Tube-OD Tube Fittings: Compression Ends Sized by Tube Outside Diameter
A tube-OD tube fitting is an instrumentation compression fitting that connects to tubing by gripping the tube's outside diameter with a single or double ferrule. There's no threading, no welding, no flaring of the tube itself. It's sized directly by OD, in both fractional-inch and metric ranges up to 2" (50 mm), and it's the end-connection family that lets you build a leak-tight, remakeable fluid or gas system straight onto standard instrumentation tubing.
Tube-OD refers to the instrumentation compression grip on a tube's outside diameter. It's not a plumbing or copper compression fitting, not tube inside diameter (ID), and not pipe (NPS) sizing either. Crestflo builds these fittings tested to ASTM F1387, in single and double ferrule versions, using materials from 316 stainless through high-nickel and titanium alloys. Every part carries a heat number. Standard items ship from ready stock, while specials are made to order, with delivery running 6–8 weeks.
Tube-OD Fittings Are Also Specified As Tube-to-Tube and O.D. Compression Fittings
Buyers and specifiers name this same end connection in several ways. It is commonly called a tube-OD fitting, a tube outside-diameter fitting, or an OD tube fitting; on drawings it appears as a "tube × tube" or tube-to-tube end, an O.D. tube fitting in period-style notation, or simply a tube-OD port. Because the seal is made by one or two ferrules, it is also written as a twin-ferrule tube-OD end, a ferrule tube-OD connection, or a compression tube-end. Where the plain projecting length of tube is the reference, the same connection is called a tube-stub end or tube spigot end. Every one of these designations refers to the same thing: a fitting that seals onto the tube by its outside diameter.
What "Tube OD" Means and How to Measure Tube Outside Diameter
OD stands for the tube's outside diameter: the actual surface a ferrule grips and seals against. Tube-OD fittings get specified and ordered by this figure, never by the inside diameter, because the ferrule acts on the outer wall. Measure it with calipers, straight across the outside of the tube. For a nominal "1/2 inch" instrumentation tube, that reading lands at 0.500" by convention. Wall thickness then decides whatever the ID happens to be.
Two distinctions matter when reading a spec:
- Tube OD vs tube ID. Fittings are chosen by OD, not ID. Wall thickness sets the ID, so two tubes with the same OD don't always share the same bore. Values follow standard tube tables, which can be customized to your requirement. For details, see the tube sizing and wall-thickness guide.
- Tube OD vs pipe (NPS). Tube is called out by its actual OD. Pipe works differently: NPS is a nominal size, and the real OD never matches the stated number. A 1/2" tube measures 0.500" across, while 1/2" NPS pipe comes in at 0.840" OD.
| Term | Measured how | Example | Note |
|---|---|---|---|
| Tube OD | True outside diameter, across the wall | 1/2" tube = 0.500" OD | The dimension the fitting seals on |
| Tube ID | Bore; OD minus twice wall thickness | Varies with wall | Not used to order tube-OD fittings |
| Pipe (NPS) | Nominal designation, not the OD | 1/2" NPS = 0.840" OD | Nominal ≠ actual; do not mix with tube OD |
OD/ID/NPS values per standard dimensional convention; customizable to requirement.
How Tube-OD Ferrule (Compression) Fittings Seal
Tighten the nut, and one or two ferrules bite radially inward onto the tube's outside diameter. That ferrule set grips the tube tight against vibration and pull-out, and at the same time it seals the outer wall against leaks. One connection, two jobs: mechanical grip and seal. It's gaugeable, remakeable, and there's no need to thread, flare, or weld the tube.
Single vs double (twin) ferrule. A single-ferrule design uses one ferrule to both grip and seal. A double- or twin-ferrule design splits those jobs across a back ferrule, which grips, and a front ferrule, which seals, giving added tolerance to tube hardness and vibration. Crestflo supplies both, in hardened and non-hardened form to requirement. The trade-offs are covered in the single vs double ferrule guide.
For 316 instrumentation tubing, the tube has to meet its dimensional standard, which is ASTM A269 (or the alloy equivalent). It also needs to sit within OD and hardness tolerance so the ferrule can grip and gauge correctly. Pull-up gets confirmed with a gap-inspection gauge; see ferrule installation and gauging.
