1/8" Tube Fittings: Instrumentation Compression Fittings for 1/8 in. OD Tubing (316 SS & High-Nickel Alloy)

A 1/8" tube fitting is a mechanical-grip, or compression, fitting that terminates rigid tubing with a 1/8-inch (0.125") outside diameter. Advancing a nut over one or two ferrules grips the tube, forming a seal that's leak-tight and gaugeable. Crestflo manufactures the full 1/8" range, covering unions, connectors, elbows, tees, and more. Materials span 316/316L stainless through high-nickel and corrosion-resistant alloys, offered with single or double ferrules. Every piece is an ASTM F1387-qualified dimensional drop-in for the major instrumentation brands.

Standard items ship from ready stock. Specials are made to order, with delivery running 6-8 weeks. Reach out to our engineering team to request a quote or request a sample.

Wherever this document says 1/8" tube fitting, it means an instrumentation-grade stainless-steel or alloy compression fitting sized for 1/8-inch OD rigid tubing. It doesn't mean a brass or copper plumbing compression fitting. It's not a push-to-connect pneumatic fitting either, and it isn't an automotive AN or inverted-flare fitting.

What a 1/8" Compression Tube Fitting Is (Function & Scope)

A 1/8 in. Compression tube fitting seals through mechanical grip alone. Threading the nut onto the body drives the ferrule (or a twin-ferrule set) radially onto the 1/8 in. OD tube, colletting it firmly against the fitting's shoulder. The result: a metal-to-metal seal that holds pressure, won't loosen under vibration, and remakes reliably.

Since the fitting grips the tube's outside diameter, the "1/8 inch" designation always points to tube OD. It never refers to the bore or an ID.

Both single-ferrule and double-ferrule (twin-ferrule) designs are available. For the trade-offs, check single vs. Double ferrule; for parts, see the single-ferrule and ferrules pages. If a ferrule set isn't wanted, the ferrule-less gaugeable design offers an alternative. For the metric small-size equivalent, look to 1/8" DIN 2353 bite-type fittings.

Also Specified As: 1/8 Inch, 0.125", Eighth-Inch & 1/8" OD Tube Fitting

People shopping for this part use a range of names: a 1/8 inch tube fitting, a 1/8" compression tube fitting, a 1/8" double-ferrule fitting, a 1/8" instrumentation fitting, or a 1/8" stainless steel tube fitting. You'll also see it written backwards or in decimal form, as in "tube fitting 1/8 inch," "0.125 inch fitting," "1/8 od tube fitting," or "eighth-inch tube fitting." Whatever the label, it's the same imperial-fractional fitting, built to grip 1/8-inch OD rigid tube.

1/8" Tube Fitting Dimensions: 0.125" OD & ~3.18 mm (Not 3 mm)

Tube OD is what actually controls the fit. A 1/8" fitting takes nominal 0.125-inch OD tubing, roughly 3.18 mm, but that's an imperial-fractional size. It won't interchange with true 3 mm metric tube or fittings. Crestflo carries genuine metric sizes, sold separately: 6 mm, 8 mm, 10 mm, 12 mm. Check the tube fitting size chart for the full cross-size reference.

Nut hex sizes, assembly gaps, and gauging data all follow the applicable standard, with dimensions and pressure values open to customization as needed. Want the 1/8" dimensional guide? Just request it from our engineering team.

1/8" Fitting Configurations: Union, Connector, Elbow, Tee, Cross & Reducer

Every shape in the tube-fittings family comes in 1/8" OD. Pick the configuration you need below; each one links to its own page, where you'll find end options and ordering details.

The 1/8 × 1/4 reducing union and the 1/8 tube × 1/4 NPT connector count among the more common small-bore transitions. You'll find both covered on the reducer and connector pages.

1/8" Thread & End Connections: NPT, BSPP, BSPT, JIC 37° & SAE-ORB

1/8" fittings aren't limited to tube-to-tube connections. They also come with threaded and welded ends, built to match whatever port they're mating with. One connector shows up in requests more than most: the 1/8" tube × 1/8" MNPT male connector.

Thread dimensions match the applicable standard, but you can customize them to what you need. For details, check the thread identification guide.

