Imperial (Fractional-Inch) Tube Fittings: Stainless & High-Alloy Instrumentation
Imperial tube fittings are instrumentation compression tube fittings sized by fractional-inch tube outside diameter (OD), 1/16", 1/4", 1/2" and so on, rather than by millimeter. One or two ferrules grip the tube OD to form a leak-tight, gas-tight seal, no brazing or flaring required. Crestflo makes imperial (fractional-inch) instrumentation tube fittings from 1/16" up to 2" (50 mm) OD in 316/316L and exotic alloys, both single and double ferrule. They're performance-qualified to ASTM F1387 as a dimensional drop-in for the major inch-based twin-ferrule systems. Heat-number traceable, and built to serve the US market.
"Imperial" here refers only to the fractional-inch tube-OD sizing convention used for instrumentation fittings. It has nothing to do with the Imperial Eastman brand, and it doesn't mean copper-plumbing or pneumatic push-in fittings.
Request a quote · Standard items ship from stock · specials are made to order · delivery runs 6-8 weeks · Crestflo.
Also Known As: Fractional, Inch-Size, ANSI/US-Standard & Inch-OD Tube Fittings
Specify the same part across drawings, purchase orders and RFQs, and you'll find it called a dozen different things. One buyer writes fractional tube fitting; another puts fractional-inch tube fitting, inch tube fitting or inch-size tube fitting. Somebody else prefers imperial compression fitting or fractional compression fitting. The end style shapes the label too: imperial double-ferrule or single-ferrule fitting, fractional ferrule fitting, inch-OD or fractional-OD tube fitting. Tender language leans on ANSI / US-standard tube fitting, and interchange notes often say imperial Swagelok-compatible tube fitting (technically that's an interchange, or an equivalent, as explained below). Every one of these names points to the same thing: an instrumentation compression fitting sized to a fractional-inch tube OD.
Imperial Tube Fitting Sizes (1/16″–2″): Fractional OD, Decimal & Metric Equivalents
Crestflo fittings are sized to the tube's outside diameter, or OD, not the inside diameter, and not a nominal pipe bore. The company stocks the full fractional ladder, from 1/16" right up to a 2" (50.8 mm) OD cap. A fractional-inch OD almost never lines up with a whole millimeter: a 1/2" fitting measures 12.70 mm, not 12 mm. So imperial and metric stay separate systems, and you need to pick the fitting that matches your tube OD exactly. The OD figures given here are the standard published tube-fitting dimensions. Fittings can be customized to requirement, sizes such as 70 mm, 12 mm, and 8 mm are made to order as needed.
| Fractional OD | Decimal inch | Metric equivalent (mm) | Per-size page |
|---|---|---|---|
| 1/16" | 0.0625 | 1.59 | — |
| 1/8" | 0.125 | 3.18 | 1/8 inch tube fittings |
| 3/16" | 0.1875 | 4.76 | — |
| 1/4" | 0.250 | 6.35 | 1/4 inch tube fittings |
| 5/16" | 0.3125 | 7.94 | — |
| 3/8" | 0.375 | 9.53 | 3/8 inch tube fittings |
| 1/2" | 0.500 | 12.70 | 1/2 inch tube fittings |
| 5/8" | 0.625 | 15.88 | 5/8 inch tube fittings |
| 3/4" | 0.750 | 19.05 | 3/4 inch tube fittings |
| 7/8" | 0.875 | 22.23 | — |
| 1" | 1.000 | 25.40 | 1 inch tube fittings |
| 1-1/4" | 1.250 | 31.75 | — |
| 1-1/2" | 1.500 | 38.10 | — |
| 2" | 2.000 | 50.80 | — |
For the complete conversion asset, see the full tube-fitting size chart (actual OD). If your tubing runs metric, switch to metric (mm-OD) tube fittings instead. Sizing always follows tube OD, without exception.
