How to choose SMC metal fittings for pneumatic tubing systems
What SMC metal fittings are used for
SMC metal fittings are used to connect pneumatic tubing to threaded ports, manifolds, valves, actuators, and other fluid-control components in industrial automation systems. In most purchasing or maintenance situations, the practical question is not simply which fitting looks right, but which SMC series matches the tube, thread, temperature, fluid, and installation environment.
The main distinction is that SMC offers more than one relevant fitting family. KQB2 is commonly described in SMC literature as a metal one-touch fitting with electroless nickel-plated brass parts. KQG2 is a stainless steel 316 one-touch fitting intended for more demanding corrosion-resistance and cleanliness requirements. Standard KQ2 fittings are also widely used in pneumatic tubing systems, but they should not be treated as the same as a dedicated all-metal or stainless steel series.
This guide compares these options without assuming that one series is always better than another. The correct fitting depends on operating pressure, temperature, tubing material, thread form, chemical exposure, hygiene requirements, maintenance access, and whether the system handles air, water, steam, or another compatible fluid.
For related hardware topics, see the Metal Fittings category.
The main SMC fitting series to compare
SMC fitting names can be easy to confuse because different series may look similar in photos or supplier listings. In practice, the series code matters. It affects body material, seal arrangement, available thread standards, tube compatibility, and temperature capability. Before selecting a replacement or specifying a new fitting, confirm the full part number rather than relying on a product image or a shortened description.
| Series | Common description | Typical material emphasis | Typical selection reason |
|---|---|---|---|
| KQB2 | Metal one-touch fitting | Electroless nickel-plated brass parts | Higher temperature capability than many standard resin-body push-in fittings and good general industrial durability |
| KQG2 | Stainless steel 316 one-touch fitting | SUS316 stainless steel except seal parts | Corrosion resistance, grease-free applications, steam suitability, and cleaner environments |
| KQ2 | General one-touch fitting | Mixed resin and metal components depending on model | General pneumatic tubing where standard pressure and temperature conditions apply |
According to SMC published information, KQB2 is compact and lightweight, with electroless nickel-plated brass components as a standard specification. SMC also lists a maximum operating temperature of 150°C and tube sizes from 3.2 mm to 16 mm, or 1/8 inch to 1/2 inch, depending on the model range. SMC literature for KQG2 describes the series as stainless steel 316 except for seal parts, grease-free, suitable for steam, and compatible with tubing materials including FEP, PFA, nylon, soft nylon, polyurethane, and polyolefin. For KQ2, SMC catalog data commonly lists an operating pressure range from low vacuum to 1 MPa for many models, but its temperature and fluid limits are generally narrower than those of KQB2 or KQG2.
Material choice is the first engineering decision
The main difference between SMC metal fittings is not just the shape of the connector. It is the material. A brass fitting with electroless nickel plating and a stainless steel 316 fitting are both metal products, but they are selected for different reasons.
Nickel-plated brass for durable general use
KQB2 fittings are often chosen when the system needs the convenience of a one-touch tubing connection with a more robust metal body specification than a basic resin fitting. Electroless nickel plating can improve surface consistency and provide additional protection for brass parts in many factory environments. The KQB2 series is also relevant where temperature fluctuation is part of the duty, because SMC lists it for fluid temperatures up to 150°C.
Brass-based fittings still need to be checked against the actual fluid, cleaning chemicals, humidity, and surrounding environment. Metal construction does not automatically mean chemical compatibility. If the fitting will be exposed to aggressive fluids, washdown chemicals, saline mist, or high-purity process media, stainless steel may be the more suitable starting point.
Stainless steel 316 for corrosion and cleanliness concerns
KQG2 fittings put the emphasis on stainless steel 316 construction. SUS316 is commonly specified where corrosion resistance carries more weight than minimum purchase cost. SMC also describes KQG2 as grease-free and suitable for steam, which makes the series relevant for certain food-related, medical-equipment, analytical, and clean-process applications. SMC describes some KQG2-F and KQB2-F variants as FDA-compliant versions for food industry applications, but that point must be verified at the exact part-number level because seals and gaskets are part of the compliance question.
