Duolok® tube fittings use a two-ferrule design to ensure leak-tight performance in analytical and process instrumentation applications where leak-tight performance can have a significant impact on quality, productivity and cost. They are designed and manufactured to interchange and intermix with Swagelok® and Parker A-Lok™ tube fittings.

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  • MAWP: 12,000 psig*
  • Temperature Range: -325 to 1200°F
  • Materials: 316 SS, Alloy 400, C-276, and brass. 
  • Fractional Sizes: 1/16 to 2 in.
  • Metric Sizes: 3 to 25 mm
  • Adapters: NPT, ISO/BSP, SAE/MS, AN, weld, tube stub, port connectors, hose, and ORFS.
  • Shapes: Straight, elbows, tees, crosses, caps, plugs
  • Special fittings: Bulkhead and extended (long), bored through, positionable, calibration and dielectric fittings.
  • Accessories: Stop collars, bulkhead locks, nut and ferrule sets, tools and accessories.

*Duolok tube fittings are rated to the maximum allowable pressure rating of the tubing it connects.


Duolok tube fittings consist of four precision-machined components: body, front ferrule, back ferrule, and nut. During make-up, the double-ferrule design, with the staged sequential swaging action of the ferrules, compensates for the variation in tubing materials, hardness, and thickness of the tube wall to provide leak-tight connections in an extensive range of applications.

Through critical interaction of precision-machined fitting components with the process tube, a leak-tight seal is achieved. The simple geometric rotation of the Duolok nut provides the axial thrust necessary to "coin" the ferrules into the outside diameter of the tube. To eliminate any potential stress on an existing system, the tube fittings have been designed to not transmit installation torque from the tube fittings to the tube.

During the rotary movement of the nut, the internal surface of the nut meets with the rear surface of the rear ferrule to axially move the rear ferrule forward against the back radius of the front ferrule. Simultaneously, the front ferrule is driven forward into the angular section of the fitting body to form a primary metal-to-metal seal. The back ferrule roll-in locking action occurs on the outside diameter of the tube to complete the sealing action and secure the tube within the fitting.

The controlled ferrule drive prevents body distortion and helps compensate for exposure to system variables such as vibration, pressure pulsation and thermal expansion or contraction.


316 stainless steel fittings are manufactured from cold-finished bar stock in accordance with ASTM A-276 and ASTM A-479. Elbows, tees and crosses, are machined from close-grained 316 stainless steel forgings in accordance with ASTM A-182. All components are heat code traceable with certified material test reports (CMTRs) available on request. More information about material standards.


Duolok tube fittings are completely assembled and individually bagged to assure the highest levels of quality and cleanliness. The protective bags eliminate contamination (tubing burrs, dirt, etc.). They are packaged in convenient, small-lot quantities for easy procurement and handling. Box labels are color-coded according to the tube fittings’ material. Labels include the part number, material, product description and picture of the product.

Factory-Installed PTFE Tape

NPT thread and BSP tapered thread fittings are available with factory-installed high-quality Teflon tape. Factory-taped fittings reduce costs and ensure that threads are properly taped for more leak-tight installations.

Tube Fittings Installation Training

SSP and SSP distributors provide extensive installation training. Courses cover:

  • Intermixing and interchanging
  • Swaging and quality assurance
  • Remaking and reusing tube fittings
  • Handling cutting and preparing tubing
  • Tube bending

Quality Certification

Duolok tube fittings are manufactured in accordance with SSP’s Quality System, which has been certified to conform to the ISO 9001:2000 Quality Standard. In addition, all fittings are manufactured to the technical design specifications and rigid quality control standards of the SSP Instrumentation Division. SSP’s continuing commitment to quality is reflected throughout the company in its personnel, policies, equipment, products and service.


Duolok tube fittings have been approved by:

Tools and Accessories

SSP offers tools and accessories to support tube fitting assembly and installation and tube cutting, preparations and bending.