O-Ring Face Seal Fittings (VCO-Type Interchange): Stainless & High-Nickel Alloy
An O-ring face seal fitting is a gaugeable instrumentation connection. Its seal is an elastomer O-ring, held in a machined face groove and squeezed against a flat mating face once the nut is torqued down to a metal-to-metal stop. The result is a leak-tight joint that can be opened and remade in service. Crestflo builds O-ring face seal fittings that are dimensionally interchangeable with the VCO-type pattern and qualified to ASTM F1387, in 316/316L and high-nickel alloys, in both imperial and metric sizes, with a full range of O-ring compounds and end connections. On this page, "O-ring face seal fitting" refers specifically to an instrumentation and process fitting: stainless or alloy, gaugeable, sealed with an elastomer face seal. It doesn't mean the automotive or hydraulic SAE J1453 ORFS flat-face fitting, and it isn't a static equipment face gasket either.
Also Specified As: VCO Fittings, O-Ring Face Seal Couplings & Elastomer Face Seals
On instrument-panel drawings, sampling-system specs and purchase orders the same connection is written many ways. Engineers and buyers commonly specify it as an O-ring face seal fitting, a VCO fitting or VCO O-ring face seal fitting, an O-ring face seal coupling, an elastomer face seal fitting, a gaugeable O-ring face seal, or an O-ring face seal tube fitting. By configuration it appears as a VCO union, male connector, female connector, elbow, tee, cross, cap, plug or reducing union; by component as a VCO gland, body or nut, and as a replacement O-ring. All of these designate the one entity covered here: the gaugeable, elastomer-sealed face-seal fitting whose metal-gasket sibling is the VCR metal-gasket face seal fitting.
How an O-Ring Face Seal (VCO) Fitting Seals: Gaugeable, Remakeable Elastomer Joint
A polymer O-ring sits in a groove machined into the fitting face, and the seal forms when that ring gets compressed against a flat sealing surface as the body and nut draw together to a solid metal-to-metal stop. Make-up ends on that positive stop, which is what makes the joint gaugeable. The assembler tightens finger-tight, then wrenches to the stop for a repeatable, inspectable seal every time. Threads do a different job entirely: they just clamp the two halves together. No sealant, no role in sealing. Because the O-ring is a discrete, field-replaceable part, the connection can be broken and remade over and over. Renewing the seal is as simple as fitting a fresh O-ring. That's exactly why these fittings show up on lines that get opened, cleaned, or reconfigured often. The seal itself is an elastomer, not a metal gasket, so service temperature and pressure depend on the O-ring compound. The body alloy, meanwhile, sets the limits for corrosion resistance and mechanical strength.
O-Ring Face Seal vs Hydraulic ORFS vs Face Gasket: Disambiguation
Because "face seal," "O-ring" and "ORFS" get used across several fluid domains, it's worth separating the instrumentation VCO-type connection from the hydraulic O-Ring Face Seal fitting, and from a static equipment gasket. They seal on different surfaces. They follow different standards. And each serves its own domain.
| Connection | Seal | Standard | Domain | In Crestflo scope? |
|---|---|---|---|---|
| VCO-type O-ring face seal | Elastomer O-ring on a flat gland face | Industry VCO-type geometry | Instrumentation / process | Yes |
| Hydraulic ORFS (O-Lok) | O-ring in a flat face, UNF nut | SAE J1453 / ISO 8434-3 | Hydraulic / high-pressure hose | No, hydraulic, not listed |
| Static face gasket | Sheet or ring gasket between flanges | Equipment-specific | Static equipment sealing | No, not a fitting |
Crestflo's O-ring face seal line falls under the instrumentation VCO-type family. That's distinct from the hydraulic ORFS/O-Lok connection to SAE J1453, which belongs to a separate high-pressure hose family and isn't covered here.
O-Ring Face Seal Fitting Anatomy: Body, Gland, Nut & Replaceable O-Ring
Three parts make up an O-ring face seal fitting: a body, or gland, machined with the face groove; a nut that pulls the joint down to its stop; and the O-ring itself, made of elastomer. The O-ring gets renewed at service. Glands come with a range of end connections, though, which means an entire sampling or analytical loop can be built from a single matched alloy family, all under common heat traceability.