Imperial Ferrule & End Connections: Single, Double Ferrule, NPT & Tube-OD Ends
Ferrules do the real work here, not thread torque. That's where the seal and the grip actually come from. Crestflo makes its imperial fittings in both single and double (twin) ferrule styles, offering hardened or non-hardened ferrules depending on what the job calls for. Standalone ferrules are available too, along with a single-ferrule option. Ends come as tube-OD (fitting) or threaded, and that includes NPT to ASME B1.20.1, plus a range of other national, international and custom threads.
| End type | Governing standard | Detail page |
|---|---|---|
| Tube-OD (fitting) end | Sized to tube OD | tube-OD fittings |
| NPT | ASME B1.20.1 | NPT tube fittings |
| NPTF (dryseal) | ASME B1.20.3 | NPTF |
| BSPT / BSPP | ISO 7 / ISO 228 | BSPT · BSPP |
| JIC 37° flare | ISO 8434-2 / SAE J514 | JIC 37° |
| SAE-ORB | SAE J1926 | SAE-ORB |
Not sure which thread you're dealing with? Check the thread identifier or work through the thread identification guide. Thread dimensions follow the applicable standard, and Crestflo can customize them to your requirement.
Imperial Tube Fitting Types: Unions, Elbows, Tees, Connectors & Bulkheads
Every common configuration comes in fractional-inch sizes. Each one is engineered and supplied on its own dedicated line.
| Configuration | Description | Detail page |
|---|---|---|
| Union / reducing union | Straight tube-to-tube join; steps one fractional size to another | union |
| Male / female connector | Tube-to-threaded-pipe transition | male connector · female connector |
| Elbow (45° / 90°) | Union, male or female directional change | elbow |
| Tee / cross | Three- or four-way junction | tee · cross |
| Reducer / adapter | Size or end-type change | reducer · adapter |
| Bulkhead union / port connector | Panel or wall pass-through | bulkhead union · port connector |
| Cap / plug | Tube-end and port closure | cap · plug |
Imperial Tube Fitting Materials: 316/316L, 6-Moly, Duplex, Inconel & Monel
Crestflo covers imperial fittings across the entire stainless and corrosion-resistant-alloy range. No brass. Standard product comes from India, though US or European melt is available on request, and full material traceability is provided either way. Bar and forging stock meet ASTM A479 / A276. The fitting comes in instrumentation form only.
| Alloy family | Grades | Grade page |
|---|---|---|
| Austenitic stainless | 304/304L, 316/316L, 904L | 316/316L stainless · 304/304L · 904L |
| Duplex / super-austenitic | Duplex 2205, Super Duplex 2507, 254 SMO (6-Moly), AL-6XN, Alloy 20 | duplex 2205 · super duplex 2507 · 254 SMO |
| Nickel alloys | Inconel 625/825, Monel 400, Hastelloy C-276 | Inconel 625 · Monel 400 · Hastelloy C-276 |
| Titanium | Titanium Gr2 | titanium Gr2 |
Alloy compositions follow the applicable ASTM/UNS specification. They're customizable to requirement. For sour service, Crestflo supplies imperial fittings that meet NACE MR0175 / ISO 15156 in qualifying alloys.
Imperial vs Metric Tube Fittings: Why You Cannot Mix Inch and Millimeter
Imperial fittings are sized by fractional-inch OD, while metric fittings are sized by millimeter OD. These are two separate systems. A 1/2" fitting (12.70 mm) isn't the same as a 12 mm fitting, and the ferrule swage geometries don't match either. Pick one convention per tubing run. Never mix inch and metric ferrules, nuts or bodies.
| Imperial (fractional) | Metric | |
|---|---|---|
| Sized by | Fractional-inch tube OD | Millimeter tube OD |
| Example | 1/2" = 12.70 mm | 12 mm = 0.472" |
| Ferrule interchangeable with the other? | No | No |
| Choose when | Existing inch-based (US) system | Existing millimeter-based system |
For the full breakdown, read imperial vs metric tube fittings, or make the switch to Crestflo's metric tube fittings line.