Tube-OD Fitting Sizes: Fractional (1/16″–2″) and Metric (3–50 mm)
Tube-OD ends come in the full instrumentation range, fractional-inch and metric alike, up to 2" / 50 mm OD. Every common size gets its own detail page. The complete dimensioned table lives on Crestflo's tube fitting size chart, and you'll find the two systems compared side by side in metric vs imperial tube fittings.
| OD (fractional) | OD (metric, nearest) | Detail page | Typical use |
|---|---|---|---|
| 1/16″ | ~1.6 mm | part of imperial-fractional range | micro-bore sampling |
| 1/8″ | ~3 mm | 1/8 inch tube fittings | analyzer, gauge lines |
| 1/4″ | ~6 mm | 1/4 inch tube fittings | most common impulse line |
| 3/8″ | ~10 mm | 3/8 inch tube fittings | instrument air, hydraulics |
| 1/2″ | ~12 mm | 1/2 inch tube fittings | supply and process lines |
| 5/8″ | ~16 mm | 5/8 inch tube fittings | larger flow runs |
| 3/4″ | ~19 mm | 3/4 inch tube fittings | higher-flow headers |
| 1″ | ~25 mm | 1 inch tube fittings | main supply |
| 1-1/4″ – 2″ | 32–50 mm | made-to-order in range | large-bore instrument runs |
Metric OD ends are stocked and made across the same range. Dedicated pages cover 6 mm, 8 mm, 10 mm and 12 mm, and the broader metric program sits on the metric tube fittings page. Intermediate fractional sizes (3/16″, 5/16″, 7/8″) and the full mm set (3–50 mm) are made-to-order within the range as well; figures follow standard OD tables and can be tailored to requirement.
Tube-OD Fitting Shapes and End Combinations
Every common fitting geometry comes with a tube-OD end, whether that's tube-OD by tube-OD or tube-OD paired with a threaded or weld end on the other side. One family can handle tube-to-tube, tube-to-port, and tube-to-thread transitions across a panel.
| Shape | Tube-OD × Tube-OD | Tube-OD × NPT/BSP | Tube-OD × weld | Product page |
|---|---|---|---|---|
| Union | Yes | — | — | union |
| Elbow | Yes | Yes | Yes | elbow |
| Tee | Yes | Yes | — | tee |
| Cross | Yes | — | — | cross |
| Reducer | Yes | — | — | reducer |
| Male connector | — | Yes | — | male connector |
| Female connector | — | Yes | — | female connector |
| Bulkhead union | Yes | — | — | bulkhead union |
| Port connector | Yes | — | — | port connector |
| Cap / plug | Yes | — | — | cap · plug |
| Nut and ferrule | — | — | — | nut and ferrule |
Opposite-end thread options include NPT, NPTF, BSPT, BSPP and ISO-parallel. Weld options include socket, butt and orbital. Thread dimensions follow their governing standards; NPT's, for instance, is ASME B1.20.1. Crestflo lists them all in its thread identification guide.
Alloys for Tube-OD Compression Fittings: 316, 904L, Duplex, Inconel, Hastelloy and Titanium
Tube-OD fittings come across the full instrumentation alloy range, with ferrules matched or complemented to the body alloy. Grade choice comes down to service conditions, chloride, sour (NACE), high-temperature, or high-purity, and the grade education on each material page links out to its instrumentation forms. For high-purity and ultra-high-purity work, tube-OD ends are supplied as electropolished (EP) fittings in 316L. These pair with electropolished tubing, and when a welded joint makes more sense, with orbital-weld fittings.
| Family | Grades | Material page | Sour service (NACE) |
|---|---|---|---|
| Austenitic stainless | 304/304L, 316/316L, 904L | 316/316L · 904L | Compliant in appropriate grades |
| Duplex / super-austenitic | Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, Alloy 20 | duplex 2205 · super duplex 2507 | Yes, for chloride/sour |
| Nickel | Inconel 625/825, Monel 400, Hastelloy C-276, Nickel 200/201 | hastelloy C-276 · inconel 625 | Yes, high-corrosion service |
| Titanium | Titanium Gr2 | titanium Gr2 | Seawater / high-chloride |
The full grade list runs from 304/304L and 316/316L through 904L, Duplex 2205, and Super Duplex 2507. It also covers 254 SMO, AL-6XN, Alloy 20, Inconel 600/601/625/718/825, Incoloy 800/800H/825, Monel 400/K500, Hastelloy C-276/C-22/B-2/B-3/X, Nickel 200/201, and Titanium Gr2. To match a grade to a service, use material selection by service, or just browse the full materials library. Compositions follow the applicable alloy standard, and they can be customized to your requirement.