VCO Fitting Configurations: Unions, Connectors, Elbows, Tees, Crosses, Caps & Plugs
The O-ring face seal pattern comes standard across the whole fitting lineup, so you can build an entire run using one seal type and one alloy. Which configurations and O-ring compounds are kept in stock versus made to order depends on the build, and that gets confirmed at quotation.
| Configuration | Description | Typical use |
|---|---|---|
| Union | Straight O-ring face seal joint, two like ends | Inline connection of two tube runs |
| Reducing union | Union stepping between two sizes | Size transition in a face-seal run |
| Male connector | Face-seal end to male pipe thread | Face seal to threaded port |
| Female connector | Face-seal end to female pipe thread | Face seal to threaded instrument |
| Union elbow | 90° change of direction, two face-seal ends | Routing around obstructions |
| Male elbow | 90° face seal to male thread | Cornered thread take-off |
| Union tee | Three-way branch, face-seal ends | Branch / instrument take-off |
| Union cross | Four-way junction, face-seal ends | Manifolding of sample lines |
| Cap | Closes a male face-seal end | Sealing an unused port |
| Plug | Closes a female face-seal end | Sealing an unused body |
| Weld gland | Face-seal gland with tube/pipe weld end | Welded system tie-in |
| Replacement O-ring | Field-replaceable seal element | Servicing an existing joint |
VCO End Connections & Threads: NPT, Tube, Weld & Metric
O-ring face seal fittings don't stop at the face-seal ends. They come with whatever process connections a system actually needs: male and female NPT to ASME B1.20.1, BSPT and BSPP when called for, and tube-OD ends for tube-weld or tube-stub tie-ins. That includes the male tube weld connector, or MTW-style, gland end. Socket-weld and butt-weld gland ends round out the lineup. All of it's available in imperial and metric.
O-Ring Face Seal Fitting Sizes & Dimensions (1/8″–1″) With Metric Equivalents
O-ring face seal fittings come in the usual instrumentation sizes, with metric tube equivalents available alongside the imperial range. When ordering, specify the nominal size, end connection, body alloy, and O-ring compound.
| Nominal size (in) | Tube OD (in) | Metric equivalent (mm) | Typical ends |
|---|---|---|---|
| 1/8 | 0.125 | 3.18 | Weld, tube-stub, NPT |
| 1/4 | 0.250 | 6.35 | Weld, tube-stub, NPT |
| 3/8 | 0.375 | 9.53 | Weld, tube-stub, NPT |
| 1/2 | 0.500 | 12.70 | Weld, tube-stub, NPT |
| 3/4 | 0.750 | 19.05 | Weld, tube-stub, NPT |
| 1 | 1.000 | 25.40 | Weld, tube-stub, NPT |
Tube-OD values follow standard tube tables, customizable to whatever the job needs. Gland series identifiers and the full ordering nomenclature? Proprietary to each build. For the ordering-code guide and size chart, reach out to Crestflo's engineering team through the RFQ form.
VCO O-Ring Materials & Temperature: Viton (FKM), Buna-N, EPDM, Silicone & Kalrez (FFKM)
An O-ring face seal fitting uses a discrete elastomer O-ring as its sealing element. That's why the seal material gets picked for the media and the temperature, while the body alloy is chosen separately, for corrosion resistance. The alloy can handle far wider temperature swings than the application will ever ask of it. It's the O-ring compound that actually sets the service temperature limit, not the metal. For oxygen service and anything bordering on cryogenic, oxygen-clean O-rings and low-temperature compounds are available.
| O-ring compound | Typical temperature range | Typical media / notes |
|---|---|---|
| Buna-N (NBR) | approx. −35 °C to 120 °C | General air, water, hydraulic and light hydrocarbon |
| FKM (Viton) | approx. −20 °C to 200 °C | Broad chemical and hydrocarbon, higher temperature |
| EPDM | approx. −50 °C to 150 °C | Steam and polar-media service |
| Silicone | approx. −55 °C to 200 °C | Wide-temperature, low-reactivity service |
| FFKM (Kalrez) | up to approx. 260 °C | Aggressive chemistries and high temperature |
Temperature limits follow the applicable seal-material standards, but they can be adjusted to fit specific requirements. Compound selection is laid out in full in the seal-material temperature guide. Every size has a replacement O-ring on hand in the specified compound, so an existing joint can be re-sealed right in the field.
O-Ring Face Seal Pressure & Temperature Ratings
An O-ring face seal handles moderate pressure and vacuum duty. The pressure it can actually take depends on size, O-ring compound, and body grade, while temperature limits come from the O-ring itself, as shown in the table above. Ratings follow the applicable standard, and they can be tailored to what's needed. When service pressure or temperature pushes past what the elastomer can handle, step up to a metal seal. Crestflo's VCR metal-gasket face seal fitting covers ultra-high-purity, vacuum, and high-temperature gas service. For high-pressure work, use cone-and-thread high-pressure fittings instead.
