Hydraulics & Fluid Power Reference Library · Measure & Identify

How to Use a Thread Pitch Gauge and Caliper

Thread diameter and pitch, measured with calipers and a thread pitch gauge, are the first steps in identifying a hydraulic fitting. This sheet shows how to take each measurement and avoid common misreads.

Ref HY-0064ASME B1.1ISO 261

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

What it is

Most fitting families share some thread sizes, so you need both the thread diameter and the pitch. Calipers measure the diameter. A thread pitch gauge is a set of toothed blades, each marked with threads per inch (TPI) or pitch in millimeters. You combine these with the seat angle and sealing method to name the fitting. The Fitting Thread Identification Chart lists the thread sizes by family.

How to use it

Thread diameter

Clean the threads.
Male thread: measure across the crests (major diameter) with the caliper jaws square to the axis.
Female thread: measure across the crests inside (minor diameter) with the inside jaws.
Measure in two places 90° apart. Take the larger reading for a male thread.
Tapered threads: measure at the same point each time, such as the fourth or fifth full thread from the end, and note that they are tapered.

Pitch

Try the blade that looks closest. Seat it along the thread axis.
The correct blade sits fully in the threads with no light and no rocking.
Check the blades on either side. Close pitches look alike.
Note whether the gauge is inch (TPI) or metric (mm).

Measurement guide

Measure	Tool	Tip
Male major diameter	Caliper outside jaws	Square to axis; check two places
Female minor diameter	Caliper inside jaws	Reads smaller than the male size; compare with the minor diameter
TPI	Inch pitch gauge	Full blade contact, no light
Metric pitch	Metric pitch gauge	Check inch blades too; some sizes are close
Taper	Caliper at two points along the thread	A change in diameter means a tapered thread
Conversion	Arithmetic	TPI = 25.4 ÷ pitch in mm

Key points

Some inch and metric pitches are very close. For example, 16 TPI is a 1.5875 mm pitch, close to 1.5 mm. Check both gauges over a long length.
Worn or damaged crests measure small. Measure an undamaged part of the thread.
Thread diameter alone does not give the family. Add seat angle and sealing method.
Pipe threads (NPT, BSPT) are tapered. Straight threads (UN, BSPP, metric) are not.

Common mistakes

Measuring a male thread at an angle.
Choosing a pitch blade that rocks.
Using only an inch gauge on a metric fitting.
Skipping the taper check and calling an NPT thread straight.

Governing standards: ASME B1.1 (unified inch screw threads); ISO 261 (metric thread general plan); ASME B1.20.1 (NPT); ISO 228-1 (BSPP).

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.

More in Measure & Identify

← All Hydraulics & Fluid Power sheets