Tube-OD Fitting Pressure and Temperature Ratings
The working pressure of a tube-OD fitting is set by the tube it grips, not by the fitting alone: rating derates by tube OD × wall thickness × alloy, so a thicker wall or smaller OD carries more pressure. Allowable working pressures follow standard tube working-pressure tables and ASME B31.3 allowable stresses for the alloy; values per applicable standard, customizable to requirement. Work the numbers for a specific OD and wall in pressure-temperature derating alongside the tube sizing guide.
An all-metal tube-OD compression end doesn't carry any elastomer in the seal path. That means the body alloy sets the temperature limit, not an O-ring, so service stretches well past soft-seat ends, provided the alloy allows it. Exact limits come from the alloy's published temperature-derating data. They're customizable to requirement.
Standards, Certifications and Traceability for Tube-OD Fittings
Every Crestflo tube-OD compression fitting is tested to ASTM F1387, the standard that governs performance qualification for compression tube fittings. For sour service, fittings ship compliant with NACE MR0175 / ISO 15156, made in alloys chosen for that duty. The company's certified to ISO 9001, 14001, and 45001, with PED approval as well. A heat/lot number gets stamped on every part. Standard documentation is EN 10204 MTC 3.1; 3.2 is available on request.
| Item | What it covers | Applies to tube-OD end? |
|---|---|---|
| ASTM F1387 | Performance qualification of compression tube fittings | Yes, the fitting itself |
| ASTM A269 | Dimensions/quality of the tubing gripped | Yes, the mating tube |
| NACE MR0175 / ISO 15156 | Sour-service material requirements | Yes, in appropriate alloys |
| EN 10204 MTC | Material test certification (3.1 / 3.2) | Yes, traceability |
| ISO 9001 / 14001 / 45001 & PED | Quality, environment, safety, pressure equipment | Company-wide |
Testing covers hydraulic proof, burst and impulse, pneumatic, vibration, thermal cycling and pull-out. PMI, in-house NDT, and destructive testing all take place at ISO/IEC 17025-accredited labs. Third-party inspection is available on request.
Certificates are held in quality. Origin and traceability detail can be found on US-origin compliance and material traceability, for anyone at Crestflo who needs it.
Tube-OD Tube Fittings from Crestflo: Request a Quote
Crestflo carries the full tube-OD compression lineup: unions, elbows, tees, crosses, reducers, connectors, bulkheads, caps and plugs, along with nuts and ferrules. Sizes run fractional and metric OD, with single and double ferrule options, in materials spanning 316 stainless to exotic alloys. All of it sits within the broader tube fittings range and pairs with Crestflo instrumentation tubing.
Stock and delivery run on one rule: standard items ship from ready stock, specials get made to order, and lead time runs 6–8 weeks. Fittings come from India, though US or European melt is available if you request it, and full material traceability comes standard. Datasheets and CAD files? Available to customers on request.
- Request a quote, send OD size, alloy, configuration and quantity.
- Request a sample, evaluate fit and finish before you commit.
Request your Swagelok® or Parker® cross-reference from our engineering team through the cross-reference desk.
US inquiries: sales@crestflousa.com · +1 346 594 9005 (Houston, TX).
Why Crestflo for Tube-OD Compression Fittings
Crestflo tube-OD fittings are built as a drop-in interchange for Swagelok, Parker, Hoke, Hy-Lok and Fitok tube-OD ends. That's dimensional interchange backed by ASTM F1387 performance qualification, and most alternatives can't point to that kind of proof. Pair that interchange with a 6-8 week delivery window, against the 20-plus weeks typical of the majors, and you can see why procurement and MRO teams qualify Crestflo as a second source.