1/8" Fitting Materials: 316/316L Stainless, Duplex, Monel, Inconel & Hastelloy

Stainless 316/316L is the default alloy. Push into corrosion, high temperature, or sour service, and you'll need to move up the alloy ladder. Crestflo doesn't offer 1/8" instrumentation fittings in brass.

Alloy compositions follow the applicable ASTM/UNS specifications. Browse all instrumentation fitting materials, or compare grades directly using 316 vs 316L and material selection by service.

1/8" Ratings, Testing & Traceability: ASTM F1387, MTC 3.1 & NACE

Crestflo 1/8" compression tube fittings meet ASTM F1387, the standard covering mechanical performance for instrumentation gaugeable tube fittings. Wall thickness and alloy set the working-pressure rating, following standard tube-pressure tables under ASME B31.3, and these figures can be adjusted to fit your specific requirement. Run the numbers with the tube pressure calculator to model your case. Sour service isn't a problem either: fittings come in NACE MR0175 / ISO 15156-compliant alloys, paired with the right ferrule.

Every part's heat- and lot-number marked for full traceability, and it ships with EN 10204 MTC 3.1 as standard, 3.2 on request. Testing covers hydraulic proof, burst and impulse, pneumatic, flexure and rotary fatigue, vibration, thermal cycling, pull-out and vacuum, with PMI and in-house NDT built in. Destructive testing is carried out at ISO/IEC 17025-accredited labs. Third-party inspection is available on request. Crestflo holds ISO 9001, ISO 14001, ISO 45001 and PED certification, and the certificates are kept in quality.

Applications for 1/8" Tube Fittings: Analytical, Instrument Air & Semiconductor

A 1/8" bore is built for low-volume work: signal lines, sample lines, pilot lines. Think analytical and sampling systems and gas chromatography, instrument air and pilot lines, hydraulic sensing, semiconductor process gas, and hydrogen service. Pick the alloy and ferrule based on what's actually flowing through the line, and check the port thread against the instrument before you commit.

Why Crestflo for 1/8" Tube Fittings

Crestflo is the instrumentation division of a four-decade-old export house recognized by the Government of India, with a proven track record delivering across the Middle East. It's engineering depth put behind a new, focused instrumentation brand. Our 1/8" fittings drop straight in dimensionally for Swagelok®, Parker®, Hoke®, Hy-Lok® and Fitok® 1/8" fittings. That's backed by ASTM F1387 performance qualification, not a look-alike claim. Standard items ship from ready stock. Specials are made to order on a 6-8 week schedule, and they're cost-competitive even after US duties, with small minimum order quantities and a private-label program available. We offer both imperial and metric sizes, hardened or non-hardened ferrules, and finishes such as electropolish, passivation and silver anti-galling plating. Standard product comes from India; US or European melt is available on request, with full material traceability. We Engineer Confidence. Need to match a competitor part? Request your Swagelok/Parker cross-reference from our engineering team, no interchange table required.

Swagelok® belongs to Swagelok Company as a registered trademark. Parker® is owned by Parker Hannifin Corporation, Hoke® by Circor/Hoke, Inc., Hy-Lok® by Hy-Lok Corporation, and Fitok® by FITOK Group. Crestflo isn't affiliated with any of these companies, doesn't endorse them, and isn't sponsored by them. Every reference here exists solely to identify dimensional interchange, nothing more.

Order 1/8" Tube Fittings: Request a Quote

Whether it's standard or made-to-order, every 1/8" fitting starts with an RFQ. Give us your tube OD and wall, configuration, end thread, alloy, and quantity, and our engineering team will send back an ordering-code guide along with a lead time. Distributors and OEMs can also ask about the private-label program and line card. Datasheets and CAD files are available on request.

Contact: sales@crestflousa.com · +1 346 594 9005 · Houston, TX. Request a quote · request a sample · contact us.

Reviewed by the Crestflo Engineering Team

Frequently Asked Questions: 1/8" Tube Fittings

Related 1/8" Tube Fitting Resources

Step up to 1/4" tube fittings or 3/8", review the full tube fitting size chart, compare metric vs. imperial tube fittings, or return to the tube fittings family hub. For the tube itself, see instrumentation tubing.