VCO (O-Ring) vs VCR (Metal Gasket) vs Compression: Which Face Seal to Choose
It really comes down to a few things: how often you'll open the joint, the purity and vacuum level you need, and the temperature and pressure involved. If you're servicing the connection often and don't need ultra-high purity, an O-ring face seal is the cheaper, practical option. Need the highest purity and vacuum performance instead? The metal-gasket VCR-type is the standard for that. A compression fitting works on a different principle: it seals against the tube's outer diameter, not against a flat face.
| Attribute | O-ring face seal (VCO) | Metal-gasket (VCR) | Compression (double-ferrule) |
|---|---|---|---|
| Seal element | Elastomer O-ring | Deformable metal gasket | Ferrules gripping tube OD |
| Remakeable | Yes, renew O-ring | Yes, new gasket each make-up | Reconnectable, ferrules stay set |
| Temperature ceiling | Set by O-ring compound | High, metal seal | Set by tube and packing |
| Pressure | Moderate | Higher | Low to high by series |
| Cleanliness / vacuum | Medium-purity | Ultra-high-purity, low outgassing | General service |
| Best use | Frequently-opened medium-purity lines | UHP gas, vacuum, high-temperature | General tube connection |
Route by service: for UHP and vacuum choose the VCR metal-gasket face seal; for general tube connection choose compression tube fittings.
O-Ring Face Seal Fitting Materials: 316L Stainless, Inconel, Hastelloy, Monel & Duplex
316/316L stainless steel serves as the default body alloy. For aggressive or corrosive media, high-nickel and other corrosion-resistant alloys step in. Each alloy is offered across the entire O-ring face seal line, and every one links to its own material page, where you'll find composition and property data.
| Family | Representative grades | When to choose |
|---|---|---|
| Stainless | 316/316L, 904L | General process and medium-purity service |
| Duplex / super-austenitic | Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN | Chloride and aggressive-media exposure |
| Nickel alloys | Inconel 625, Inconel 825, Monel 400, Hastelloy C-276 | Corrosive gas, acidic and high-temperature media |
| Titanium | Titanium Gr2 | Light weight, high corrosion resistance |
Crestflo supplies O-ring face seal fittings in stainless steel and corrosion-resistant alloys only. Carbon steel, brass and aluminum are off the table. Origin is Indian, though US or European melt can be arranged on request, and full material traceability comes standard.
Standards, Certifications, Testing & Traceability
Crestflo holds ISO 9001, 14001, and 45001 certification, along with PED, and carries the standard approvals for instrumentation fittings, including ASTM F1387 performance qualification, the anchor for interchangeability. Bar and tube raw material comes from the applicable ASTM specifications. Sour-service grades are also available, meeting NACE MR0175 / ISO 15156 requirements. Testing is extensive: hydraulic proof, burst and impulse, pneumatic, vibration, thermal cycling, pull-out, and vacuum, plus positive material identification (PMI). In-house NDT is standard practice; destructive testing takes place at ISO/IEC 17025-accredited labs, and third-party inspection can be arranged on request. Every part carries a heat or lot number, so material traceability isn't ever in question. EN 10204 MTC 3.1 ships as standard, with 3.2 available on request. Datasheets and CAD drawings go out to customers on request.
O-Ring Face Seal Fitting Applications: Analytical, Semiconductor, Hydrogen & Instrument Air
Plants reach for the remakeable, low-particle elastomer face seal wherever a joint gets opened and reconfigured often. That covers analytical and sampling systems, semiconductor support and medium-purity gas lines, hydrogen and specialty-gas systems, and instrument air and general process work, anywhere a serviceable seal is the better fit. For higher purity or vacuum, pair it with the metal-gasket VCR-type on the same panel.
VCO Interchange & Swagelok Alternative
Crestflo's O-ring face seal fittings are dimensional equivalents, built to interchange with existing VCO-type fittings on the standard face-seal geometry, and that claim rests on ASTM F1387 performance qualification. VCO® is a registered trademark of Swagelok Company. O-Lok® belongs to Parker Hannifin as a trademark. Crestflo isn't affiliated with, endorsed by, or sponsored by either company, and we describe our parts strictly as interchange or equivalent, never as genuine or compatible product. Need to confirm interchange for your incumbent part numbers? Send your Swagelok® VCO catalog part list to our engineering team at the cross-reference desk, or run it yourself through the interchange lookup.
Why Crestflo for O-Ring Face Seal Fittings
Crestflo manufactures and supplies O-ring face seal fittings to the US market. It's the instrumentation division of an export house that's been running for four decades, recognized by the Government of India, with a delivery track record across the Middle East, and now it brings drop-in instrumentation fittings to American buyers. These fittings come as engineered drop-in replacements built to ASTM F1387, offered in both imperial and metric sizes, with an alloy range that runs deeper than what most commodity sellers reach. Every part ships with a full range of O-ring compounds and a heat-number MTC. Standard items sit in ready stock. Specials are made to order, with delivery running 6-8 weeks. Small minimum order quantities and private-label programs make it practical to standardize a sampling or analytical system without waiting on a major supplier's lead time. We Engineer Confidence.