Crestflo is the instrumentation division of a four-decade-old, Government-of-India-recognized export house with a delivery track record across the Middle East. Decades of manufacturing and QA sit behind a brand built specifically for instrumentation. The offer includes small MOQ and private-label programs, imperial and metric options on every fitting, both hardened and non-hardened ferrules, and a heat number plus MTC on every part. Take a look at lead time, private label, and testing and inspection. We Engineer Confidence.
Frequently Asked Questions: Tube-OD Tube Fittings
What does "tube OD" mean on a fitting?
OD stands for the tube's outside diameter, the surface the ferrule grips and seals against. You specify and order tube-OD fittings by that outside measurement, not by the tube's inside diameter, since the ferrule works on the outer wall.
How do OD compression fittings work?
Tighten the nut and it drives one or two ferrules radially onto the tube's outside diameter. That single motion forms a mechanical grip you can gauge and remake, plus a leak-tight seal, all in one step. There's no threading, no flaring, no welding the tube. A gap-inspection gauge confirms the pull-up.
What tube-OD sizes are available?
Fractional-inch OD runs from 1/16 up to 2″, and metric OD goes as high as 50 mm. The common sizes get their own dedicated pages: 1/8″–1″ and 6–12 mm. Everything else, whether intermediate or larger, is made to order within the range. Values follow standard OD tables, and they're customizable to whatever you need.
Single or double ferrule, which should I use?
A double (twin) ferrule comes in hardened and non-hardened versions, same as a single. Split the grip and seal across two ferrules and you gain tolerance for tube hardness variation and vibration. Which one to use comes down to tube hardness, vibration, and pressure, details are in our single-vs-double-ferrule guide.
What is the OD of 1/2" SS tubing?
Instrument tube marked "1/2 inch" has an OD of 0.500" by dimensional convention. The inside diameter depends on wall thickness after that. You'll find the full set of OD and wall values on the tubing size chart from Crestflo.
Is tube OD the same as pipe size?
Tube's called out by its actual outside diameter. Pipe, on the other hand, uses a nominal size (NPS), and the real OD doesn't match the stated number. Take 1/2" tube: that's 0.500" OD. But 1/2" NPS pipe comes in at 0.840" OD.
Do Crestflo tube-OD fittings interchange with Swagelok or Parker?
Yes. They match up dimensionally with Swagelok, Parker, Hoke, Hy-Lok and Fitok tube-OD ends, and it's backed by ASTM F1387 performance qualification. Ask our engineering team for your part-specific cross-reference. Swagelok® and Parker® are trademarks of their respective owners.
What alloys can tube-OD fittings be made in?
We work across 304, 316 and 904L stainless, Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN and Alloy 20, along with Inconel, Incoloy, Monel and Hastelloy. Nickel 200/201 and Titanium Gr2 fill out the range. Ferrules aren't an afterthought either; they're matched to the body alloy every time.
Are tube-OD fittings suitable for sour (NACE) service?
Yes, these come NACE MR0175 / ISO 15156 compliant, and they're available in the appropriate alloys for sour and high-corrosion service.
What certification and traceability come with them?
Every part carries a heat/lot number. EN 10204 MTC 3.1 comes as standard, and we'll supply 3.2 on request. Everything's produced under a certified system covering ISO 9001, 14001, 45001, and PED.
What is the lead time and minimum order?
Standard items ship straight from stock. Specials, though, are made to order, and delivery runs six to eight weeks. Small minimum order quantities aren't a problem, and private-label options are available as well.
Are these plumbing compression fittings?
No. These are instrumentation-grade metal fittings built for high-pressure fluid and gas systems. They're not the copper or plastic compression fittings you'd find in a plumbing job.
Related Tube-OD Fitting Pages and Guides
- All end types: the tube fittings hub covers everything, with dedicated pages for NPT ends, metric, and imperial-fractional sizing.
- Sizing: start with the tube fitting size chart, then check tube sizing and wall thickness if you're unsure about tolerances. Comparing standards? The metric vs imperial guide lays it out plainly.
- Ferrules: not all ferrules behave the same. Single vs double ferrule walks through the basic distinction, hardened vs non-hardened ferrules gets into material choice, and ferrule installation and gauging handles the actual fitting work.
- Interchange: before swapping parts across manufacturers, run them through Crestflo's cross-reference desk. Pair that with mixing tube fitting brands for the caveats worth knowing first.