How to Identify Hydraulic Fittings

You have a leaking hydraulic fitting and no idea what standard it is. This is one of the most common problems in equipment maintenance. Here is how to identify it without guessing.

Step 1: Thread Type - Tapered or Parallel?

Try threading a known parallel (straight) nut onto the fitting. If it threads on and stays snug the entire length, the threads are parallel (straight). If it gets progressively tighter, the threads are tapered.

Tapered threads: NPT, BSPT Parallel threads: JIC, ORFS, BSPP, DIN, JIS

This single test eliminates half the options immediately.

Step 2: Measure the Thread Pitch

Use a thread pitch gauge or ruler. Count the threads per inch (TPI) for inch-based systems, or measure the distance between thread peaks in millimeters for metric.

NPT uses US thread pitches (14, 11.5, 8 TPI, etc.) BSP uses Whitworth pitches (14, 11, 8 TPI, etc.), similar but not identical to NPT Metric (DIN, JIS) uses mm pitch (1.0, 1.5, 2.0 mm, etc.)

If the TPI does not match any standard chart exactly, you may have a worn fitting or a less common standard.

Step 3: Check the Seal Method

How does the fitting seal?

Thread seal (tape/compound needed): NPT, BSPT. The tapered threads deform together to seal. 37-degree flare: JIC. Look for a cone-shaped flare at the end of the tube. O-ring face seal: ORFS. Look for a flat face with an O-ring groove. O-ring boss: SAE straight thread with O-ring on a shoulder. Bonded seal (washer): DIN, some JIS. Look for a metal-rubber bonded washer on the face. Parallel with washer or gasket: BSPP with bonded seal.

Step 4: Measure the Thread Diameter

Measure the outside diameter of the male thread (or the inside diameter of the female thread) with calipers.

Compare against the nominal size chart for the standard you identified in steps 1-3. Note that nominal sizes are not the actual measured diameter. A 1/2 inch NPT male thread measures approximately 0.840 inches OD.

The fitting standard pages on GageRef include sizing tables with actual measured dimensions for each standard.

When You Still Cannot Tell

If the fitting does not match any standard you can identify, it may be a proprietary design, a metric DIN or JIS fitting common on imported equipment, or a worn/damaged fitting that does not measure correctly.

Bring the fitting to a hydraulic supply shop. They have thread identification kits with calibrated gauges for every standard and can identify it in minutes.