Imperial Swagelok, Parker & Hy-Lok Cross-Reference (Drop-In Interchange)
Crestflo imperial twin-ferrule fittings match the dimensions of the major inch-based twin-ferrule systems. They're backed by ASTM F1387 performance qualification, the kind of interchange proof most competitors never publish.
Instead of printing a static part-number table, our engineering team puts together your exact interchange list against your own part numbers. Request a Swagelok cross-reference, a Parker A-Lok cross-reference, or a Hy-Lok cross-reference from the master interchange desk. (Swagelok® is a trademark of Swagelok Company and Parker® of Parker Hannifin; Crestflo isn't affiliated with or endorsed by them.)
Standards, Certifications & Traceability
Crestflo holds ISO 9001, 14001, and 45001 certification, along with PED. It also carries the standard approvals required for instrumentation tube fittings.
| Item | Standard / reference | Scope |
|---|---|---|
| Fitting performance qualification | ASTM F1387 | Dimensional interchange + performance |
| Bar / forging stock | ASTM A479 / A276 | Fitting body material |
| NPT thread | ASME B1.20.1 | Threaded ends |
| Sour-service compliance | NACE MR0175 / ISO 15156 | Qualifying alloys |
| Material test certificate | EN 10204 3.1 (3.2 on request) | Every shipment |
| Traceability | Heat / lot number on every part | Full lot linkage |
| Verification | PMI, in-house NDT | Confirms alloy + integrity |
Full details appear in the ASTM F1387 performance qualification and the EN 10204 3.1 material test certificate (MTC). Crestflo keeps these certificates on file in our quality documentation. Datasheets and CAD models go to customers on request.
Order Imperial Tube Fittings: RFQ, Sample & Private Label
Crestflo supplies and manufactures imperial tube fittings for customers across the US. Standard items ship from stock; specials are made to order, with delivery running 6-8 weeks. Distributors can request the imperial tube fittings catalog or line card. OEMs looking into private-label work can ask about the private-label program. End users have two options: request a quote or request a sample. For a specification discussion, reach the engineering team directly here, by emailing sales@crestflousa.com, or by calling +1 346 594 9005 in Houston, TX.
Why Crestflo for Imperial Instrumentation Fittings
Crestflo is the instrumentation arm of an export house that's been in business for four decades and carries Government of India recognition, with a delivery record built across the Middle East. That background is what makes a 6-8 week lead time possible, against the 20-plus weeks typical of the major suppliers. It also stands behind ASTM F1387 drop-in proof, small MOQs, private-label support, in-house CNC, VMC and EDM machining, heat-number traceability backed by EN 10204 3.1 MTC, and prices that hold up even after US duties. Every fitting ships in both imperial and metric sizes. Origin is Indian, though US or European melt can be arranged on request, with full material traceability throughout. We Engineer Confidence.
Imperial Tube Fitting Applications: Instrumentation, Hydraulic Power, Instrument Air & Sampling
Imperial tube fittings connect inch-sized instrumentation tube runs wherever process, pilot or sample fluid is carried, instrument and transmitter hook-ups, analytical sampling systems, instrument air distribution, and instrument hydraulic-power circuits, where they serve as imperial hydraulic tube fittings. Here "hydraulic" means instrumentation hydraulic-power service, pilot lines, actuators and small-bore control circuits at instrument working pressures, not domestic plumbing, water supply or push-in fittings. For corrosive, high-pressure or sour duty the same fractional sizes move to oil and gas service in the matching alloy.
Frequently Asked Questions
What are imperial tube fittings?
Imperial tube fittings are instrumentation compression fittings sized to a fractional-inch tube OD, say 1/4" or 1/2", rather than to millimeters. One or two ferrules grip the outside of the tube to form a leak-tight, gas-tight seal. No brazing. No flaring.
