Hydraulic fitting identification: thread, mating part and sealing method

The most common connection problem in the field is not the wrong part, but the wrong match. BSPP parallel, BSPT tapered, NPT tapered, UNF/JIC and metric threads look alike, but they differ in angle, pitch, diameter and sealing method. Forcing two incompatible threads together comes back as a leak at the first pressurization, and often as permanent thread damage. Technical consulting exists to eliminate this uncertainty before the part is ordered.

How is the thread standard identified?

1. Outside diameter: The major diameter of a male thread, or the inside diameter of a female thread, is measured with a caliper. On its own it is not enough to decide, but it narrows down the candidate standards.
2. Pitch: The number of threads per inch, or the pitch in millimeters, is determined with a thread pitch gauge.
3. Thread angle: The BSP family uses the 55° Whitworth profile, while NPT, UNF and metric threads have a 60° profile. The key difference between BSP and NPT threads of the same pitch is the profile angle.
4. Tapered or parallel: If the diameter measured at the start and end of the thread differs, the thread is tapered (BSPT or NPT, 1:16 taper). On a parallel thread, the diameter is the same along its length.
5. Sealing surface: Look for the 37° flare seat on JIC, the 24° cone on DIN 2353/ISO 8434-1 connections, the flat face with an O-ring groove on ORFS, and the seal face on parallel-thread ports.

Correct matching

Once the standard is identified, the mating part is selected from the same standard. If two different standards need to meet, for example connecting a JIC-ended hose to a pump with a BSP port, a suitable conversion adapter is used instead of forcing the connection. Transitions not available in catalogs can be custom-made from a sample or technical drawing.

Selecting the sealing method

O-ring

On ORFS connections, the O-ring sits in a groove in the face and seals by being pressed against the flat face of the mating part. On SAE O-ring boss (ORB) connections, it sits in the neck where the thread ends and is compressed by the chamfer at the port mouth. The O-ring material must be compatible with the fluid and temperature.

Bonded seal

It consists of a metal washer with a rubber ring vulcanized to its inside diameter, and seals on the face of parallel-thread ports such as BSPP and metric. The port face must be flat, clean and free of scratches.

Cone seat

On JIC 37° and 24° cone connections, sealing is achieved through metal-to-metal contact. A nick or scratch on the cone surface is a direct leak path; sealing tape is never wrapped on these surfaces.

Thread sealing

On BSPT and NPT tapered threads, sealing is achieved by the threads wedging into each other together with a suitable thread sealant. The first one or two threads are left bare so that the sealant does not get into the system. This method does not provide reliable sealing on parallel threads.

Installation recommendations

• Seal faces should be kept clean, and connection axes should not be forced.
• On stainless steel threads, a suitable anti-seize compound can reduce the risk of galling.
• A suitable tightening method should be used; over-tightening damages the cone and thread, while under-tightening causes leaks.
• On hose connections, bend radius and twisting should be checked.
• A controlled leak check should be carried out after initial commissioning.

Remote preliminary identification: what should you send?

Clear photos of the part from different angles, the outside diameter measured with a caliper (even better if the caliper reading is visible in the photo), the pitch measured with a thread pitch gauge if available, and the make and model of the equipment the part is fitted to. With this information, a preliminary identification can be made for most connections; if a definitive identification is needed, the sample can be sent to Tuzla, Istanbul.

Applications by sector

The need for identification arises in any plant where equipment of different origins operates side by side: foreign-made deck and steering equipment on ships and in shipyards, mixed-standard hydraulic lines on offshore facilities, lines in rolling mills and paper and board mills where old and new machines are used together, and correct connection selection in defense, aviation, chemical, food and fire suppression systems.

From identification to supply

If the identified part is in stock, it is typically ready to ship within 1-3 business days. Standard parts not in stock are scheduled within an average of 5-10 business days, and custom conversion adapters within an average of 10-20 business days after technical approval.