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Metric Ports (ISO 6149 and ISO 9974)

ISO 6149 and ISO 9974 are the two common metric straight-thread port systems. ISO 6149 seals with an O-ring in the port chamfer; ISO 9974 seals on a flat spot face with an elastomer or metal seal on the stud end.

Ref HY-0019ISO 6149ISO 9974

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Both systems use metric fine straight threads (M-series) in the port, with the seal made away from the threads. ISO 6149 is the metric version of the SAE O-ring boss port. The port has a chamfer that holds an O-ring on the stud end. Part 1 covers the port, Part 2 the heavy-duty (S series) stud end and Part 3 the light-duty (L series) stud end. ISO 9974 ports have a flat spot face with no O-ring chamfer. The seal is on the stud end: an elastomer sealing ring (often called type E) or a metal-to-metal edge (type B). ISO 9974 is the metric match to the ISO 1179 BSPP port system.

How to identify

Measure the thread with a caliper and pitch gauge. Common sizes include M10x1, M12x1.5, M14x1.5, M16x1.5, M18x1.5, M22x1.5 and M27x2. Then look at the port. An ISO 6149 port has a chamfer for the O-ring and an identification ring or ridge machined on the spot face. An ISO 9974 port has a plain flat face. Stud ends for ISO 6149 carry an O-ring at the base of the thread; ISO 9974 stud ends have a bonded sealing ring or a sharp sealing edge. See the fitting thread identification chart.

Port system comparison

Item	ISO 6149	ISO 9974
Thread	Metric fine, straight	Metric fine, straight
Port feature	Chamfer for O-ring; identification ring on face	Flat spot face, no chamfer
Seal location	O-ring on stud end, in port chamfer	Elastomer ring or metal edge on stud end, on spot face
Stud end standards	ISO 6149-2 (S series), ISO 6149-3 (L series)	ISO 9974-2 (type E), ISO 9974-3 (type B)
Inch counterpart	SAE J1926 and J1453 O-ring boss	Similar to ISO 1179 BSPP style
Adjustable stud ends	Yes, with locknut and backup washer	Usually non-adjustable

Key points

Thread size alone does not tell the port type. Check the port face for the identification ring.
An ISO 6149 stud in an ISO 9974 port will not seal because there is no chamfer for the O-ring.
Use the O-ring compound and size listed for the stud end. Replace O-rings and sealing rings each time.
Tightening values and pressure ratings are in the fitting maker's data and the ISO standards.
Metric straight threads look close to some UN threads. Measure pitch in millimeters to confirm.

Common mistakes

Using thread sealant or tape on straight-thread ports.
Reusing a flattened bonded seal.
Mixing up ISO 6149 and ISO 9974 parts with the same thread size.
Overtightening an adjustable stud and cutting the O-ring.

Governing standards: ISO 6149-1, -2 and -3 (metric ports and stud ends with O-ring seal); ISO 9974-1, -2 and -3 (metric ports and stud ends with elastomer or metal seal).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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