As a practical rule, use KQB2 when plated brass construction and the listed temperature range match the duty. Consider KQG2 when stainless steel 316, corrosion resistance, steam, grease-free assembly, or higher cleanliness expectations are central to the application.
Check tube material, tube size, and thread form together
A common selection error is choosing a fitting by tube outside diameter alone. The tube may fit into the connector, but the assembly can still be wrong if the tubing material, bend load, thread type, or seal method does not match the system.
SMC information for KQB2 and KQG2 lists compatibility with several tubing materials, including FEP, PFA, nylon, soft nylon, polyurethane, and polyolefin. This does not mean every tube grade, wall thickness, or operating condition is automatically acceptable. Tubing hardness, outside-diameter tolerance, temperature exposure, and fluid absorption can all affect sealing and pull-out resistance. For maintenance work, the safest approach is to match the original tube specification or review the tube manufacturer’s datasheet together with the fitting catalog.
Thread selection is just as important. SMC metal fitting ranges are available with several thread families, including metric threads and common pipe-thread formats such as R, Rc, G, UNF, and NPT, depending on the series and regional catalog. These thread types are not interchangeable. A fitting that appears to start into a port can still damage the thread, leak under pressure, or create a weak joint if the standard is wrong. On imported or mixed-origin equipment, confirm both the machine port and the fitting thread before replacement.
- Match tube outside diameter in millimeters or inches, not by visual estimate.
- Confirm tube material and tube temperature rating, not only the fitting temperature rating.
- Check whether the thread is tapered or parallel and whether a gasket or sealant is required.
- Use the complete SMC part number to confirm body style, thread, tube size, and seal option.
- Do not assume that a fitting suitable for air is automatically suitable for water, steam, or chemicals.
Pressure, temperature, and fluid limits must be read as a system
Fitting selection should not be based on one headline rating. A pneumatic connection is limited by the weakest element in the chain: fitting, tube, thread engagement, seal, valve port, actuator, and surrounding temperature. SMC catalog data for general KQ2 one-touch fittings commonly lists use from vacuum conditions to 1 MPa in many models. KQB2 emphasizes a fluid temperature range up to 150°C. KQG2 is positioned around stainless steel 316 construction and steam suitability. These points are useful, but they are starting checks rather than full design approval.
For example, a KQB2 fitting may be listed for a high fluid temperature, while the selected polyurethane tube may have a lower allowable temperature. A stainless steel fitting may be suitable for steam, but the connected tubing, gasket, and thread seal must also be suitable. A fitting may hold air pressure in a static installation, yet vibration, side loading, repeated tube removal, or surge pressure can reduce reliability.
Safety standards for pneumatic systems, including ISO 4414, address hazards related to pneumatic fluid power systems and their components. In practical terms, engineers should consider secure assembly, pressure release, maintenance access, energy isolation, and component suitability. When a fitting is used on machinery, selection is part of the wider safety design, not only a purchasing decision. See also: Buying Guides.
Installation details that affect leakage and service life
Even a correctly specified SMC metal fitting can leak or fail early if it is installed poorly. Push-in fittings rely on a clean tube end, correct insertion depth, and a sealing surface that is not scratched or ovalized. Threaded fittings also need proper tightening practice. Over-tightening can damage the port, while under-tightening can allow movement or leakage.
Tube preparation
Cut tubing squarely with a proper tube cutter. Avoid crushed, angled, or burred tube ends. Inspect the outside diameter near the insertion point for scratches, flattening, embedded chips, or heat damage. Push the tube fully into the fitting until it reaches the internal stop, then pull back lightly to confirm retention. If the tube has been removed several times, trim it back to fresh material before reassembly.
Thread assembly
Use the correct wrench flats and avoid applying torque through the push-in body when the catalog specifies a thread section or base for tightening. Keep sealing tape, sealant, and metal chips out of the air line. If a fitting is supplied with pre-applied sealant, do not assume that adding more sealant is helpful. Excess sealant can enter valves or restrictors and create downstream problems.
