggressive-media service. Full material traceability comes standard with every order.
What thread and end connections are available in 6 mm?
Terminations run the gamut: tube-by-tube ends, NPT, NPTF, BSPT, BSPP, ISO parallel, JIC 37°, SAE-ORB, socket-weld, and butt-weld. Thread dimensions follow the applicable standards, and we'll adjust them to fit your requirements.
What certifications and traceability come with 6 mm fittings?
Crestflo holds ISO 9001 / 14001 / 45001 certification along with PED approval, and its 6 mm fittings meet ASTM F1387 and carry NACE MR0175 / ISO 15156 approvals. Every part gets a heat/lot number. EN 10204 3.1 material test certificates come standard; 3.2 versions aren't automatic, but they're available on request.
Where are Crestflo 6 mm fittings made?
They serve US clients but are Indian in origin, with US or European melt available on request and full material traceability behind it. The standard product comes from India; the US and European melt options exist for cases where domestic-melt or TAA sourcing is required, and that distinction is spelled out plainly on the US origin and compliance page.
What is the lead time and minimum order for 6 mm fittings?
Ready stock covers standard items. Specials are made to order, with delivery running six to eight weeks. Minimum order quantities stay small, and private-label options are available.
Are these instrumentation fittings rather than pneumatic or plumbing fittings?
That's right. These fittings are stainless-steel and alloy compression types built for instrumentation service, sized for 6 mm OD tubing. They're not push-to-connect fittings, not air-line fittings, and not copper plumbing fittings.
Related 6 mm Tube Fitting Resources
- Tube fittings hub, full instrumentation fitting range
- Tube fitting size chart, metric and imperial sizes
- Metric tube fittings and metric vs imperial tube fittings cover the basics side by side.
- Nearby sizes worth checking: 8 mm, 10 mm, 12 mm, and 1/4 inch.
- For cross-checking parts, see the Swagelok cross-reference; for materials, the 316/316L material page has what you need.