Request a Quote for O-Ring Face Seal Fittings
Send us your sizes, configuration, body alloy, O-ring compound and end connection, and we'll get you a fast quote. You can also order a sample, or set up a distributor, OEM or private-label program. Request a quote · request a sample · private-label program · become a distributor. Reach US sales at sales@crestflousa.com or +1 346 594 9005; we're based in Houston, TX. Need to confirm brand interchange? Request your Swagelok® VCO cross-reference from the cross-reference desk.
Frequently Asked Questions: O-Ring Face Seal (VCO) Fittings
What are VCO fittings?
VCO fittings fall into the O-ring face seal category: gaugeable instrumentation fittings that seal by compressing a replaceable elastomer O-ring into a machined face groove, pressed against a flat face and torqued down to a metal-to-metal stop. "VCO®" is Swagelok Company's registered trademark for the pattern, though it's Crestflo that manufactures dimensionally interchangeable O-ring face seal fittings in stainless steel and high-nickel alloys.
What is an O-ring face seal?
An O-ring face seal is a face-seal joint that uses a polymer O-ring, not a metal gasket, as the sealing element. It sits in a groove and gets squeezed between two flat faces as the nut is drawn down to its stop. The elastomer seal isn't permanent. It can be serviced and remade easily, and its temperature and pressure limits depend entirely on the O-ring compound.
How is a VCO fitting different from a VCR fitting?
A VCO-type fitting relies on a replaceable elastomer O-ring. That keeps it cheap and makes it a good fit for medium-purity lines you open often, though temperature and pressure limits depend on the compound used. A VCR-type fitting instead uses a deformable metal gasket to form a metal-to-metal seal, which suits ultra-high-purity, vacuum and high-temperature service.
Is there a size chart for O-ring face seals?
O-ring face seal fittings come in both imperial and metric, typically 1/8″ to 1″ with matching metric tube sizes, as laid out in the size table above. Order by nominal size, end connection, body alloy and O-ring compound. For the full size chart and ordering-code guide, ask the engineering team.
How do I measure or order an O-ring face seal fitting?
Give the nominal size, the end connection, the body alloy and the O-ring compound, then confirm the make-up gauges out to the metal-to-metal stop. Send Crestflo your VCO part number or drawing, and you'll get back an interchange quote matched to that size and configuration.
What O-ring materials are available and what temperature do they take?
Several elastomers are available: Buna-N (NBR), FKM (Viton), EPDM, silicone and FFKM (Kalrez), each offered in oxygen-clean and low-temperature versions. It's the compound that sets the temperature range, not a single fixed limit. Low-temperature grades reach down to about −55 °C. FFKM, at the other end, pushes as high as 260 °C. Both figures follow applicable seal-material standards, and they can be tailored to whatever the job requires.
Do your O-ring face seal fittings interchange with Swagelok VCO?
They are dimensional equivalents built to interchange with the VCO-type pattern, and performance is qualified under ASTM F1387. VCO® is a registered trademark of Swagelok Company; Crestflo isn't affiliated with or endorsed by it. Request your cross-reference from the cross-reference desk to confirm your incumbent part numbers.
Are O-ring face seal fittings NACE compliant for sour service?
Yes. O-ring face seal fittings are available in NACE MR0175 / ISO 15156-compliant alloys for sour-service applications, with the O-ring compound matched to the media itself. Just note the sour-service requirement on the RFQ, and you'll receive the correct alloy and compound.
What certifications and traceability come with an order?
Crestflo holds ISO 9001, 14001, and 45001 certification, along with PED, and carries the standard approvals for instrumentation fittings, including ASTM F1387 interchange qualification. Every part is marked with a heat or lot number. EN 10204 MTC 3.1 comes as standard, and 3.2 is available on request. PMI and in-house NDT round out the testing.
What is the lead time and minimum order?
Standard items ship from stock. Specials are made to order, with delivery running six to eight weeks. MOQs stay small, and private-label options are available. Send your sizes, configurations, alloy, and O-ring compound to the RFQ desk, and we'll get you a quote.
Where are Crestflo O-ring face seal fittings made?
Crestflo O-ring face seal fittings come from India, made by the instrumentation division of an export house that's been recognized by the Government of India for four decades, and shipped on to the US market. Need US or European melt instead? That's available on request, with full material traceability for cases where domestic-melt sourcing is a requirement.