Layout and maintenance access
Leave enough straight tube length before a bend so the fitting is not continuously side-loaded. In compact manifolds, check that the release collar can be reached during maintenance. Where equipment vibrates, support the tubing so the fitting is not used as a mechanical bracket. In hot areas, verify that nearby radiant heat does not push the tube beyond its own rating.
When to consider alternatives to SMC metal fittings
SMC metal fittings are widely used in industrial pneumatic systems, but they are not the right answer for every connection. A compression fitting may be preferred where repeated tube removal is not expected and mechanical retention is the main priority. A welded or sanitary connection may be more suitable in high-purity process lines. A special chemical fitting may be required for aggressive fluids that are not covered by standard pneumatic catalogs.
Some SMC fitting listings also state that certain products are not intended for potable water systems. That distinction matters. General industry water, cooling water, process water, steam, and potable water can fall under different material, health, and regulatory requirements. If drinking-water contact is involved, do not rely on a general pneumatic fitting description.
For replacement work, identify the original fitting series and full part number first, then compare the current duty against the original machine design. If the system has changed, such as higher temperature, a new cleaning chemical, a different tubing material, or more frequent cycling, the replacement should be re-evaluated rather than copied automatically.
A practical selection checklist
- Identify the fitting function: straight connector, elbow, tee, bulkhead, reducer, plug-in connector, or manifold connection.
- Confirm the SMC series: KQB2 for metal one-touch, KQG2 for stainless steel 316, or another series if the catalog points elsewhere.
- Match tube outside diameter and unit system: metric and inch sizes should not be mixed by approximation.
- Verify tubing material: FEP, PFA, nylon, soft nylon, polyurethane, and polyolefin have different temperature and flexibility behavior.
- Confirm thread type and port standard: M, R, Rc, G, UNF, and NPT are application-specific.
- Check pressure, vacuum, surge pressure, and proof-pressure requirements against the complete catalog entry.
- Check ambient and fluid temperature, including nearby heat sources and cleaning cycles.
- Review fluid compatibility, especially for water, steam, oils, chemicals, or food-related media.
- Confirm seal material and any compliance claim at the exact part-number level.
- Plan installation access so the tube can be inserted, removed, inspected, and replaced safely.
Frequently asked questions
Are SMC metal fittings the same as stainless steel fittings?
No. Some SMC metal fittings, such as KQB2, use electroless nickel-plated brass parts. KQG2 is the stainless steel 316 one-touch fitting series. Both are metal options, but they are not equivalent in corrosion resistance, material cost, or application focus.
Can SMC metal fittings be used with polyurethane tubing?
SMC literature for KQB2 and KQG2 includes polyurethane among the applicable tubing materials. The tubing grade, temperature, pressure, and fluid must still be checked. A fitting rating does not override the tube manufacturer’s limits.
Which SMC fitting is better for steam?
SMC describes KQG2 stainless steel 316 one-touch fittings as suitable for steam. That does not mean every connected tube, seal, or threaded joint is automatically suitable. Steam service should be checked at the full assembly level.
Are SMC KQB2 fittings suitable for high temperature areas?
KQB2 is listed by SMC with a maximum operating temperature of 150°C, making it relevant for warmer industrial environments. The complete system still needs review because tubing, seals, nearby components, and pressure conditions may have lower limits.
Can these fittings be used for potable water?
Do not assume so. Some SMC product pages state that certain fitting products are not intended for potable water systems. Potable water use may require specific material approvals and local code compliance.
Bottom line
Choosing SMC metal fittings is less about finding a universal connector and more about matching a precise part number to a precise operating condition. KQB2 is a strong candidate for general industrial metal one-touch applications where plated brass construction and higher temperature capability are appropriate. KQG2 is the more relevant choice when stainless steel 316, corrosion resistance, grease-free construction, or steam suitability is required. In every case, tube material, thread standard, pressure, temperature, fluid, and installation method should be reviewed together before the fitting is installed.