What sizes do imperial tube fittings come in?
Crestflo supplies fractional OD from 1/16" up to 2" (50.8 mm), covering 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 7/8, 1, 1-1/4, 1-1/2 and 2 inch. Fittings are matched to the tube OD. For decimal and metric equivalents, check the full size chart.
What is the difference between imperial and metric tube fittings?
Imperial fittings use fractional-inch OD; metric fittings use millimeter OD. The two systems don't line up: a 1/2" fitting measures 12.70 mm, not 12 mm. Pick one convention and stick with it for a given tubing run. For a closer look at the difference, see imperial vs metric tube fittings.
Can I mix imperial and metric ferrules or bodies?
No. Don't mix inch and metric ferrules, nuts, or bodies; the swage geometries aren't the same. Pick one convention and stick with it for the whole system. Working with metric tube? Use the metric tube fittings line instead.
Do Crestflo imperial fittings drop into Swagelok or Parker systems?
Yes. Crestflo's imperial twin-ferrule fittings match up dimensionally with inch-based Swagelok® and Parker® systems, backed by ASTM F1387 performance qualification. Ask our engineering team for your part-number cross-reference; they'll get you what you need. Swagelok® is a trademark of Swagelok Company, and Parker® belongs to Parker Hannifin. Crestflo isn't affiliated with either company, nor does it carry their endorsement.
Do imperial fittings use single or double ferrules?
Both options are on offer. Crestflo supplies imperial fittings in single and double (twin) ferrule designs, and you can specify hardened or non-hardened ferrules to suit your needs. For a closer look at the difference, see single vs double ferrule.
What materials are available in imperial sizes?
316/316L is the standard choice. But the lineup runs far wider: 304/304L, 904L, duplex 2205, super duplex 2507, 254 SMO (6-Moly), AL-6XN, Alloy 20, Inconel and Incoloy grades, Monel 400, Hastelloy C-276, and Titanium Gr2. Every composition follows the applicable ASTM/UNS specification. Take a look at the full range on Crestflo's materials index.
What thread ends are available on imperial fittings?
We offer tube-OD ends as well as threaded ends: NPT (ASME B1.20.1), NPTF, BSPT, BSPP, JIC 37°, SAE-ORB, and other national, international, or custom threads. If you're not sure which one you have, check the thread identification guide.
What is the pressure rating of an imperial tube fitting?
Working pressure isn't just a function of the fitting. It depends on tube OD, wall thickness and alloy, and it's calculated per ASME B31.3. Ratings in qualifying alloys meet NACE compliance. The fitting series has been performance-qualified to ASTM F1387. For a specific figure, use the tube-pressure calculator; values follow the applicable standard and can be adjusted to your requirement.
Are imperial fittings made in the USA?
Crestflo serves the US market. Our standard product comes from India and is built as a drop-in replacement, shipped in 6-8 weeks. Need domestic melt or TAA eligibility? US or European melt is available on request. You'll find the specifics on our US origin and compliance page. We don't claim "made in USA."
What certifications and traceability come with them?
Crestflo holds ISO 9001 / 14001 / 45001 and PED certification. Each part carries its own heat and lot number and ships with an EN 10204 3.1 material test certificate, with a 3.2 available on request. That's paired with positive material identification (PMI) and in-house NDT. Destructive testing takes place at ISO/IEC 17025-accredited labs, and third-party inspection can be arranged on request.
Related Pages
- All instrumentation tube fittings
- Tube-fitting size chart (actual OD)
- Browse Metric (mm) tube fittings at Crestflo, or start with the imperial vs metric tube fittings guide if you're not sure which system your line uses.
- Need imperial sizes instead? Crestflo carries 1/4 inch tube fittings, 1/2 inch tube fittings, and 1 inch tube fittings.
- 316/316L stainless material
- ASTM F1387 interchange qualification · Swagelok cross-reference