How to Use a Thread Pitch Gauge on Hydraulic Fittings

What You Need

You need two thread pitch gauges: one imperial/inch (covering 8-32 TPI) and one metric (covering 0.5-3.0mm pitch). Most hardware stores sell combination sets. You also need a caliper or ruler to measure the thread outside diameter. The combination of pitch and diameter uniquely identifies most hydraulic threads.

Step 1: Check Taper

Before measuring pitch, determine if the thread is tapered or straight. Hold the fitting at eye level and look along the thread. If the diameter visibly narrows toward the end, it is tapered (likely NPT or BSPT). If the diameter is uniform from start to finish, it is straight (JIC, ORB, ORFS, BSPP, or DIN metric).

You can also hold a straightedge against the threads. On a straight thread, the straightedge touches all thread crests. On a tapered thread, a gap appears at one end.

Step 2: Measure Pitch

Fan out the pitch gauge blades and try them against the threads one at a time. The correct blade will drop into the thread grooves and sit flush with no light visible between the blade teeth and the thread crests. If you see light, that pitch is wrong.

For imperial threads, the gauge reads in TPI (threads per inch). For metric, it reads in mm between threads. Write down the exact pitch that matches.

Step 3: Measure Diameter

With a caliper, measure the outside diameter of the male thread (or the inside diameter of the female thread). Measure across the crests of the threads. This measurement combined with pitch identifies the thread.

For NPT, measure at the fourth thread from the end (the hand-tight engagement point) since the diameter changes along the taper.

Step 4: Cross-Reference

With taper (yes/no), pitch, and diameter, you can identify the thread:

Tapered, 60-degree, imperial pitch: NPT Tapered, 55-degree, Whitworth pitch: BSPT Straight, 60-degree, UNF pitch, O-ring groove: SAE ORB Straight, 60-degree, UNF pitch, 37-degree flare seat: JIC Straight, 60-degree, UNF pitch, flat face: ORFS Straight, 55-degree, Whitworth pitch: BSPP Straight, 60-degree, metric pitch: DIN metric

The thread angle (55 vs 60 degrees) is the key differentiator between BSP and SAE/metric families. BSP uses 55-degree Whitworth form. JIC, ORB, ORFS, and DIN all use 60-degree thread form.